Effects of the addition of trace amounts of Sc on the microstructure and mechanical properties of Al-11.6Si alloys

The effects of the addition of trace amounts of scandium on the microstructure and mechanical properties of Al-11.6Si alloy were investigated. The results show that the iron-bearing phase of Al-11.6Si alloy without the addition of Sc in the aluminum matrix was present in the acicular form. This acicular iron-bearing phase is a hard and brittle intermetallic compound that is easily broken during the solidification process, causing serious damage in the mechanical properties (especially the ductility) of the alloy. During thermal exposure to high temperature at 250 °C for 100 h, the Al-11.6Si alloy without the addition of Sc undergoes recovery and recrystallization, reducing the mechanical strength. However, trace amounts of Sc transform the acicular iron-bearing phase of Al-11.6Si alloy into the chinese-script iron-bearing phase, thereby reducing the length and the amount of the iron-bearing phase. These factors enhance the ductility of Al-11.6Si alloy with Sc addition. In addition, it was remarkably found that a high density of L12-ordered Al3Sc precipitates are formed during thermal exposure at 250 °C for 100 h in Al-11.6Si alloy with Sc addition. The precipitation of L12-ordered Al3Sc precipitates effectively inhibited grain growth and hindered the movement of dislocations. These factors led to the Sc-containing alloy having the best mechanical properties (strength and ductility) than Al-11.6Si alloy without Sc addition during the following thermal exposure at 250 °C.