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  • 1.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    All-oxide ceramic matrix composites2001Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This work has concerned the preparation and properties of all-oxide composites. The most common examples of such materials are composites of oxide particles in an oxide matrix, continuous oxide fibres in an oxide matrix and layered composites consisting of alternate layers of different oxides. In the case of continuous fibre composites, the matrix and fibres can be of the same oxide since the mechanical properties of the composite are to a large extent influenced by the interface between the two. All-oxide composites are of interest as possible high temperature materials since they are inherently oxidation resistant. The emphasis of the thesis is on continuous fibre composites. The thesis, consisting of an extensive introductory review and six appended papers (listed in appendix 2), covers a number of aspects of these composites including the properties of candidate oxide constituents, methods of composite preparation, microstructure and mechanical properties. The first paper describes attempts made to prepare alumina composites reinforced with monocrystalline (sapphire) fibres using hot isostatic pressing. The second paper reports on the thermal expansion behaviour of candidate oxides and the consequences of differences in thermal expansion of composite constituents with respect to residual thermal stresses in the composite. Paper VI reports measurements made of the stress-strain and fracture behaviour of experimental sapphire fibre/alumina matrix composites. The materials studied included both unidirectional and 0/90° cross-ply composites, all with a thin layer of zirconia at the fibre/matrix interface designed to adjust the interfacial properties to provide optimum composite stress-strain behaviour. The results could be linked to measurements made of interfacial properties. Papers III, IV and V concern the stress strain behaviour of a commercial all-oxide composite consisting of fine-diameter, polycrystalline oxide fibres in a porous, aluminosilicate matrix. In these composites the fibres were woven in a 0/90° geometry and tested in both the 0/90° and ±45° orientation. The emphasis of the study was on the notch-sensitivity of test specimens containing a central circular hole. Aspects studied included failure mechanisms, the effects of notch size and the effects of high temperature thermal exposure on microstructure and strength degradation. The observed behaviour could be described successfully in terms of a simple model based on fracture mechanics.

  • 2.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Preparation and properties of sapphire/alumina long fibre composites1999Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The research described in the thesis concerns the preparation and properties of oxide/oxide composites and in particular composites consisting of an oxide matrix reinforced with continuous oxide fibres and intended for use as structural materials at very high temperatures. For this application particular attention must be paid to the behaviour of the fibre/matrix interface and to the properties of the fibre. The research has involved two main aspects (i) a thorough review of the physical and mechanical properties of candidate oxides with emphasis on elastic properties and creep properties and (ii) the experimental development of methods to produce continuous fibre reinforced oxide/oxide composites.

  • 3.
    Antti, Marta-Lena
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Babushkin, O
    Luleå tekniska universitet.
    Shen, Z
    Nygren, M
    Warren, R
    Thermal expansion behaviour of high melting point oxides1999In: Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795, Vol. 164-165, p. 279-282Article in journal (Refereed)
    Abstract [en]

    The thermal expansion characteristics of the reinforcement and matrix constituents in ceramic composites have a strong influence on a number of aspects of the composite performance. Thus the toughness and strength of the composite can be influenced by local residual stresses due to the thermal expansion mismatch of different phases. Moreover, in individual polycrystalline phases, local stress variations from grain to grain, generated as a result of anisotropy of thermal expansion in combination with anisotropy of elasticity can influence strength and toughness in similar ways. In the present study, reported data on the thermal expansion coefficients of oxides having potential as constituents in high temperature composites are reviewed and complemented with additional measurements made using high temperature X-ray diffraction

  • 4.
    Antti, Marta-Lena
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Cheng, Y-B
    Monash University, Melbourne, VIC.
    Odén, Magnus
    Synthesis and phase development in the Ct-Al-N system2009In: Mechanical properties and processing of ceramic binary, ternary and composite systems: a collection of papers presented at the 32nd International Conference on Advanced Ceramics and Composites, January 27-February 1, 2008, Daytona Beach, Florida / [ed] Jonathan Salem; Greg Hilmas; William Fahrenholtz, Hoboken, NJ: John Wiley & Sons, 2009, p. 3-12Conference paper (Refereed)
    Abstract [en]

    The ternary nitride system Cr-Al-N has been investigated by sintering different powder compositions. The powder compositions belong to four groups, AIN- + Cr-powder (5 compositions between 20-90 molar% AlN), Al- + Cr 2N-powder (5 compositions between 15-80 molar% Cr2N), AlN-+ Cr2N-powder (50- and 90 molar% Cr2N) and Al- + Cr-powder. The powders were dry mixed and pressed into pellets by uniaxial pressing followed by cold isostatic pressing (ClP). Sintering took place in a graphite lined reaction bonding furnace under nitrogen atmosphere at three different temperatures, 1350°C, 1500°C and 1800°C and in an alumina tube furnace in order to avoid access to carbon. Holding times were varied, from 2 hours up to 72 hours. The phase development was evaluated by thermal analysis and XRD. CrAlN was formed at 1350°C but decomposed at higher temperatures. Both pure Al and Cr-powder were prone to react with carbon in the graphite furnace. Thermal analysis showed a sublimation of Cr2N at temperatures around 1050°C and nitridation of pure Al-powder between 680-750°C and of pure Cr-powder between 610-1080°C. Samples with pure Al-powder showed a very large expansion due to melting of aluminium in combination with nitridation. AIN was found to be more stable than Cr 2N at higher temperatures and longer holding times. The mixtures of Al-+Cr-powder produced an intermediate Al-Cr-phase.

  • 5.
    Antti, Marta-Lena
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Frisk, Lars
    Warren, Richard
    Luleå tekniska universitet.
    P, P Vincenzini
    Synthesising of a model composite in the oxide/oxide system: alumina fibre reinforced alumina matrix1999In: Advanced structural fiber composites, TECHNA Srl , 1999Conference paper (Refereed)
    Abstract [en]

    Ceramic matrix composites have attracted big interest as potential material for high temperature applications. To avoid one of the main problems, namely oxidation at higher temperatures, interest has increased for systems where both reinforcement and matrix material are ceramic oxides. This work presents a possible method of producing long fibre reinforced ceramic matrix composites. The method was applied to single crystal aluminium oxide fibres in a polycrystalline alumina matrix and involves cladding individual fibres with the matrix material in the form of pre-sintered tubes with appropriately small diameter. The composite is then hot isostatically pressed (HIP), by putting several fibre-tube pairs in a container of required diameter. A HIP cycle was found that led to full densification of the composite without visual damage to the fibres. The method produces composites with an ideal uniform fibre spacing; samples of cylindrical shape suitable, for example, for uniaxial testing can be produced

  • 6.
    Antti, Marta-Lena
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Kero, Ida
    Cheng, Y-B
    Monash University, Melbourne, VIC.
    Tegman, Ragnar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Phase reactions in a hot pressed TiC/Si powder mixture2012In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 38, no 3, p. 1999-2003Article in journal (Refereed)
    Abstract [en]

    This work investigated the possibility of producing dense Ti3SiC2 by hot pressing TiC/Si powders. A hot press with graphite heating elements was used for densification and the phase reactions of some hot pressed samples were further evaluated by pressureless heating in a dilatometer. The density and phase composition of the heat treated samples were evaluated using Archimedes principle and by x-ray diffractometry respectively. Hot pressing resulted in a low Ti3SiC2 yield; the main phases were TiC and TiSi2 regardless of starting powder composition, temperature, holding time or pressure. A second heating without pressure resulted in Ti3SiC2 formation, but only in samples initially hot pressed at 1300 °C or lower. At higher hot pressing temperatures, thin oxide layers on particle surfaces were locked into the structure. Acting as diffusion barriers, they prevented the Ti3SiC2 forming reaction. In hot pressed samples the density was significantly higher than in samples sintered without pressure

