Structural evolution and micromechanical properties of ternary Al–Ag–Ge alloy solidified under microgravity condition

Microgravity solidification provides a particular opportunity to produce an extraordinary microstructure and optimized mechanical properties. To shed further light on the influence of rapid solidification mechanism on mechanical properties, both the microgravity solidification mechanism and resultant micromechanical properties of ternary Al₅₇Ag₁₂Ge₃₁ alloy were analyzed by means of drop tube, nanoindentation and frictional sliding techniques, which was compared with equilibrium solidification condition. The solidification pathways changed with the decrease of droplet size, owing to a larger cooling rate and a higher undercooling. Consequently, the microstructure transformed from dendrites plus two-phase eutectic to two-phase eutectics, eventually to anomalous ternary eutectic, while the thickness of surface (Ge) layer decreased. The micromechanical properties of rapidly solidified alloy droplets were evidently improved with the decrease of droplet size, which is mainly ascribed to the microstructure refinement and the homogenous distribution especially of hardening (Ge) phase. The measured microhardness, yield strength, strain hardening exponent, pile-up resistance and friction coefficient were analyzed as a function of droplet size.