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Eutectic Modification of Al-Si casting alloys
Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering. Department of Material Science, Saarland University, Saarbrücken, Germany.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Aluminum alloys with silicon as the major alloying element are the most widely used aluminum casting alloys. The eutectic phase in these alloys is formed by hard and brittle silicon plates in an aluminum matrix. Such silicon plates can act as crack propagation paths deteriorating the toughness of the material. To enhance ductility, silicon can be modified to a coral-like microstructure by addition of a modifying agent. Amongst the elements proposed as modifiers, only strontium, sodium and europium induce a plate-tocoral transition, while others such as ytterbium, only refine the silicon plates. The exact mechanism for the remarkable plate-to-coral change, and the reason why certain elements only refine the structure, is still not completely understood.

In this investigation, atom probe tomography and transmission electron microscopy were used to analyze and compare the crystal structure and the distribution of solute atoms in silicon at the atomic level. An unmodified alloy and alloys modified by strontium, sodium, europium and ytterbium were studied. Elements inducing silicon plate-to-coral transition were found to contain nanometer sized clusters at the defects in silicon with stoichiometries corresponding to compounds formed at the ternary eutectic reaction of each system. In contrast, the addition of ytterbium, that only refines the silicon plates, is unable to form clusters in silicon. We propose that the formation of ternary compound clusters AlSiNa, Al2Si2Sr and Al2Si2Eu at the silicon / liquid interface during solidification restrict silicon growth. The formation of clusters on silicon facets create growth steps and increase growth direction diversity. The incorporation of clusters in silicon explains the high density of crystallographic defects and the structural modification from plates to corals.

The parallel lattice plane-normals 011Si // 0001Al2Si2Eu, 011Si // 610Al2Si2Eu and 111Si // 610Al2Si2Eu were found between Al2Si2Eu and silicon, and absent between Al2Si2Yb and silicon. We propose a favorable heterogeneous formation of Al2Si2Eu on silicon. The misfit between 011Si and 0002Al2Si2X interplanar spacings shows a consistent trend with the potency of modification for several elements such as strontium, sodium, europium, calcium, barium, ytterbium and yttrium.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2019. , p. 82
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2014
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:liu:diva-160235DOI: 10.3384/diss.diva-160235ISBN: 9789175190075 (print)OAI: oai:DiVA.org:liu-160235DiVA, id: diva2:1351010
Public defence
2019-10-31, Mott, F Building, Campus Valla, Linköping, 14:00 (English)
Opponent
Supervisors
Available from: 2019-09-13 Created: 2019-09-13 Last updated: 2019-09-16Bibliographically approved
List of papers
1. Comparison of segregations formed in unmodified and Sr-modified Al-Si alloys studied by atom probe tomography and transmission electron microscopy
Open this publication in new window or tab >>Comparison of segregations formed in unmodified and Sr-modified Al-Si alloys studied by atom probe tomography and transmission electron microscopy
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2014 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 611, p. 410-421Article in journal (Refereed) Published
Abstract [en]

The mechanical properties of Al-7 wt.% Si can be enhanced by structural modifications of its eutectic phase. Addition of low concentrations of certain elements, in this case 150 wt-ppm Sr, is enough to cause a transition from a coarse plate-like Si structure to a finer coralline one. To fully understand the operating mechanism of this modification, the composition of the eutectic Si phase in unmodified and Sr-modified alloys was analysed and compared by atom probe tomography and (scanning) transmission electron microscopy. The unmodified alloy showed nanometre sized Al-segregations decorating defects, while the Sr-modified sample presented three types of Al-Sr segregations: (1) rod-like segregations that promote smoothening of the Al-Si boundaries in the eutectic phase, (2) particle-like segregations comparable to the ones seen in the unmodified alloy, and (3) planar segregations favouring the formation of twin boundaries. Al and Sr solubilities in Si after solidification were determined to be 430 +/- 160 at-ppm and 40 +/- 10 at-ppm, respectively. Sr predominantly segregates to the Si phase confirming its importance in the modification of the eutectic growth.

Place, publisher, year, edition, pages
Elsevier, 2014
Keywords
Aluminium-silicon alloys; Strontium modification; Atom probe tomography; Transmission electron microscopy; Microstructure
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-109356 (URN)10.1016/j.jallcom.2014.05.121 (DOI)000338932400064 ()
Available from: 2014-08-15 Created: 2014-08-15 Last updated: 2019-09-13Bibliographically approved
2. Cluster formation at the Si/liquid interface in Sr and Na modified Al-Si alloys
Open this publication in new window or tab >>Cluster formation at the Si/liquid interface in Sr and Na modified Al-Si alloys
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2016 (English)In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 117, p. 16-19Article in journal (Refereed) Published
Abstract [en]

Atom probe tomography was used to compare Na and Sr modified Al-Si hypoeutectic alloys. Both Na and Sr promote the formation of nanometre-sized clusters in the Si eutectic phase. Compositional analyses of the clusters show an Al:Sr ratio of 2.92 +/- 0.46 and an Al:Na ratio of 1.07 +/- 0.23. It is proposed that SrAl2Si2 and NaAlSi clusters are formed at the Si/liquid interface and take part in the modification process by altering the eutectic Si growth.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2016
Keywords
Eutectic solidification; Atom probe tomography; Aluminium alloys; Eutectic modification; Transmission electron microscopy
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-127548 (URN)10.1016/j.scriptamat.2016.02.018 (DOI)000373547500004 ()
Note

