Precipitation and strengthening in aluminum-germanium-silicon alloys.

摘要

The focus of this work is to study the precipitation hardening behavior of three Al-Ge-Si alloys and the effect of precipitates on their mechanical responses under forward and reverse loading conditions. Because of the misfit strain cancellation of Ge and Si elements in the aluminum matrix the ternary alloys have finer, denser and more evenly distributed precipitates than in the Al-Ge or Al-Si binary alloys. For the same total alloy content the ternary alloys have smaller precipitate spacing and therefore higher yield stress according to the Orowan theory [1], make them candidates for structural applications.; Experimental and analytical work has been conducted to understand the effect of precipitates on the mechanical behavior of the Al-Ge-Si alloys. The material samples were first solution heat treated and then artificially aged. Their precipitate characteristics, such as precipitate shape and size, were examined through TEM. Tensile and tension/compression tests of the aged materials were conducted to obtain their mechanical properties. Based on the measured precipitate information a combined hardening model was constructed to explain the mechanical strengthening mechanisms in these alloys. Although the yield stress modeling result is satisfactory, the combined model over-predicts the monotonic strain hardening of the materials. Various sources affecting material response during reverse loading were also analyzed. Besides the precipitate effect grain boundary and grain texture are the other important factors. For the Al-1%Ge-Si material in this study, grain boundary obstacles accounts for 20% of the backstress while precipitates contribute the remaining 80%. It was found that the magnitude of the Bauschinger factor relates to the volume fraction of precipitate, while the Bauschinger strain correlates better with particle spacing.