  • 7.
    Antti, Marta-Lena
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lara-Curzio, E.
    Effect of notches, specimen size, and fiber orientation on the monotonic tensile behavior of composites at ambient and elevated temperatures2001In: Ceramic Engineering and Science Proceedings, ISSN 0196-6219, E-ISSN 1940-6339, Vol. 22, no 3, p. 643-650Article in journal (Refereed)
    Abstract [en]

    The effect of circular central holes on the monotonic tensile stress-strain behaviour and strength of a continuous fibre-reinforced oxide/oxide composite was investigated. The material used consisted of 12 layers of un-coated plain weave fabric of Nextel 720, with either 0/90 or plus or minus 45 degrees fibre orientation with respect to the loading direction, embedded in an aluminosilicate matrix. 100 and 200-mm long straight-sided specimens with central holes were tested for ratios of hole diameter to width (a/w) between 0.1 and 0.4, and width values between 3 mm and 25 mm. It was found that as-processed specimens with both fibre architectures were notch insensitive at ambient temperature, but exhibited mild notch sensitivity at elevated temperatures. It was also found that the strength of the material was retained after heat treatments at 1000 C, but that it decreased significantly after exposures at 100 C for periods of time of 20 and 100 h. The strength of test specimens with 0/90 fibre orientation was found to decrease with increasing width, but no size effects on strength were observed for specimens with plus or minus 45 degrees fibre orientation. These results are discussed in relation to the microstructure of the material and its evolution at elevated temperatures.

  • 8.
    Antti, Marta-Lena
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lara-Curzio, E.
    Ferber, M K
    Analysis of damage evolution in continuous fiber-reinforced oxide /oxide composites under cyclic loading, using infrared thermography2001In: Ceramic Engineering and Science Proceedings, ISSN 0196-6219, E-ISSN 1940-6339, Vol. 22, no 3, p. 711-716Article in journal (Refereed)
    Abstract [en]

    The evolution of damage in continuous fiber-reinforced oxide/oxide matrix composites was investigated at ambient temperature using an infra-red camera and a servohydraulic testing machine. The material used in this study consisted of 12 layers of un-coated fabric of Nextel 720 (either 0/90 or plus/minus45 deg fiber orientation with respect to the loading direction) and an aluminosilicate matrix. Straight-sided specimens 100 mm long and 12.5 mm wide with center holes were tested for a/w between 0.1 and 0.4. It was possible to record the onset of "necking", the propagation of cracks in the matrix, and the rotation of fiber bundles during matrix crack growth. These processes were recorded using a fast-speed digital image recorder/analyzer, and the sequence of events is presented.

  • 9.
    Antti, Marta-Lena
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lara-Curzio, E.
    Oak Ridge National Laboratory.
    Warren, R
    Malmö University.
    Thermal degradation of an oxide fibre (Nextel 720)/aluminosilicate composite2004In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 24, no 3, p. 565-578Article in journal (Refereed)
    Abstract [en]

    The effect of thermal exposure on the microstructure and tensile stress-strain behaviour has been investigated for composites of woven continuous oxide fibres (Nextel 720) in a porous aluminosilicate matrix. The tensile tests were carried out on straight-sided, centre hole notched plates with 0/90° and ±45° orientations. The as-received material was slightly notch sensitive in that the net section fracture stress decreased somewhat with increasing hole diameter but much less than predicted for an ideally elastic, fully notch-sensitive material. After exposure at 1100°C and for long time at 1000°C in air the composite was embrittled. In the 0/90 composite this resulted in a reduced fracture strength, a reduced strain to failure as well as a reduced fracture toughness and damage zone size. After exposure for 100 h at 1100 °C (the most extreme exposure applied) the material also became significantly more notch sensitive and had failure characteristics similar to those of a monolithic ceramic. The ±45 composite was also embrittled which resulted in a reduced strain to failure but an increase in fracture strength. Density measurements and observations on the microstructure and fracture surfaces indicated that the embrittlement was due mainly to localised densification of the matrix and an increase in fibre/matrix bonding.

  • 10.
    Antti, Marta-Lena
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    LePagne, D
    Holmqvist, M.
    Paquien, J-N
    Warren, Richard
    Micromechanics of failure in an alumina fibre reinforced alumina matrix composite2000In: Composites - from fundamentals to exploitation: [the premier composites conference in Europe ; proceedings] / ECCM9, 4 - 7 June 2000, Brighton Conference Centre, UK, London: Institute of Materials , 2000Conference paper (Refereed)
  • 11.
    Ekman, Jonas
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Martin-Torres, Javier
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Emami, Reza
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Törlind, Peter
    Luleå University of Technology, Department of Business Administration, Technology and Social Sciences, Innovation and Design.
    Kuhn, Thomas
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Nilsson, Hans
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Minami, Ichiro
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Öhrwall Rönnbäck, Anna
    Gustafsson, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Zorzano Mier, Maria-Paz
    Milz, Mathias
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Grahn, Mattias
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Parida, Vinit
    Luleå University of Technology, Department of Business Administration, Technology and Social Sciences, Innovation and Design.
    Behar, Etienne
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering.
    Wolf, Veronika
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Dordlofva, Christo
    Luleå University of Technology, Department of Business Administration, Technology and Social Sciences, Innovation and Design.
    Mendaza de Cal, Maria Teresa
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Jamali, Maryam
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Roos, Tobias
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Ottemark, Rikard
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Nieto, Chris
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Soria Salinas, Álvaro Tomás
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Vázquez Martín, Sandra
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Nyberg, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Neikter, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lindwall, Angelica
    Luleå University of Technology, Department of Business Administration, Technology and Social Sciences, Innovation and Design.
    Fakhardji, Wissam
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Projekt: Rymdforskarskolan2015Other (Other (popular science, discussion, etc.))
    Abstract [en]

    The Graduate School of Space Technology

  • 12.
    Engström, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Mouzon, Johanne
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Córdoba, J.M.
    Instituto de Ciencia de Materiales de Sevilla, Código del Centro.
    Tegman, Ragnar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Synthesis of a TiCN – SiC polyhedron and elongated crystals nanopowder at low nitrogen concentration2012In: Materials letters (General ed.), ISSN 0167-577X, E-ISSN 1873-4979, Vol. 81, p. 148-150Article in journal (Refereed)
    Abstract [en]

    At room temperature diluted TiCl4 and CCl4 were reduced by sodium particles and mixed with a polycarbomethylsilane (PCS) solution to yield a precursor. It was dried and subsequently annealed at 1300 °C, 1400 °C and 1450 °C in a tube furnace using argon with 10 ppm N2. After the 1450 °C annealing a nanocrystalline powder of TiC0.5 N0.5–SiC polyhedron and elongated crystals was obtained. At the low nitrogen concentration during annealing a gradual nitration is proposed. It is promoted by carbon gaseous species, precursor oxidation, a sufficient temperature and a summarised nitrogen surplus compared to the titanium and carbon amount.

  • 13.
    Escalera, Edwin
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Odén, Magnus
    Thermal treatment and phase formation in kaolinite and illite based clays from tropical regions of Bolivia2012In: 6th EEIGM International Conference Advanced Materials Research: 7th and 8th November, 2011 EEIGM, Nancy, France, Bristol: IOP Publishing Ltd , 2012Conference paper (Refereed)
    Abstract [en]

    The aim of this study was to compare the thermal behaviour of clays containing illite and kaolinite in various proportions. The clays contained small amounts of K and Fe, which act as fluxing agents. In order to investigate the phase formations during heating, the samples were examined in a differential scanning calorimeter at temperatures up to 1300°C. The thermal expansion of the samples was determined by dilatometer measurements from room temperature up to 1150°C. Phases were identified using x-ray diffraction and scanning electron microscopy. In all samples, most of the kaolinite was transformed into metakaolinite during heating up to 650°C, while illite remained unchanged up to 950°C. There was no influence of K and Fe on dehydroxylation. Metakaolinite formed at temperatures above 950°C leading to a Si-Al spinel. Furthermore, mullite was formed in the temperature interval 1050-1150°C. In this temperature range, the mechanism of mullite formation depended on the amount of K and Fe in the samples, changing the temperature of formation of mullite. It was observed by x-ray diffraction that most of the illite was transformed into a Si-Al spinel phase at 1050°C, and during further heating transformed into mullite. An increased amount of illite in the clays slightly decreased the melting temperature. The dilatometer measurements showed expansion and shrinkage for the dehydroxylation and spinel-phase formation, respectively.

