Formation of TiAlSi intermetallics during heating Ti-A356 Al mixed powder compact at semisolid temperature

Abstract

• The local breakage of oxide films determine the formation of τ1 and τ2 crystals and their morphologies. • Both τ1 and τ2 crystals have an intrinsic plate morphology, but τ2 crystals were thinner due to their more anisotropic growth. • τ1 lamellar domains generate during growth of τ2 crystals accompanied by decrease in density of SFs. • τ1 crystals coarsen at the expense of τ2 crystals, leading all reaction products to finally convert into large τ1 plates. Due to the similarity in composition and crystal structure of some TiAlSi ternary intermetallics, it is quite difficult to accurately confirm their physic characteristics, and thus, to tailor microstructure to improve mechanical properties of metal materials related to the TiAlSi intermetallics. Therefore, the formation process of TiAlSi intermetallics was investigated during heating Ti-A356 mixed powder compact at a A356 alloy semisolid temperature of 595 °C. The results indicated that two kinds of intermetallics of τ1 (tetragonal, I41/amd) and τ2 (orthorhombic, Cmcm) generated, and both the concentration and supply direction of Ti atoms, which are related to local breakage of oxide films on neighboring Ti and A356 alloy powders, determined the phase and morphology of the intermetallics. τ1 and τ2 crystals all had an intrinsic plate morphology, but τ2 crystals were thinner due to more anisotropic growth resulted from their high density of stacking faults (SFs). τ1 lamellar domains generated during growth of τ2 crystals, which were accompanied by decrease in density of SFs. The combined effect of size difference and internal energy difference between τ1 and τ2 crystals made large τ1 crystals coarsen at the expense of τ2 crystals in Ostwald ripening, leading them to finally evolve into an agglomerate of large τ1 plates.