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

Abstract

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 230nm 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 311MPa at room temperature to 208MPa 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.