  • 14.
    Escalera, Edwin
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ballem, Mohammed A.
    Nanostructured Materials, Department of Physics, Chemistry and Biology, Linköping University.
    Cordoba, José M.
    Nanostructured Materials, Department of Physics, Chemistry and Biology, Linköping University.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Odén, Magnus
    Synthesis of homogeneously dispersed cobalt nanoparticles in the pores of functionalized SBA-15 silica2012In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 221, p. 359-364Article in journal (Refereed)
    Abstract [en]

    Cobalt nanoparticles were prepared at room temperature by reducing cobalt sulfate heptahydrate with sodium borohydride using functionalized SBA-15 mesoporous silica as a hard template. It was found that both external and internal fuctionalization of silica walls play a crucial role on the infiltration and reaction of the reagents in the silica framework. Subsequent heat treatment of the impregnated silica at 500 °C in air or nitrogen atmospheres leads to growth of crystals of the deposited cobalt and formation of cobalt oxide and cobalt nanoparticles, respectively. Dissolution of the silica template by NaOH resulted in well dispersed Co and Co3O4 nanoparticles ranging in size between 2 and 4 nm. The functionalization of the silica was studied by FTIR, N2-physisorption, and thermogravimetric techniques and the obtained nanoparticles were characterized by XRD, TEM and EDX analysis.

  • 15.
    Escalera, Edwin
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Garcia, Gustavo
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Terán, R
    Chemistry Department, San Simon University.
    Tegman, Ragnar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Odén, Magnus
    The production of porous brick material from diatomaceous earth and Brazil nut shell ash2015In: Construction and Building Materials, ISSN 0950-0618, E-ISSN 1879-0526, Vol. 98, p. 257-264Article in journal (Refereed)
    Abstract [en]

    Diatomaceous earth was mixed with Brazil nut shell ash (BNS ash) in different amounts between 0 and 30 wt% and sintered at temperatures between 750 and 950 °C. The BNS ash contains 33 wt% K2O and 11 wt% CaO mainly in carbonate form. The addition of BNS ash into the diatomaceous earth caused significant changes of the microstructure after sintering. The BNS ash addition produces lightweight porous bricks with acceptable strength at lower sintering temperature. The best combination of strength and porosity was achieved for a mixture of 10 wt% of BNS ash in the diatomaceous earth sintered at 850 °C. The achieved high porosity was 49%, density 1.06 g/cm3, thermal conductivity 0.20 W/(m K) and the compressive strength was 8.5 MPa.

  • 16.
    Escalera, Edwin
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Tegman, Ragnar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Odén, Magnus
    Linköpings universitet.
    High temperature phase evolution of Bolivian kaolinitic–illitic clays heated to 1250 °C2014In: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053, Vol. 101, p. 100-105Article in journal (Refereed)
    Abstract [en]

    The thermal behaviour of two types of clays collected from different locations in Bolivia has been studied. The clays contain kaolinite, illite, quartz and small amounts of microcline. The phase evolutions have been characterized from room temperature to 1250 °C. For both clays, kaolinite is completely transformed into metakaolinite when heated up to 650 °C. During further heating to 1050 °C, illite undergoes total dehydroxylation. Mullite is formed in the temperature interval of 1050–1150 °C and its formation rate is dependent on the amount of K and Fe present in the clays. The clay with higher amounts of K (3.2 mass %) and Fe (5.6 mass%) has an onset temperature for sintering at about 900 °C and an onset temperature for liquid formation at 1080 °C. This is about 50 °C lower onset temperature for sintering and 94 °C lower onset temperature for liquid formation when compared with the clay with lower amounts of K (2.3 mass %) and Fe (1.6 mass %).

  • 17.
    Fargas, Gemma
    et al.
    CIEFMA/EEBE, Departament de Ciència dels Materials i Enginyeria Metallúrgica, Universitat Politècnica de Catalunya, Barcelona.
    Roa, Joan Josep
    CIEFMA/EEBE, Departament de Ciència dels Materials i Enginyeria Metallúrgica, Universitat Politècnica de Catalunya, Barcelona.
    Sefer, Birhan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. CIEFMA/EEBE, Departament de Ciència dels Materials i Enginyeria Metallúrgica, Universitat Politècnica de Catalunya, Barcelona.
    Pederson, Robert
    Department of Engineering Science, University West.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mateo, Antonio
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Oxidation behavior of Ti6Al4V alloy exposed to isothermal and cyclic thermal treatments2017In: METAL 2017: 26th International Conference on Metallurgy and Materials, Conference Proceedings, Ostrava: TANGER Ltd. , 2017, p. 1573-1579Conference paper (Refereed)
    Abstract [en]

    One of the most common titanium alloys for aerospace industry is Ti6Al4V (usually designed as Ti64) which is used for manufacturing aero-engine components, such as fan discs, compressor discs, blades and stators. The maximum service temperature for this alloy is limited partly because of degradation of mechanical properties at elevated temperatures (above 480 °C). During the first stage of oxidation the oxide scale is protective, whereas after prolonged oxidation time it loses its protective nature and favours higher diffusion of oxygen through the oxide. In the present study, cyclic thermal treatments were performed in air at 500 and 700 °C, up to 500 hours, and compared with similar studies carried out on isothermal oxidation conditions. The evolution of the surface oxidation was analyzed by metallographic techniques and X-ray diffraction, together with a detailed advanced characterization of the microstructure by Scanning Electron Microscopy and Focus Ions Beam. The results point out that the cyclic thermal treatments induced a strong increase of the weight gain compared to isothermal treatments. The analysis of the oxide scale revealed not only the presence of rutile, at 700 °C, but also anatase and TiOx at 500 °C for both isothermal and cyclic thermal treatments. At 700 °C, thermal stress caused by cyclic thermal treatments promoted the fracture of the oxide after the first 20 hours.

  • 18.
    Gaddam, Raghuveer
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Influence of alpha–case layer on the low cycle fatigue properties of Ti–6Al–2Sn–4Zr–2Mo alloy2014In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 599, p. 51-56Article in journal (Refereed)
    Abstract [en]

    Strain–controlled low cycle fatigue properties of Ti–6Al–2Sn–4Zr–2Mo with different thickness of alpha–case layers were investigated. Results show that at strain amplitudes 0.3 and 0.4%, the fatigue life of the alloy is reduced for the specimens with alpha–case layer compared to the ones without any alpha–case. It was noted that with a 2 μm thick alpha–case layer the low cycle fatigue life is reduced about 50% at the higher strain amplitude. The degrading effect of the alpha–case layer on fatigue life increased with increasing thickness. The alpha–case layer at the surface is enriched with oxygen making the surface harder and brittle, which results in easier crack initiation and thus decrease in fatigue life.

