Microscopic mechanism of mechanical property improvement of the casted Al-7Si-0.4Mg-0.3Y alloy

Abstract

This paper investigates the effect of different casting conditions on the microstructure of Al-7Si-0.4 Mg alloys with 0.3 wt.% rare earth Y addition and the effect of T6 heat treatment on their mechanical properties. The experimental studies showed that the addition of 0.3 wt.% rare earth Y at 30 min or 1200 k resulted in the optimum microstructure and mechanical properties of the alloys, and the addition of Y led to the formation of Y-containing nanoparticles on eutectic Si, which were re-solidified by T6 heat treatment and precipitated a more stable orthorhombic cubic structure of Al2Si2Y compounds at the grain boundaries. According to the growth kinetic relationship, the addition of Y replaces the nucleation center of the original eutectic Si, and the preferential precipitation of the Y-containing phase destroys the heterogeneous nucleation conditions of the original eutectic Si, which leads to the growth of eutectic Si towards the original growth direction close to 60°, and generates more heterogeneous twins at the interface and gradually builds up, which has an inhibitory effect on the growth of eutectic Si. The fracture morphology and energy spectra show that the precipitated Y-containing coexist in these precipitates will lead to local stress concentration resulting in microcracks and ultimately fracture, which will change the mechanical properties of the alloy significantly.

Keywords

• Al-Si alloy
• T6 heat treatment
• Microstructure
• Lattice parameters
• Mechanism