Structure and properties of cast Al-Si based alloy with Zr-V-Ti additions and its evaluation of high temperature performance

A hypoeutectic Al-Si based alloy with Cu, Mg and additions of Zr, V and Ti, was evaluated to determine its suitability for applications at higher temperatures. The as-cast alloy consists of α-Al matrix, Al-Si eutectic and Al2Cu, Al5Mg9Si8Cu2 and Al14Mg4FeSi6/Al5FeSi, intermetallic phases, plus two distinct Zr-V-Ti-based phases. Solutionizing treatment leads to dissolution of Al5Mg8Si6Cu2 and Al2Cu and the partial dissolution of Zr-V-Ti-based intermetallics. Integration of aging kinetics with mechanical properties data allowed to establish different aging parameters leading to T6 peak-aging and T7 over-aging conditions corresponding to hardness of 96 and 68 HRF respectively. T7 condition enabled higher hardness retention than T6 condition; the alloy softening was observed at 260 °C versus 240 °C after T6 treatment. TEM characterization of the alloy in the T6 condition revealed GP zones, and rod-shaped trialuminide Al3(Zr, V, Ti) with average size of 230 nm as well as S-phase (Al2CuMg) precipitates. After T6 treatment followed by isothermal annealing at 475 °C, the alloy showed hardness retention improvement by approximately 30%. Subsequent TEM analysis revealed a higher density of the Al3(Zr, V, Ti) rod-shaped precipitates along with formation of new lath shaped Q′-phase AlSiCuMg precipitates. Two over-aging reactions between 250-350 °C and 440-460 °C were associated with the stability of Cu-Mg and Al-Zr-V-Ti based nanoprecipitates respectively. Temperature of 250 °C was established as the transition temperature above which degradation of mechanical properties was observed from UTS of approximately 311 MPa at room temperature to 208 MPa at 250 °C. The results indicate that additions of Zr, V and Ti improve yield strength and cyclic yield strength at room temperature but lower ductility, while slightly improve elevated temperature hardness with almost no effect on tensile strength.