Precipitation of ScAl<sub>3</sub> Phase during Solutionizing and Ageing of Hypoeutectic Al-Sc Alloys and its Impact to Mechanical Properties

Abstract

In this paper, precipitation of ScAl3 phase during solutionizing and ageing and its impact to mechanical properties of hypoeutectic Al-Sc alloy and effect of Sc addition amount were investigated by TEM and SEM microstructure observation and mechanical property testing. During solutionizing at 640oC for 24h, for Al-0.2wt.%Sc and Al-0.4wt.%Sc alloy, ScAl3 particles formed in the course of solidification become smaller, indicating, as a whole, Sc is partially dissolved into Al solution. Simultaneously, precipitation of two types of ScAl3 particles are observed by TEM. One has a size of 200~300nm, and the other is much small, 5~20nm in size. Though re-dissolution of Sc solute into Al solution and precipitation of large and fine ScAl3 particles occur in the solutionizing course, but the strength and hardness are decreased. The key reason for it is thought to be the softening effect of high level of vacancies in matrix lattice from the high solutionizing temperature. After further aging at 300oC for 3h, a great number of fine ScAl3 particles are precipitated in the Al matrix, which leads to a considerable precipitation hardening effect, thus the strength and hardness are increased obviously. Increasing the Sc content in the alloy results in a considerable rise in as-cast and as-aged strength and hardness, due to the solution strengthening and precipitation hardening respectively.