  • 19. Gaddam, Raghuveer
    et al.
    Hörnqvist, M.
    Department of Applied Physics, Chalmers University of Technology.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Influence of High-pressure gaseous Hydrogen on the low-cycle fatigue and fatigue crack growth properties of a cast titanium alloy2014In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 612, p. 354-362Article in journal (Refereed)
    Abstract [en]

    In the present study, the effect of gaseous hydrogen on the fatigue properties of a commonly used aerospace titanium alloy (Ti–6Al–4 V) was studied. The low-cycle fatigue and fatigue crack growth properties were investigated at room temperature in ambient air and 15 MPa gaseous hydrogen. Results showed that the low-cycle fatigue life was significantly reduced in hydrogen, and the detrimental effect was larger at higher strain amplitudes. The fatigue crack growth rate in hydrogen remained unaffected below a critical stress intensity ΔK⁎≈17 MPa√m, while beyond this value, the fatigue crack growth rate fluctuated and increased with increasing ΔK. Fractography analysis clearly showed that gaseous hydrogen mainly affected the fatigue crack growth rate. On the fracture surfaces, striations were noted over the entire crack growth region in air, whereas in hydrogen striations were noted at stress intensities lower than ΔK⁎. Above ΔK⁎, secondary cracks and brittle flat surfaces with features similar to crack arrest marks were mostly observed in hydrogen. Microstructural analysis along the crack growth direction showed that the crack followed a transgranular path in air, i.e. through α colonies. In hydrogen, the crack also grew along the prior β grain boundaries and at α/β interface within the α colonies. Thereby, the detrimental effect of hydrogen in cast titanium alloy was attributed to a change in the fracture process during crack propagation

  • 20.
    Gaddam, Raghuveer
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Hörnqvist, Magnus
    GKN Aerospace Engine Systems.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Fatigue crack growth behaviour of forged Ti-6Al-4V in gaseous hydrogen2014In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 78, p. 378-383Article in journal (Refereed)
    Abstract [en]

    Fatigue crack growth (FCG) tests were performed to evaluate the fatigue behaviour of forged Ti-6Al-4V in air and high-pressure gaseous hydrogen (15 MPa) at room temperature. The results indicate that the effect of gaseous hydrogen is dependent on the stress intensity factor (ΔK). The FCG rate was unaffected by hydrogen below a critical stress intensity, ΔK* ≈ 20 MPa√m. Above ΔK*, the FCG rate fluctuated and subsequently accelerated at higher ΔK values. The observed behaviour is attributed to the change in the fracture processes. A hypothesis is proposed that describes the FCG behaviour in gaseous hydrogen.

  • 21.
    Gaddam, Raghuveer
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Hörnqvist, Magnus
    GKN Aerospace Engine Systems.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Influence of hydrogen environment on fatigue crack growth in forged Ti-6Al-4V: fractographic analysis2013In: 7th EEIGM International Conference on Advanced Materials Research: 21–22 March 2013, LTU, Luleå, Sweden, IOP Publishing Ltd , 2013, article id 1210Conference paper (Refereed)
    Abstract [en]

    In this study, the influence of hydrogen environment (15 MPa) on the fatigue crack growth in forged Ti-6A1-4V at room temperature is investigated. It is observed that at 21 < ΔK > 25 MPa√m, there exists a change of fatigue crack growth rate (FCGR) in hydrogen environment, and it is accelerated at ΔK > 25MPa√m. FCGR in hydrogen environment is dependent on the stress intensity levels (ΔK). Detailed fractographic analysis of the fracture surfaces were performed at different ΔK using high-resolution scanning electron microscope (HR-SEM). Fatigue striations were observed in air and hydrogen at ΔK < 21MPa√m. At ΔK > 21MPa√m, secondary cracks were observed in hydrogen environment. The differences in appearances of fracture surfaces in air and hydrogen are discussed.

  • 22. Gaddam, Raghuveer
    et al.
    Sefer, Birhan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Oxidation and alpha–case formation in Ti–6Al–2Sn–4Zr–2Mo alloy2015In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189, Vol. 99, p. 166-174Article in journal (Refereed)
    Abstract [en]

    Isothermal heat treatments in ambient air were performed on wrought Ti–6Al–2Sn–4Zr–2Mo (Ti–6242) material at 500, 593 and 700 °C for times up to 500 hours. In presence of oxygen at elevated temperatures simultaneous reactions occurred in Ti–6242 alloy, which resulted in formation of an oxide scale and a layer with higher oxygen concentration (termed as alpha–case). Total weight gain analysis showed that there was a transition in the oxidation kinetics. At 500 °C, the oxidation kinetics obeyed cubic relationship up to 200 hours and thereafter changed to parabolic at prolonged exposure times. At 593 °C, it followed parabolic relationship. After heat treatment at 700 °C, the oxidation obeyed parabolic relationship up to 200 hours and thereafter changed to linear at prolonged exposure times. The observed transition is believed to be due to the differences observed in the oxide scale. The activation energy for parabolic oxidation was estimated to be 157 kJ/mol. In addition, alpha–case layer was evaluated using optical microscope, electron probe micro analyser and microhardness tester. The thickness of the alpha–case layer was found to be a function of temperature and time, increasing proportionally, and following parabolic relationship. The activation energy for formation of alpha–case layer was estimated to be 153 kJ/mol.

  • 23.
    Gaddam, Raghuveer
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sefer, Birhan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Study of alpha case depth in Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V2013In: 7th EEIGM International Conference on Advanced Materials Research: 21–22 March 2013, LTU, Luleå, Sweden, IOP Publishing Ltd , 2013, article id 12002Conference paper (Refereed)
    Abstract [en]

    Titanium alloys, mostly Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) and Ti-6Al-4V (Ti-64) are used in aero engine applications, because they possess high specific strength. The future concept in designing aircraft engines results in higher pressure, which increases the efficiency of aircraft engines by achieving high thrust and lowering the fuel consumption. Nevertheless, higher pressure in the engine means increase of service temperature. These conditions enforce new requirements on the materials used for manufacturing the engine components (compressors). Ti-6242 is mostly used in compressors where the service temperature is in the range of 400-450°C. It is well known that titanium alloys above 480°C for longer service time have tendency to form a hard and brittle oxygen stabilized surface layer (α-case). This layer has impact on the mechanical properties of the surface, by lowering the tensile ductility and the fatigue resistance. Factors that contribute for growth of α-case are: presence of oxygen, exposure time, temperature and pressure. In order to extend the service temperature of titanium alloys, it is required to understand the formation of α-case at high temperatures for long exposure times. In the present study, isothermal oxidation experiments in air were performed on forged Ti-6242 alloy at 500°C and 593°C up to 500 hours. Similar studies were also performed on Ti-64 sheet at 593°C and 700°C. Alpha case depths for both alloys were quantified using metallography techniques and compared.

  • 24.
    Gaddam, Raghuveer
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Åkerfeldt, Pia
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Influence of hydrogen environment on the mechanical properties of cast and electron beam melted Ti-6Al-4V2012In: Ti-2011: proceedings of the 12th World Conference on Titanium, June 19-24, 2011, China National Convention Center (CNCC), Beijing / [ed] Lian Zhou, Beijing: Social Sciences Academic Press (China), 2012, Vol. 3, p. 1885-1888Conference paper (Refereed)
    Abstract [en]

    In order to save weight in a certain engine application the possibility of replacing the currently used material with cast Ti-6Al-4V is investigated here. The working environment for this particular engine part is pure hydrogen gas at high pressure. Therefore selected mechanical properties such as tensile and low cycle fatigue (LCF) in air and hydrogen atmosphere have been studied for cast Ti-6Al-4V. In addition to cast Ti-6Al-4Vt the corresponding mechanical properties of a more recently developed additive manufacturing method, electron beam melting (EBM), is also investigated in hydrogen and compared with cast Ti-6Al-4V. Cast Ti-6Al-4V showed lower yield strength and lower ultimate tensile strength in hydrogen compared with air. However, no significant change in the ductility was observed. The LCF was significantly reduced in the hydrogen atmosphere, mostly at high strain range (π 2%). The EBM Ti-6Al-4V in hydrogen showed higher yield strength, higher ultimate strength and higher ductility as well as improved fatigue life compared with cast Ti-6Al-4V under the same test conditions. Microstructural and fractographic characterization were also performed and the results are included.

