STRUCTURE EVOLUTION OF DIRECTIONALLY SOLIDIFIED Ti--43Al--3Si ALLOY II. Microstructure Evolution in the Steady--State Growth Region

Microstructure evolutions in the steady--state growth region of directionally solidified Ti--43Al--3Si (atomic fraction, %) alloy were studied at growth rates of 3---100 μm/s. There are four main phase transitions during directional solidification: L→Ti₅Si₃, L→α+Ti₅Si₃, α→α₂(Ti₃Al)+γ(TiAl) and α₂→γ+Ti₅Si₃, especially, the eutectic reaction of α and Ti₅Si₃ is the typical solidification behavior. When the growth rate exceeds 20 μm/s, another phase transition of L→γ+Ti₅Si₃ also happens. With increase of growth rate, the steady--state growth microstructure changes from coarse cells to fine cells, to cellular dendrites and then to dendrites. The Ti₅Si₃ phase stabilizing $\alpha$ phase distributes in the α matrix at a low growth rate, and in the solidified γ matrix at a relatively high growth rate, which is unfavorable in seeding. When the growth rate is 10 μm/s, the distance needed to reach steady--state region becomes shorter, which is favourable in seeding for the present alloy.