Precipitation kinetics and hardening mechanism in Al-Si solid solutions processed by high pressure solution treatment

A dense uniform distribution of nanoscale Si precipitates in Al-Si binary alloys were achieved using high pressure solution treatment and aging treatment. Precipitation kinetics, hardening effect as well as supersaturation-growth-hardening relationships during aging treatment were quantitatively investigated. The results reveal that there exists a critical supersaturation value (0.026) during precipitation process, which divides the size versus time curve into two stages corresponding to different slopes. The sudden variation in the curve around the critical point indicates a great change in the growth rate of second phase, which is assumed to be related to the inconstancy of diffusion coefficient over a wide supersaturation range. The dense nanoscale precipitates could lead to extraordinarily high precipitation strengthening, which is combined with solid solution hardening caused by residual Si solute atoms in Al matrix, resulting in much more complicated age hardening response than that in low solubility alloys. Ultra-high hardness (~ 130 HV) is achieved in Al-7Si alloy with optimized microstructure. This work provides a promising approach for achieving high performance alloys with nanoscale secondary phase as well as insights into the underlying mechanism of microstructure evolution during aging process.