  • 25. Kero, Ida
    et al.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Odén, Magnus
    Linköpings universitet.
    Preparation and firing of a TiC/Si powder mixture2009In: 5th International EEIGM/AMASE/FORGEMAT Conference on Advanced Materials Research, Bristol: IOP Publishing Ltd , 2009Conference paper (Refereed)
    Abstract [en]

    This paper describes how the preparation and heat treatment of TiC/Si powders influences the phase reactions during firing. The powders are prepared by milling and some effects of powder preparation are discussed. A solid state displacement reaction according to: 3TiC + 2Si → Ti3SiC2 + SiC is a priori expected to take place during heat treatment. The firing procedure is investigated with respect to the effect of heat treatment time and temperature on the phases produced, especially Ti3SiC2. Samples were heat treated in a graphite lined furnace. Heat treated samples are analysed by x-ray diffraction, scanning electron microscope and energy dispersive spectroscopy. Ti3SiC2, TiC and SiC are dominant in the final products. The highest amount of Ti3SiC2 is achieved for short holding times (2-4 hours) at high temperatures (1350-1400°C). Ti3SiC2 appears to decompose at elevated temperatures or extended times, through a Ti3SiC2 → TiC + Si(g) type reaction. The activation energy of Ti3SiC2 phase formation is determined to be 289 kJ/mol, using the Mehl-Avrami-Johnson model.

  • 26. Kero, Ida
    et al.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Odén, Magnus
    Linköpings universitet.
    Synthesis of Ti3SiC2 by reaction of TiC and Si powders2009In: Mechanical properties and processing of ceramic binary, ternary and composite systems: a collection of papers presented at the 32nd International Conference on Advanced Ceramics and Composites, January 27-February 1, 2008, Daytona Beach, Florida / [ed] Jonathan Salem; Greg Hilmas; William Fahrenholtz, Hoboken, NJ: John Wiley & Sons, 2009, p. 21-30Conference paper (Refereed)
    Abstract [en]

    The MAX phase Ti3SiC2 has been synthesized from starting powder mixtures which do not include pure titanium. The presence of pure titanium in a powder is problematic because of its oxidizing, and in the form of a finely divided powder, explosive nature. The aim of this study was to evaluate the synthesis of bulk polycrystalline samples of Ti3SiC2 from a starting powder mixture which is more suited for large scale production. Titanium silicon carbide MAX phase was synthesized by pressureless sintering of ball milled TiC and Si powders of six different compositions. The sintering reactions were evaluated in situ by dilatometer analysis under flowing argon gas. The as-sintered samples were evaluated using mainly x-ray diffraction (XRD) analysis. This study showed that titanium carbide, silicon carbide and titanium disilicide were present as intermediate or secondary phases in the samples.Our results indicate that TiSi2 is an intermediate phase to the formation of Ti3SiC2 when excess Si is present. The excess of silicon also proved beneficial for the synthesis of the MAX phase and there is a Si content which is optimal with respect to the maximum MAX phase content of the final product. The Ti3SiC2 was found to decompose into TiC and gaseous Si at high temperatures.

  • 27.
    Kero, Ida
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Odén, Magnus
    Ti3SiC2 Synthesis by Powder Metallurgical Methods2007In: European School of Materials Science and Engineering, fourth research conference: preceedings of the fourth meeting of the European School of Materials Sciencie and Engineering / [ed] Marc Anglada, Barcelona: CPDA-ETSEIB , 2007Conference paper (Refereed)
    Abstract [en]

    Titanium silicon carbide MAX phase was synthesised by a powder metallurgical method from ball milled TiC/Si powders of two different compositions, with TiC/Si ratios of 3:2 and 3:2.2 respectively. The cold pressed samples were analysed by dilatometry under flowing argon or sintered under vacuum for different times. The sintered samples were evaluated using x-ray diffraction (XRD). This study showed that titanium carbide was always present as a secondary phase and silicon carbide accompanied the Ti3SiC2 formation. Titanium silicide was observed in the samples of TiC/Si ratio 3:2 and is suggested to be an intermediate phase to the Ti3SiC2 formation in these samples. The melting of Si is essentially concurrent with the MAX phase formation in these samples and the evaporation of Si with its decomposition. The activation energy of Ti3SiC2 formation from 3TiC/2Si starting powders was determined to be 289 kJ/mol, using the Mehl-Avrami-Johnson model.The samples of TiC/Si ratio 3:2.2 extra silicon resulted in Ti3SiC2 of higher purity and the Si of the samples did not melt or evaporate. Furthermore, the thermochemical stability of the samples was increased by the extra silicon.

  • 28.
    Kero, Ida
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Tegman, Ragnar
    Luleå tekniska universitet.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Carbon atmosphere effect on on Ti3SiC2 based composites made from TiC/Si powders2010In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 36, no 4, p. 1259-1263Article in journal (Refereed)
    Abstract [en]

    The previous termeffect of carbonnext term activity and CO pressure in the furnace previous termatmospherenext term is investigated with respect to the phase reactions during heat treatment of TiC/Si powders. Special attention is given to the production and decomposition of Ti3SiC2. Samples were heated in graphite and alumina furnaces, connected to a dilatometer which enabled the in-situ analysis of the phase reactions. The phase compositions of the heat treated samples were determined by x-ray diffraction. The reducing previous termatmospherenext term of the graphite furnace enhanced the reactivity of the starting powder and enabled phase reactions to take place at a lower temperature than in the alumina furnace. TiSi2 and SiC phases formed at temperatures below the melting point of Si and were continuously consumed at higher temperatures. Ti3SiC2 formed at the melting point of Si regardless of furnace previous termatmosphere.next term No decomposition of the Ti3SiC2 was observed in either furnace.

  • 29. Kero, Ida
    et al.
    Tegman, Ragnar
    Luleå tekniska universitet.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effect of the amounts of silicon on the in situ synthesis of Ti3SiC2 based composites made from TiC/Si powder mixtures2010In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 36, no 1, p. 375-379Article in journal (Refereed)
    Abstract [en]

    This paper describes how variations in silicon content and heat treatment influence on the phase composition of TiC/Si powder mixtures during heat treatment. The sintering procedure is investigated with respect to the phases produced with a particular emphasis on maximisation of the Ti3SiC2 content. The heat treated products have been analysed using scanning electron microscopy, energy dispersive spectroscopy and x-ray diffractometry. In the heat treated products Ti3SiC2 and TiC are the dominant phases and SiC and TiSi2 are found in smaller (0-30vol%) amounts. The composition was found to depend on both Si content and heat treatment temperature and time. The silicon content can be optimised with respect to maximum Ti3SiC2 production, but its value changes with different temperature programs. The highest amount (56 vol%) of Ti3SiC2 is achieved by heat treating powder mixtures of initial composition 3TiC/2.2Si at 1350° for 1 h. Heat treatment at lower temperatures (1250-1300 °C) results in an incomplete reaction and production of TiSi2.

  • 30. Kero, Ida
    et al.
    Tegman, Ragnar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Phase reactions associated with the formation of Ti3SiC2 from TiC/Si powders2011In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 37, no 7, p. 2615-2619Article in journal (Refereed)
  • 31.
    Melk, Latifa
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Anglada, Marc
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona, Universitat Politècnica de Catalunya, Universidad de Extremadura.
    Material removal mechanisms by EDM of zirconia reinforced MWCNT nanocomposites2016In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 42, no 5, p. 5792-5801Article in journal (Refereed)
    Abstract [en]

    Several composites of tetragonal zirconia polycrystals doped with 3 mol% yttria (3Y-TZP) and multiwalled carbon nanotubes (MWCNT) with concentrations from 0.5 to 4 wt% CNT were processed, spark plasma sintered, and characterised for a wide range of mechanical, electrical and thermal properties. In particular, a strong increase in electrical conductivity at room temperature was found with only 0.5 wt% CNT. However, the thermal conductivity was decreasing with increasing CNT content. Electrical discharge machining (EDM) using die sinking was carried out using the composites of 1 and 2 wt% CNT as workpieces. It was shown that both compositions could be successfully machined by EDM. The surface integrity and the subsurface were studied by SEM/FIB in order to determine the material removal mechanisms, which were found to be associated to spalling and melting/evaporation. Raman Spectroscopy was used to evaluate the damage of CNTs after EDM.

