Sc-induced great increase in high temperature strength of Al-Si-Cu heat-resistant alloy

The effect of 0.2 wt% Sc addition on the high temperature strength and microstructure evolution of Al-11.5 wt%Si-4.0 wt%Cu alloy in different states (as-cast, T6 and LTHE (long-time-heat-exposure at 300 °C for 100 h)) was studied in detail. Tensile test at RT (room temperature) and at 300 °C was used to examine the variation of strength. OM (optical microscope), SEM and TEM/HRTEM were used to characterize the microstructure. It is found that the value of σ0.2 (the yield strength) at 300 °C of the alloy with Sc-micro-alloying can be increased significantly, especially in T6 and LTHE states. During the ageing treatment, Sc-micro-alloying greatly increases the number densities of θ′′ and θ′ dispersoid particles, thus σ0.2 at 300 °C of the samples in T6 state can be increased from 86 MPa to 116 MPa. By Elements-Mapping, it is found that Sc and Si co-exist inside θ′ precipitates. This co-existence improves the thermo-stability of θ′ phase and hence suppresses the coarsening behavior of θ′ particles and phase transformation from θ′ to θ during the LTHE treatment. Therefore, the value of σ0.2 at 300 °C of the alloy with Sc-micro-alloying in LTHE state still keep at a very high level of 99 MPa, with an increment of 36 MPa compared with the base alloy.