Dissertation/Thesis Abstract

Characterization of the Structure and Precipitation Process in Al-Mg-Si and Al-Mg-GE Casting Alloys
by Boyko, Viktoriya, Eng.D., Technische Universitaet Berlin (Germany), 2015, 153; 10694721
Abstract (Summary)

The Al-Mg-Si alloys recently applied in foundry show an excellent combination of strength and plasticity, but information about composition structure phases and the effect of heat treatment on the precipitation process are rather scanty. Even less information is available for the Al-Mg-Ge system. Therefore, the aim of this thesis is to provide the knowledge about structure, composition and precipitates in nine Al-Mg-Si and Al-Mg-Ge casting alloys with and without Mn, Li and Sc+Zr additions. Three conditions, as cast, solution treated and aged, were investigated and the results were compared with those of three commercial alloys. The eutectic melting temperatures for both systems were recorded by differential scanning calorimetry and gave for Al-Mg-Si – 587.0°C, for Al-Mg-Ge – 629.0°C. It was shown that in as-cast state, the structure of an alloy having the nominal composition AlMg7Si3 consists of four phases: first - the Al based solid solution, second - the (Al)+(Mg2Si) eutectic, third - the primary Mg2Si crystals and fourth – the α-Al(Mn,Fe)Si phase. Similar phases were observed in the AlMg4.3Ge6.49 alloy: Al based solid solution, (Al)+(Mg2Ge) eutectic, primary Mg2Ge crystals and α-Al(Mn,Fe)Ge. After two days of storing in an as-cast condition, the solid solution in all tested alloys decomposes and forms zebra-crossing shaped precipitates. TEM examinations revealed that these precipitates nucleate heterogeneously on dislocations. The solution treatment at 575.0°C results in spheroidization of the eutectic, dissolution of the precipitates and formation of α-Al(Mn,Fe)Si dispersoids, nucleating on the surfaces of Mg2Si and Mg2Ge lamellas. In the Sc+Zr containing alloys, the formation of Al3(Sc1-xZrx) was detected after 120 min soaking. Further heating resulted in the growth of these precipitates. Aging of the Al-Mg-Si and the Al-Mg-Ge alloys leads to an increase of hardness in all studied alloys. This effect is mainly related to precipitation strengthening, via solid solution decomposition and formation of β"-phase in the Al-Mg-Si alloys and U1Ge phase in the Al-Mg-Ge alloys. Additionally, in Li-alloyed specimens, plates of β Mg2Si phase were observed together with small cubic-shaped δ' Al3Li precipitates. The obtained results were compared with those of three commercial casting alloys, namely Magsimal 59 (Al-Mg-Si system), A201.0 (Al-Cu system) and A356.0 (Al-Si-Mg system). The macro and microhardness properties of the developed alloys are lower than those of the high strength A201.0 alloy, but higher than those of A356.0 casting alloy. This demonstrates the promising potential of the Al-Mg-Si and the Al-Mg-Ge system for the design of novel casting alloys and the development of the first Li-containing casting alloys with reduced density.

Indexing (document details)
Advisor: Reimers, WalterSchneider, WolfgangBanhart, John
School: Technische Universitaet Berlin (Germany)
School Location: Germany
Source: DAI-C 81/1(E), Dissertation Abstracts International
Subjects: Materials science
Keywords: Metallurgy, Aluminum alloys
Publication Number: 10694721
ISBN: 9781392884782
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