  • 32.
    Melk, Latifa
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mouzon, Johanne
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Chemical Engineering.
    Turon-Vinas, Miquel
    CIEFMA—Department of Materials Science and Metallurgical Engineering, ETSEIB, Universitat Politècnica de Catalunya.
    Akhtar, Farid
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Anglada, Marc
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona, Universitat Politècnica de Catalunya, Universidad de Extremadura.
    Surface microstructural changes of Spark Plasma Sintered Zirconia after grinding and annealing2016In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 42, no 14, p. 15610-15617Article in journal (Refereed)
    Abstract [en]

    Spark plasma sintered zirconia (3Y-TZP) specimens have been produced of 140 nm 372 nm and 753 nm grain sizes by sintering at 1250 C, 1450 C and 1600 C, respectively. The sintered zirconia specimens were grinded using a diamond grinding disc with an average diamond particle size of about 60 µm, under a pressure of 0.9 MPa. The influence of grinding and annealing on the grain size has been analysed. It was shown that thermal etching after of ruff grinding of specimens at 1100 C for one hour induced an irregular surface layer of about a few hundred nanometres in thickness of recrystallized nano-grains, independently of the initial grain size. However, if the ground specimens were exposed to higher temperature, e.g. annealing at 1575 °C for one hour, the nano-grain layer was not observed and the final grain size was similar to that achieved by the same heat treatments on carefully polished specimens. Therefore, by appropriate grinding and thermal etching treatments, nanograined surface layer can be obtained which increases the resistance to low temperature degradation.

  • 33.
    Melk, Latifa
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Rovira, Joan Josep Roa
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Anglada, Marc
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Coefficient of friction and wear resistance of zirconia-MWCNTs composites2015In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 41, no 1 Part A, p. 459-468Article in journal (Refereed)
    Abstract [en]

    Composites of 3 mol.% yttria-doped tetragonal zirconia (3Y-TZP) reinforced with multiwalled carbon nanotubes (MWCNT) up to 2 wt. % content have been produced using spark plasma sintering (SPS). The theoretical densities of the studied composites were found to be between 99.4 and 97.4%. The addition of MWCNT content resulted in reduction of 3Y-TZP grain size from 174 to 148 nm. The effect of MWCNT on the friction coefficient (COF) was studied by performing nano- and macro-scratches using diamond Berkovich and Rockwell indenters, respectively. Moreover, the COF and the wear rate were also investigated in reciprocating sliding against a zirconia ball under a load of 5 N. The results showed that the COF decreased upon the increase in MWCNT content. However, in macro-scratch testing, there was a critical load over which brittle fracture sets in and its value decreases as the MWCNT content increases. The wear resistance was found to be decreasing very slightly for less than 1 wt. % MWCNT, while it increases strongly for the addition of 2 wt. % MWCNT under the conditions studied. The results were discussed in terms of material properties, scanning electron microscopy observations of the wear track and nanoindentation tests.

  • 34.
    Melk, Latifa
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Rovira, Joan Josep Roa
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Garcia-Marro, Fernando
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Milsom, Ben
    Department of Materials, Queen Mary College, University of London.
    Reece, Michael J.
    Department of Materials, Queen Mary College, University of London.
    Anglada, Marc
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Nanoindentation and fracture toughness of nanostructured zirconia/multi-walled carbon nanotube composites2015In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 41, no 2, p. 2453-2461Article in journal (Refereed)
    Abstract [en]

    Multi-walled carbon nanotubes (MWCNTs)/3 mol% yttria-doped tetragonal zirconia (3Y-TZP) composites were produced using spark plasma sintering (SPS) with MWCNT content ranging within 0-2 wt%. In the present paper, it was shown that the addition of MWCNTs results in a refinement of the composites microstructure. Moreover, nanoindentation tests were performed in order to monitor the change in elastic modulus and hardness with MWCNT content and it was found that both properties decrease with the addition of MWCNT content. A novel method was used to measure the true fracture toughness of the composites by producing a shallow surface sharp notch machined by ultra-short pulsed laser ablation on the surface of beam specimens. The true fracture toughness obtained on this laser machined single edge V-notch beam (SEVNB) specimens tested in four point bending was compared to the indentation fracture toughness measured using a Vickers indenter. It was found that the indentation fracture toughness increases with increasing MWCNT content, while the true fracture toughness determined with SEVNB was practically independent of the composition. Finally, it was concluded that the increase in the resistance to indentation cracking of the composites with respect to 3Y-TZP matrix cannot be associated to higher true fracture toughness. The results were discussed in terms of transformation toughening, damage induced in front of the notch tip, microstructure of the composites, and fracture toughness of 3Y-TZP.

  • 35.
    Melk, Latifa
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Turon-Vinas, Miquel
    CIEFMA—Department of Materials Science and Metallurgical Engineering, ETSEIB, Universitat Politècnica de Catalunya.
    Rovira, Joan Josep Roa
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Anglada, Marc
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona, Universitat Politècnica de Catalunya.
    The influence of unshielded small cracks in the fracture toughness of yttria and of ceria stabilised zirconia2016In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 36, no 1, p. 147-153Article in journal (Refereed)
    Abstract [en]

    The fracture toughness, KIC of two 3Y–ZrO2 with different grain size (177 and 330 nm) and 12Ce–ZrO2 were determined from a sharp micro-machined notch by Ultra-Short Pulsed Laser Ablation (UPLA) where a micro-cracked zone and non-transformed is generated in front of the notch. The notch plus the damage behaved as an unshielded edge surface crack. The fracture stress, σf of both 330 nm-3Y–ZrO2 and 12Ce–ZrO2 with similar short crack sizes were found to be comparable in despite of their different published R-curves. The results of KIC were discussed in terms of the type of cracks induced and by using a simple R-curve model. It was concluded that for the development of high strength composites with 12Ce–ZrO2 as the matrix, the relevant KIC that controls the σf with surface unshielded short cracks is much closer to the intrinsic KIC than to the indentation KIC or to the plateau KIC of long cracks.

  • 36.
    Neikter, Magnus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Forsberg, Fredrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lycksam, Henrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Pederson, Robert
    University West, Div. Welding Material .
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Microstructure and Defects in Additive Manufactured Titanium: a Comparison Between Microtomography and Optical Microscopy2017Conference paper (Other academic)
    Abstract [en]

    The aim of this work has been to compare two different analysing methods;x-ray microtomography and light optical microscopy, when it comes to defects and microstructure of additively manufactured Ti-6Al-4V. The results showthat both techniqueshave theirpros and cons:microtomography is the preferred choicefor defect detectionby analysing the full 3D sample volume, while light optical microscopy is better for analysing finer details in 2D.

  • 37.
    Neikter, Magnus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Division of Welding Technology, University West, Trollhättan .
    Åkerfeldt, Pia
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Microstructure characterisation of Ti-6Al-4V from different additive manufacturing processes2017In: IOP Conference Series: Materials Science and Engineering, ISSN 1757-8981, E-ISSN 1757-899X, Vol. 258, article id 012007Article in journal (Refereed)
    Abstract [en]

    The focus of this work has been microstructure characterisation of Ti-6Al-4V manufactured by five different additive manufacturing (AM) processes. The microstructure features being characterised are the prior β size, grain boundary α and α lath thickness. It was found that material manufactured with powder bed fusion processes has smaller prior β grains than the material from directed energy deposition processes. The AM processes with fast cooling rate render in thinner α laths and also thinner, and in some cases discontinuous, grain boundary α. Furthermore, it has been observed that material manufactured with the directed energy deposition processes has parallel bands, except for one condition when the parameters were changed, while the powder bed fusion processes do not have any parallel bands.

