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 alpha-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 degrees C versus 240 degrees C after T6 treatment. TEM characterization of the alloy in the T6 condition revealed GP zones, and rod-shaped trialuminide Al-3(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 degrees C, the alloy showed hardness retention improvement by approximately 30%. Subsequent TEM analysis revealed a higher density of the Al-3(Zr, V, Ti) rod-shaped precipitates along with formation of new lath shaped Q'-phase AlSiCuMg precipitates. Two over-aging reactions between 250-350 degrees C and 440-460 degrees C were associated with the stability of Cu-Mg and Al-Zr-V-Ti based nanoprecipitates respectively. Temperature of 250 degrees 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 degrees 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. (C) 2014 Crown Copyright and Elsevier BV. All rights reserved.