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First-principles description of planarfaults in metals and alloys
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.ORCID iD: 0000-0001-7724-8299
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Phase interface and stacking fault are two common planar defects in metallic materials. In the present thesis, the interfacial energy and the generalized stacking fault energy of random alloys are investigated using density functional theory formulated within the exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA).

The interfacial energy is one of the key physical parameters controlling the formation of the Cr-richα’ phases during the phase decomposition in Fe-Cr ferrite stainless steels. This decomposition is believed to cause the so-called “475°Cembrittlement”. Aluminum addition to ferritic stainless steels was found to effectively suppress the deleterious 475oC embrittlement. The effect of Al on the interfacial energy and the formation energy of Fe-Cr solid solutions are studied in this thesis. The interface between the decomposed Fe-rich α and Cr-richα0 phases carries a positive excess energy, which represents a barrier for the process of phase separation. Our results show that for the α-Fe70Cr20Al100-Fe100−x−yCryAlx (0≤x≤10, 55≤y≤80) interface, the Al con-tent(x) barely changes the interfacial energy. However, when Al is partitioned only in the alpha phase, i.e. for the α-Fe100−x−yCryAlx0-Fe10Cr90 (0≤x≤10, 0≤y≤25) interface, the interfacial energy increases with Al concentration due to the variation of the formation energies of the Fe-Cr alloys upon Al alloying.

The intrinsic energy barriers (IEBs) on theγ−surface (also called generalized stacking fault energy, GSFE) provide fundamental physics for understanding the plastic deformation mechanisms in face-centred cubic (fcc) metals and alloys. In this thesis, the GSFEs of the disordered Cu-X (X=Al, Zn, Ga, Ni) and Pd-X (X=Ag, Au) alloys are calculated. Studying the effect of segregation of the solutes to the stacking fault planes shows that only the local chemical composition affects the GSFEs. Based on the calculated GSFE values, the previously revealed “universal scaling law” between these IEBs is demonstrated to be well obeyed in random solid solutions. This greatly simplifies the calculations of the twinning parameters or the critical twinning stress. Adopting two twinnability measure parameters derived from the IEBs, we find that in binary Cu alloys, the addition of Al, Zn and Ga increases the twinnability, while adding Ni decreases it. Aluminum and gallium yield similar effects on the twinnability. Our theoretical predictions are in line with the available experimental data. These achievements open new possibilities in understanding and describing the plasticity of complex alloys.

We investigate theγ-surface of paramagneticγ-Fe as a function of temperature. At ambient conditions, the fcc lattice is thermodynamically unstable with respect to the hexagonal close-packed (hcp) lattice, resulting in negative intrinsic stacking fault energy (ISF). However, the unstable stacking fault energy (USF), representing the energy barrier along theγ-surface connecting the ideal fcc and the intrinsic stacking fault positions, is large and positive. The ISF is calculated to have a strong positive temperature coefficient, while the USF decreases monotonously with temperature. According to the recently developed plasticity theory, the overall effect of temperature is to move the plastic deformation mode of the paramagnetic fcc Fe from the stacking fault formation regime (T <<300K) towards maximum twinning (T≈300K) and finally to a dominating full-slip regime (T >>300K). Our predictions are discussed in connection with the available experimental observations.

The same methodology is used to establish theγ-surface of Fe-Cr-Ni alloys as a function of chemical composition and temperature. We fix the concentration of Cr at 20 at.%. Nickel is found to increase the intrinsic stacking fault (SFE), unstable stacking fault (USF) and unstable twin fault (UTF) energies. The theoretical SFE versus chemistry and temperature trends agree well with experiments. Both USF and UTF decrease with increasing temperature. The calculated IEBs are used to establish the temperature and composition dependence of the deformation modes in Fe-Cr-Ni alloys. Stacking fault formation is predicted to be the leading deformation mechanism for alloys with effective SFE below∼18mJm−2, which is in good agreement with the observed upper limit of the SFE for the TRIP (transformation-induced plasticity) mechanism. Alloys with SFE above this critical value show both twinning and full slip at room temperature and surprisingly, even the SFE is very high, twinning remains a possible deformation mode even at elevated temperatures, which is in line with observations

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , x, 61 p.
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-180650ISBN: 978-91-7595-822-4 (print)OAI: oai:DiVA.org:kth-180650DiVA: diva2:895574
Public defence
2016-01-29, Sal B2, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20160119