  • 38.
    Neikter, Magnus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Åkerfeldt, Pia
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Division of Welding Technology, University West, Trollhättan 461 32, Sweden.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Microstructural characterization and comparison of Ti-6Al-4V manufactured with different additive manufacturing processes2018In: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189Article in journal (Refereed)
    Abstract [en]

    In this work, the microstructures of Ti-6Al-4V manufactured by different additive manufacturing (AM) processes have been characterized and compared. The microstructural features that were characterized are the α lath thickness, grain boundary α (GB-α) thickness, prior β grain size and α colony size. In addition, the microhardnesses were also measured and compared. The microstructure of shaped metal deposited (SMD) Ti-6Al-4V material showed the smallest variations in α lath size, whereas the material manufactured with laser metal wire deposition-0 (LMwD-0) showed the largest variation. The prior β grain size was found to be smaller in material manufactured with powder bed fusion (PBF) as compared with corresponding material manufactured with the directed energy deposition (DED) processes. Parallel bands were only observed in materials manufactured with DED processes while being non-present in material manufactured with PBF processes.

  • 39.
    Nilsson, Erik A. A.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pettersson, Lars
    LKAB.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Effect of Si, Mn and Ti on High Temperature Resistance of a Travelling Grate in a Grate-Kiln Indurator2017In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812Article in journal (Refereed)
  • 40.
    Nilsson, Erik A. A.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Tegman, Ragnar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Deposit-induced accelerated corrosion of as-casted ACI HHII stainless steel in a Grate-Kiln induratorManuscript (preprint) (Other academic)
  • 41.
    Nilsson, Erik A.A.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pettersson, L.
    LKAB, Kiruna.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mn, Si and Ti in cast stainless steel at elevated temperature2018In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812Article in journal (Refereed)
    Abstract [en]

    ACI HH II is an austenitic castable stainless steel for high-temperature applications such as grates in a Grate-Kiln indurator. Eight different alloy compositions have been used to evaluate the oxide formation and the microstructural evolution during an isothermal heat treatment at 800°C with a holding time of 200 h in an atmosphere containing 20% O2–N2. Residual ferrite is transformed into sigma phase during the heat treatment while secondary carbides are precipitated in the austenite. Silicon will form a thinner oxide but will reduce its adherence to the steel. Additions of Ti are beneficial for the properties of the oxide layer as the oxide is well adherent and thinner, while additions of Mn increase the growth rate of the oxide and enhance the risk of growth stresses in the oxide. It is proposed that by combing the beneficial effects of both Ti and Si it is possible to suppress the malign effects of Mn additions.

  • 42.
    Nilsson, Erik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pettersson, L.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Case study of grate-chain degradation in a grate-kiln process2013In: 7th EEIGM International Conference on Advanced Materials Research: 21–22 March 2013, LTU, Luleå, Sweden, IOP Publishing Ltd , 2013, article id 12012Conference paper (Refereed)
    Abstract [en]

    Austenitic stainless steels are often used in high temperature applications due to their inherent resistance to corrosion. The grate-chain in some Grate-Kiln processes for sintering of iron pellets is made of these austenitic steels to withstand the severe environment. It has been shown however that the grate-chain is affected by several degrading mechanisms in the harsh environment of the sintering process. A grate-chain that had been used for 13 months in production was investigated in order to find the mechanisms of degradation. Results show that slag products are accumulated on the grate-chain during time and interact with the steel mainly due to the content of alkali metals. The resistance towards degradation seems to decrease with time which is suggested to be caused by the depletion of chromium.

  • 43.
    Nilsson, Erik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Tegman, Ragnar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Thermal cycling of grate-link material for iron ore pelletising process2017In: Ironmaking & steelmaking, ISSN 0301-9233, E-ISSN 1743-2812, Vol. 44, no 4, p. 269-280Article in journal (Refereed)
    Abstract [en]

    A test-rig for thermal cycling has been developed to investigate the mechanisms behind some of the damages done to a grate-link in a pelletising indurator for iron ore pellets. The results from the test-rig were compared with the degradation of grate-links that had been in service for 8 months in the indurator. The damages on the grate-links induced both in the test-rig and at service were defined as internal oxidation which facilitated propagation of intergranular stress corrosion cracking (IGSCC). Internal oxidation was initiated by spallation or removal of the oxide layers. The test-rig was able to successfully perform a lab-scale simulation of the degradation experienced in the indurator. Thermal cycling was applied to plate-formed specimens by constraining their thermal elongation to form bending and thereby stimulate the mechanisms of spallation and sensibilisation to IGSCC by internal oxidation.

  • 44.
    Norberg, Anders
    et al.
    Umeå University.
    Stöckel, Birgit
    Umeå University.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Time shifting and agile time boxes in course design2017In: International Review of Research in Open and Distance Learning, ISSN 1492-3831, E-ISSN 1492-3831, Vol. 18, no 6, p. 88-103Article in journal (Refereed)
    Abstract [en]

    The ongoing integration of Information and Communication Technologies (ICTs) into higher education courses is often called blended learning although it often relates to course design. It is usually understood in place categories, as a combination of traditional classroom-based sessions and Internet-enabled distance or online learning practices. One alternative understanding of ICT integration can be constructed of time categories, with an understanding of ICTs more as process- and project-related. Two such design frameworks are conceptually presented and then used together in a small case study in a pilot experiment in physics at the preparatory level for entering engineering programs at a university in Northern Sweden. These are a) time shift mechanisms between synchronous and asynchronous learning modes in the course process and b) agile frameworks mechanisms adapted from work process developments in the software industry. Both are here used to address common procrastination problems in flexible education. Data were collected in student interviews and analysed with qualitative content analysis. Results show student satisfaction with the work rhythm and that a feeling of presence, which enables easy interaction, can be facilitated by synchronicity.

  • 45.
    Pederson, Robert
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Gaddam, Raghuveer
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Microstructure and mechanical behavior of Cast Ti-6Al-4V with addition of boron2012In: Central European Journal of Engineering, ISSN 1896-1541, E-ISSN 2081-9927, Vol. 2, no 3, p. 347-357Article in journal (Refereed)
    Abstract [en]

    The effect of boron (between 0.06 and 0.11 wt%) on the microstructure, hardness and compression properties of cast Ti-6Al-4V was investigated. Compression properties were examined in the temperature range from room temperature to 1000ºC. It was found that the addition of boron refines the as-cast microstructure in terms of prior beta grain size and alpha colony size. This microstructural refinement led to an increase in compressive yield strength from room temperature up to 700ºC. Three different strain rates (0.001, 0.1 and 1 s-1) were evaluated during compression testing from which it was found that the compressive yield strength decreased with decreasing strain rate from 600ºC up to the beta transus temperature.

