Evolution of intermetallic phases in an Al–Si–Ti alloy during solution treatment

Abstract

Abstract A cast Al–Si–Ti alloy was solution treated at 540 °C for different periods between 0 and 72 h to understand the evolution of intermetallic phases. Only an (Al,Si)3Ti intermetallic phase with a low Si content was found in the as-cast alloy. The (Al,Si)3Ti particles were converted partly into a lamellar structure, a eutectoid phase, consisting of a Si-rich phase and an Al phase during solution treatment. The amount of the lamellar structure increased with the solution treatment time, but the composition of either constitute was kept almost unchanged regardless of the solution treatment times. The lamellar Si-rich phase is (Al,Si)2Ti (τ2) with a TiSi2 (C49-type) structure based on thermodynamic calculations and high-resolution TEM analyses. FCC Al phase is the product residing between the τ2 lamellae after the completion of transformation from (Al,Si)3Ti to τ2 phase. A near-rational orientation relationship (OR) between the Al and τ2 phases is determined as Al [110]//τ2 [100], Al ( 1 1 ¯ 1 ¯ )//τ2 [060]. The phase transformation from (Al,Si)3Ti to τ2 being a result of the diffusion of Ti and Si within the original (Al,Si)3Ti particulates as well as the Si diffusion from the Al matrix during solution treatment is proposed. The formation of the lamellar structure in the microstructure is attributed mainly to the limited diffusivity of Ti element.