Funding Agencies|German Federal Ministry of Economics and Technology [AiF 17204 N]; European Regional Development Fund (AME-Lab) [C/4-EFRE-13/2009/Br]; German Research Foundation (DFG); Federal State Government of Saarland [INST 256/298-1 FUGG]; Erasmus Mundus Doctoral Programme DocMASE of the European Commission [FPA 2011-0020]; VINNOVA Strategic Faculty Grant VINNMER Marie Curie Chair [2011-03464]; Major International (Regional) Joint Research Project from China [51420105005]

Available from: 2016-05-04 Created: 2016-05-03 Last updated: 2019-09-13
3. Eutectic modification by ternary compound cluster formation in Al-Si alloys
Open this publication in new window or tab >>Eutectic modification by ternary compound cluster formation in Al-Si alloys
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2019 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 9, article id 5506Article in journal (Refereed) Published
Abstract [en]

Al-alloys with Si as the main alloying element constitute the vast majority of Al castings used commercially today. The eutectic Si microstructure in these alloys can be modified from plate-like to coral-like by the addition of a small amount of a third element to improve ductility and toughness. In this investigation the effects of Eu and Yb are studied and their influence on the microstructure is compared to further understand this modification. The two elements impact the alloy differently, where Eu modifies Si into a coral-like structure while Yb does not. Atom probe tomography shows that Eu is present within the Si phase in the form of ternary compound Al2Si2Eu clusters, while Yb is absent in the Si phase. This indicates that the presence of ternary compound clusters within Si is a necessary condition for the formation of a coral-like structure. A crystallographic orientation relationship between Si and the Al2Si2Eu phase was found, where the following plane normals are parallel: 011(Si) //0001(Al2Si2Eu), 111(Si)//6 (7) over bar 10(Al2Si2Eu) and 011(Si)//6 (7) over bar 10(Al2Si2Eu). No crystallographic relationship was found between Si and Al2Si2Yb. The heterogeneous formation of coherent Al2Si2Eu clusters inside the Si-phase is suggested to trigger the modification of the microstructure.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2019
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:liu:diva-156563 (URN)10.1038/s41598-019-41919-2 (DOI)000462990000048 ()30940873 (PubMedID)
Note

Funding Agencies|European Regional Development Fund (ERDF); DFG; Federal State Government of Saarland [INST 256/298-1 FUGG]; Erasmus Mundus Programme of the European Commission within the Doctoral Programme DocMASE; Major International (Regional) Joint Research Project from China [51420105005]; Overseas, Hong Kong, Macao Scholars Cooperative Research Fund from China [51728101]

Available from: 2019-05-15 Created: 2019-05-15 Last updated: 2019-09-13
4. Nucleation and Growth of Eutectic Si in Al-Si Alloys with Na Addition
Open this publication in new window or tab >>Nucleation and Growth of Eutectic Si in Al-Si Alloys with Na Addition
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2015 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 46, no 3, p. 1300-1311Article in journal (Refereed) Published
Abstract [en]

Al-5 wt pct Si-based alloys with Na additions (19 and 160 ppm) have been produced by controlled sand casting and melt spinning. Entrained droplet technique and differential scanning calorimetry were employed to investigate the nucleation behavior of eutectic Si. High-resolution transmission electron microscopy and atom probe tomography were used to investigate the distribution of Na atoms within eutectic Si and at the interfaces between eutectic Si and eutectic Al. It was found that (i) only 19 ppm Na addition results into a high undercooling (49 K (49 °C)) of the entrained eutectic droplet. However, further increasing Na addition up to 160 ppm exerts no positive effect on the nucleation of eutectic Si, instead a decreased undercooling (29 K (29 °C)) was observed. (ii) Na addition suppresses the growth of eutectic Si due to the Na segregation at the interface between eutectic Si and eutectic Al, and (iii) Na addition promotes significant multiple Si twins, which can be attributed to the proposed adsorption of Na atoms at the intersection of Si twins and along the 〈112〉Si growth direction of Si. The present investigation demonstrates, for the first time, a direct observation on the distribution of Na atoms within eutectic Si and thereby provides strong experimental supports to the well-accepted impurity-induced twinning growth mechanism and poisoning of the twin plane re-entrant edge growth mechanism.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-160234 (URN)10.1007/s11661-014-2702-6 (DOI)
Available from: 2019-09-12 Created: 2019-09-12 Last updated: 2019-09-13
5. Phase Selective Sample Preparation of Al-Si alloys for Atom Probe Tomography
Open this publication in new window or tab >>Phase Selective Sample Preparation of Al-Si alloys for Atom Probe Tomography
2019 (English)In: Praktische metallographie, ISSN 0032-678X, E-ISSN 2195-8599, Vol. 56, no 2, p. 76-90Article in journal (Refereed) Published
Abstract [en]

We present how the conventional focused ion beam (FIB) lift-out method can be modified to obtain phase selective specimens for atom probe tomography (APT). The modified method combines selective deep etching with site-specific lift-out using a micromanipulator in a FIB/SEM workstation. This method is used for phase-selective sample preparation in alloys with complex microstructures such as the coral- and plate-like silicon structures in the eutectic phase of Al-Si castings. The method proves to be both, practical and robust, with a high success rate of high-quality phase-specific APT specimens.

Place, publisher, year, edition, pages
CARL HANSER VERLAG, 2019
National Category
Analytical Chemistry
Identifiers
urn:nbn:se:liu:diva-154847 (URN)10.3139/147.110557 (DOI)000458507500002 ()
Note

Funding Agencies|EU; DFG; Federal State Government of Saarland [INST 256/298-1 FUGG]; Erasmus Mundus Programme of the European Commission within the Doctoral Programme DocMASE

Available from: 2019-03-05 Created: 2019-03-05 Last updated: 2019-09-13

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