Available from: 2016-01-19 Created: 2016-01-19 Last updated: 2016-04-01Bibliographically approved
List of papers
1. The effect of Al on the 475 degrees C embrittlement of Fe-Cr alloys
Open this publication in new window or tab >>The effect of Al on the 475 degrees C embrittlement of Fe-Cr alloys
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2013 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 74, 101-106 p.Article in journal (Refereed) Published
Abstract [en]

Aluminum addition to ferritic stainless steels was found to effectively suppress the deleterious 475 degrees C embrittlement resulting from the phase decomposition in concentrated Fe-Cr alloys. With the aim of revealing the mechanism behind this prosperous phenomenon, here we investigate the effect of Al on the interfacial energy and formation energy of Fe-Cr solid solutions. The interface between the decomposed Fe-rich alpha and Cr-rich alpha' phases carries a positive excess energy, which is of significant importance on determining the process of phase separation. Using ab initio alloy theory, we show that for the alpha-Fe70Cr20Al10/alpha'-Fe100-x-yCryAlx (0 <= x <= 10, 55 <= y <= 80) interface, the Al content (x) barely changes the interfacial energy. However, for the alpha-Fe100-x-yCryAlx/alpha'-Fe10Cr90 (0 <= x <= 10, 0 <= y <= 25) interface, the interfacial energy increases with Al content due to the variation of the formation energies of the Fe-Cr alloys upon Al alloying. Our ab initio results are supported by CALPHAD calculations, and suggest that the beneficial effect of Al on ferritic steels is mainly due to its thermodynamical effect on the alpha' phase.

Place, publisher, year, edition, pages
Elsevier, 2013
Keyword
Ab initio, 475 degrees C embrittlement, Stainless steel, Interfacial energy, Phase separation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-125754 (URN)10.1016/j.commatsci.2013.03.021 (DOI)000321051700014 ()2-s2.0-84876158103 (Scopus ID)
Funder
Swedish Research CouncilEU, European Research Council, 228074
Note

QC 20130813

Available from: 2013-08-13 Created: 2013-08-13 Last updated: 2017-12-06Bibliographically approved
2. Generalized stacking fault energies of alloys
Open this publication in new window or tab >>Generalized stacking fault energies of alloys
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2014 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 26, no 26, 265005- p.Article in journal (Refereed) Published
Abstract [en]

The generalized stacking fault energy (gamma surface) provides fundamental physics for understanding the plastic deformation mechanisms. Using the ab initio exact muffin-tin orbitals method in combination with the coherent potential approximation, we calculate the. surface for the disordered Cu-Al, Cu-Zn, Cu-Ga, Cu-Ni, Pd-Ag and Pd-Au alloys. Studying the effect of segregation of the solute to the stacking fault planes shows that only the local chemical composition affects the. surface. The calculated alloying trends are discussed using the electronic band structure of the base and distorted alloys. Based on our. surface results, we demonstrate that the previous revealed 'universal scaling law' between the intrinsic energy barriers (IEBs) is well obeyed in random solid solutions. This greatly simplifies the calculations of the twinning measure parameters or the critical twinning stress. Adopting two twinnability measure parameters derived from the IEBs, we find that in binary Cu alloys, Al, Zn and Ga increase the twinnability, while Ni decreases it. Aluminum and gallium yield similar effects on the twinnability.

Keyword
stacking fault energy, random alloy, ab initio
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-147937 (URN)10.1088/0953-8984/26/26/265005 (DOI)000337349200007 ()2-s2.0-84902270010 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20140714

Available from: 2014-07-14 Created: 2014-07-10 Last updated: 2017-12-05Bibliographically approved
3. Generalized stacking fault energy of γ-Fe
Open this publication in new window or tab >>Generalized stacking fault energy of γ-Fe
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(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-180655 (URN)
Note

QS 2016

Available from: 2016-01-19 Created: 2016-01-19 Last updated: 2016-01-19Bibliographically approved
4. First-Principles prediction of the deformation modes in austenitic Fe-Cr-Ni alloys
Open this publication in new window or tab >>First-Principles prediction of the deformation modes in austenitic Fe-Cr-Ni alloys
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(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-180656 (URN)
Note

QS 2016

Available from: 2016-01-19 Created: 2016-01-19 Last updated: 2016-01-19Bibliographically approved
5. Stacking Fault Energies in austenitic stainless steels
Open this publication in new window or tab >>Stacking Fault Energies in austenitic stainless steels
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(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-180657 (URN)
Note

QS 2016

Available from: 2016-01-19 Created: 2016-01-19 Last updated: 2016-01-19Bibliographically approved

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