Influence of cooling rate on the microstructure and ageing behavior of as-cast Al-Sc-Zr alloy

摘要

Al-0.3Sc-0.15Zr alloy was cast using copper die, insulated alumina mould, and conventional investment shell mould to obtain a wide range of cooling rates. A novel method of quenching the investment shell mould along with the liquid metal in oil was also used which resulted in a significant increase in the cooling rate. The order in increasing average cooling rate is 0.16, 0.78, 1.28, 5.93, 7.69℃/s. The as-cast samples were aged isothermally at 300℃ and various temperatures for 2 h. Slow cooled samples (in alumina-insulated mould) showed the presence of as-cast primary precipitates as well as rod shaped discontinuous precipitates with high density of interfacial dislocation. The amount of as-cast precipitates decreased with increase in the cooling rate. These as-cast precipitates grew at the expense of Sc in solid solution reducing the number of precipitates formed during ageing process. This results in lower increment in hardness on ageing.