TEM Observation of Precipitates in Cast Al-7%Si-0.3%Mg Alloy Aged at 473 K

Abstract

Al-Si-Mg casting alloys are widely used as practical materials due to their g ood castability, pressure resistance, corrosion resistance and excellent mechanical properties. Many researches have investigated about Al-Si-Mg casting alloy for improving the properties mentioned above. However, few reports are available about precipitation sequence and types of precipitates using transmission electron microscopy（TEM）. Previously, we have reported the age-hardening of Al-10mass%Si-0.3mass%Mg alloy from the aspect of mechanical property improvement. It is needed to investigate the shape of precipitate, the distribution of precipitate and the structure of precipitate to understand precipitation hardening of Al-Si-Mg casting alloys. In this research, Al- 7mass%Si-0.3mass%Mg alloy was selected and investigated due to chemical composition similarity of A356. Alloy was prepared by casting and demolded at 773 K. Then, ice-water quenched. Aging treatment was conducted at the temperature of 473 K and 20 Hv of age-hardenability was obtained. With TEM observation, several precipitates were observed during aging treatment. Close inspection of SAED pattern and inverse fast Fourier transformation（IFFT）confirmed parallelogram type precipitate, Type-A precipitate, β’ precipitate, and Si phase.