Microstructural stability and fracture behavior in aluminum-silicon-germanium alloys

Recently developed age-hardenable Al-Si-Ge alloys utilize a uniform distribution of very small SiGe precipitates that have an incoherent interface with the matrix and are looped by dislocations. These features result in a high degree of hardening for a small volume fraction of precipitates.; The ductile fracture of Al-Si-Ge alloys occurs by the nucleation, growth, and coalescence of cavities. Since the ductility may be increased if the onset of cavity nucleation is delayed, it is important to study the parameters controlling cavity nucleation. The objective of this dissertation is to investigate cavity nucleation and growth associated with spherical SiGe precipitates that are one to two orders of magnitude smaller than previously studied and to analyze the results quantitatively using only experimentally determined cavity nucleation parameters and finite element calculations.; The volume fraction of voids in one of the ternary Al-Si-Ge alloys was determined as a function of strain and diameter of the SiGe precipitates through a pycnometer by measuring densities of the deformed gage and grip sections. The finite element method was employed in order to calculate stress and strain distributions around the SiGe precipitates for different precipitate diameters using both elasticity and plasticity models. This analysis allowed a quantitative comparison between the experimental data and the predictions of cavity nucleation criteria based on a model developed by Brown and Stobbs and confirms the predictions of their model. In order to explain the experimental observation that the tensile ductility decreases with increasing aging time (or precipitate diameter) while the critical strain for cavity nucleation increases, the maximum void size was determined as a function of strain for different aging times using scanning electron microscopy. These results combined with finite element calculations revealed that the initial void size in the ternary Al-Si-Ge alloy increases with increasing precipitate size leading to an increase of the void growth rate with increasing precipitate size.