  • 46.
    Ramanenka, Dmitrij
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Gustafsson, Gustaf
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Jonsén, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Characterization of high-alumina refractory bricks and modelling of hot rotary kiln behaviour2017In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 79, p. 852-864Article in journal (Refereed)
    Abstract [en]

    Rotary kilns for iron-ore pellets production are highly dependent on a well-functioning refractory brick lining. To improve the long-term capability of the lining, in-situ observations of the bricks' performance are desired, however, the high process temperatures and the size of the kiln make it difficult to study the lining during operation. By using numerical simulations as a tool, some of the problems encountered by the brick lining can be studied. Knowing material properties of the refractory bricks as input in a numerical model is therefore necessary. However, material properties are poorly documented for this type of materials, especially, at elevated temperatures. In this work three commercial aluminasilicate bricks were tested in compression until failure for a temperature range of 25–1300 °C. The purpose was to evaluate compression strength and Young's modulus in compression of the fully burned bricks at a wide range of temperatures. The data was later used for modelling of a hot rotary kiln lined with bricks by using the finite element method, whereupon load state of the lining was evaluated at steady state after the expansion of the system. The objective of the numerical modelling was to investigate trustworthiness of the model and to give insight into the stress levels that can potentially arise. It was found that for all of the investigated brick types the compression strength increased with increased temperature, having a peak in the vicinity of 1000 °C. The maximum increase was between 50 and 150 % for the different brick types. After passing 1100 °C the compression strength rapidly and considerably decreased below its as-received compression strength. Young's modulus was measured to vary between 2 and 10 GPa in the range of up to 1000 °C. The numerical results indicate that severe boundary conditions (expansion of the lining is highly restricted) can potentially lead to compression stress of up to 34 MPa in the brick lining at steady state. However, at these boundary conditions the present tensile stress was only 0.5 MPa, while tensile stresses of close to 3 MPa could be observed in the lining with mild boundary conditions. The authors conclude that the created model is trustworthy and that it has high potential for being used as a tool in further investigations of the lining in hot state.

  • 47.
    Sefer, Birhan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Department of Materials Science and Metallurgical Engineering, Universitat Politècnica de Catalunya.
    Dobryden, Illia
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Division of Surface and Corrosion Science, KTH Royal Institute of Technology.
    Almqvist, Nils
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Research and Technology Centre, GKN Aerospace Engine Systems.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Chemical Milling of Cast Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo Alloys in Hydrofluoric-Nitric Acid Solutions2017In: Corrosion, ISSN 0010-9312, E-ISSN 1938-159X, Vol. 73, no 4, p. 394-407Article in journal (Refereed)
    Abstract [en]

    The behavior of cast Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo during chemical milling in hydrofluoric-nitric (HF-HNO3) acid solutions with 1:3 and 1:11 molar ratios was investigated using electrochemical and atomic force microscopy (AFM) techniques. Faster corrosion rate in 1:3 solutions was measured for Ti-6Al-4V than for Ti-6Al-2Sn-4Zr-2Mo, whereas in 1:11 solution Ti-6Al-2Sn-4Zr-2Mo exhibited higher corrosion rate. Scanning Kelvin probe force microscopy measurements revealed difference in the Volta potential between the α-laths and the β-layers in the Widmansttäten microstructure indicating operation of microgalvanic cells between the microconstituents when in contact with HF-HNO3 solution. The AFM topography measurements demonstrated faster corrosion of the α-laths compared to the β-layers, in both alloys. In 1:3 solutions, higher α/β height difference was measured in Ti-6Al-4V, whereas in 1:11 solution, the difference was higher in Ti-6Al-2Sn-4Zr-2Mo. The results revealed that the chemical milling behavior of the two investigated alloys is controlled by the microscopic corrosion behavior of the individual microconstituents.

  • 48.
    Sefer, Birhan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Gaddam, Raghuveer
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. GKN Aerospace Engine Systems Sweden.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Study of the Alpha-Case Layer in Ti–6Al–2Sn–4Zr–2Mo and Ti–6Al–4V by Electron Probe Micro Analysis2014Conference paper (Refereed)
    Abstract [en]

    Titanium and its alloys are susceptible to oxidation when exposed to elevated temperatures and oxygen containing environments for long exposure times, e.g. in jet engines [1–3]. In such conditions oxygen rapidly reacts with titanium, stabilizing α–titanium and forming solid solution due to the high solubility of oxygen in titanium (14.5 wt.%) [4]. The oxidation results in simultaneous formation of oxide scale on top of the metal and a brittle oxygen enriched layer beneath the scale, commonly referred as alpha–case. Alpha–case layer reduces important mechanical properties such as ductility, fracture toughness, and most severe reduces the fatigue life of jet engine components when subjected to dynamical loadings [5]. Therefore, the alpha-case layer in aerospace applications is usually removed by chemical milling [1–3] or prevented by using vacuum environments and high temperature coatings [1–3,6–9]. In the present study alpha–case in Ti–6Al–2Sn–4Zr–2Mo and Ti–6Al–4V alloys was developed by performing isothermal heat treatments at 700 °C in ambient air for 500 hours. The developed alpha–case layer was evaluated metallographically and by using instrumental techniques. It was found that the alpha–case development is a function of alloy composition and microstructure. The oxygen and the main alloying elements concentration profiles were measured using Electron Probe Micro Analyzer (EPMA) in both alloys. Based on the analysis of the concentration profiles an increase of the amount of alpha phase in the two alloys was found as a result of beta to alpha phase transformation.

  • 49.
    Sefer, Birhan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Gaddam, Raghuveer
    Rovira, Joan Josep Roa
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Mateo, Antonio
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Chemical milling effect on the low cycle fatigue properties of cast Ti-6Al-2Sn-4Zr-2Mo alloy2016In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 92, no 1, p. 193-202Article in journal (Refereed)
    Abstract [en]

    The current research work presents the chemical milling effect on the low cycle fatigue properties of cast Ti-6Al-2Sn-4Zr-2Mo alloy. Chemical milling treatment is one of the final steps in manufacturing titanium alloy components that removes the brittle alpha-case layer formed during various thermal processes. The treatment includes immersion of the components in solutions containing hydrofluoric (HF) and nitric (HNO3) acids in relevant molar ratios. Although this treatment demonstrates advantages in handling components with complex net geometries, it may have detrimental effects on the surface, by introducing pitting and/or intergranular corrosion and thereby adversely affecting in particular the fatigue strength. The first series of specimens were tested in as-machined condition. Two more series were, prior to fatigue testing, subjected to 5 and 60 minutes chemical milling treatment. It was found that the fatigue lives were substantially decreased for the chemically treated specimens. The fractographic investigation of all mechanically tested samples revealed multiple fatigue crack initiation sites in the chemically milled samples. These cracks were located either at the prior beta grain boundary or the prior beta grain boundary triple joints. The prior beta grain boundaries were found to have deep ditch-like appearance which depth increased with increasing milling time. These ditch-like grain boundaries acts as stress raisers and thereby promote early fatigue crack initiation and thus lower fatigue life.

  • 50.
    Sefer, Birhan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Rovira, Joan Josep Roa
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Mateo, Antonio
    Department of Materials Science and Engineering, Universitat Politècnica de Catalunya, Barcelona.
    Pederson, Robert
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Antti, Marta-Lena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Evaluation of the bulk and alpha-case properties in Ti-6Al-4V at micro- and nano-metric length scale2016In: 13th World Conference on Titanium: August 16-20, 2015 * Manchester Grand Hyatt * San Diego, California / [ed] V. Venkatesh; A.L. Pilchak; J.E. Allison; S. Ankem; R. Boyer; J. Christodoulou; H.L. Fraser; M.A. Imam; J. Kosaka; H.J. Rack; A. Chaterjee; A. Woodfield, John Wiley and Sons , 2016, p. 1619-1624, article id 271Conference paper (Refereed)
    Abstract [en]

    In the present study the hardness of individual alpha (α)-Ti grains in Ti-6Al-4V was measured by nanoindentation using Berkovich tip indenter. Additionally, alpha-case layer was induced by performing isothermal heat treatment at 700°C in air for 500 hours. The average hardness of the α-Ti grains found in the bulk material and in the alpha-case layer were 6.7 ± 0.7 GPa and 9.4 ± 1.4 GPa, respectively. The high hardness of the α-Ti grains in the alpha-case layer is due to solid solution strengthening caused by interstitial oxygen diffusion. The thickness of the developed alpha-case layer was estimated metallographically and compared with that measured from a hardness profile performed along the layer. Moreover, electron back-scattered diffraction was used to determine the local crystallographic orientation, the texture of the alloy microstructure, as well as phase fraction changes, where the nanoindentation measurements were performed.

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