Microstructural evolution and mechanical properties of Al–Si–Cu–(Ge)–(Mg) alloy solidified under high pressure

Abstract

The influence of high pressure on the microstructural evolution and mechanical properties of Al–Si–Cu–(Ge)–(Mg) alloys was investigated. With the increased solidification pressure, the secondary dendrite arm spacing (SDAS) of Al decreases, and the eutectic Si undergoes a transformation from the coarse acicular morphology to the coral-like morphology. The well-refined microstructure is attributed to the accelerated nucleation rate and the restrained crystal growth rate under high pressure. The morphological transformation of eutectic Si is ascribed to the synergetic effect of the high undercooling and Cu segregation at the outer surfaces of Si. Besides, the study shows that multiple nano-sized particles emerge in the alloys solidified under high pressure due to the high solid solubility of elements and increased nuclei. These nano-sized particles act as dominating strengthening phases leading to enhanced hardness and compressive strength compared with alloys solidified under atmospheric pressure. The as-cast alloy MG solidified under 4 GPa has the highest mechanical properties with a hardness of 135.2 HV and compressive stress of 721 MPa. The strengthening contribution of variable nano-sized particles was discussed. These results indicate that high pressure can be used as an effective method to introduce structural refinement and nano-sized strengthening phases, thus providing a potential way to improve mechanical properties of Al–Si–Cu-based alloys.