Influence of Cu additions on the morphology of GeSi precipitates in an Al-Ge-Si alloy |
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Authors: | A K Mukhopadhyay |
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Affiliation: | (1) the Defence Metallurgical Research Laboratory, 500 058 Kanchanbagh, Hyderabad, India |
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Abstract: | Combined additions of Ge and Si to Al are known to produce higher precipitation hardening than that which occurs in the constituent
binaries, when the total amounts of alloying atoms are the same for all the alloys investigated. In the resultant Al-Ge-Si
alloys, the diamond cubic precipitates contain both Ge and Si and are designated as GeSi. During artificial aging at 160 °C,
the GeSi precipitates are commonly present in three forms, i.e., equiaxed, 〈100〉Al lath, and triangular plate. The equiaxed form is the dominant one of the three. This article examines the influence of varying
amounts (i.e., 2 to 4 wt pct) of Cu additions on the morphology of GeSi precipitates formed in an Al-2.6 wt pct Ge-1.04 wt pct Si alloy
during artificial aging at 160 °C. It is shown that Cu additions have the remarkable effect of maximizing the nucleation frequency
of the 〈100〉Al lath form and simultaneously suppressing the nucleation of the equiaxed and the plate forms of the GeSi precipitates. Increasing
Cu additions also increase the homogeneity and cause refinement of the 〈100〉Al laths. These results are discussed in light of (1) the critical requirement of vacancies for the nucleation and growth of
GeSi precipitates having an atomic volume larger than Al and (2) the crystallographic nature of the negative dilation strains
that develop locally in the Cu-rich regions of the Al matrix. It is further shown that, in the alloys containing increased
levels (i.e., exceeding about 2.5 wt pct) of Cu, the precipitation of ϑ′ (metastable ϑ-Al2Cu) phase occurs, and that the nucleation of Cu-rich ϑ′ precipitates occurs upon the 〈100〉Al laths of GeSi. The latter effect is discussed in terms of the attainment of both the nucleation site and the necessary solute
supersaturation at the 〈100〉Al GeSi/α-Al interfaces. |
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