Phase transformation and microstructure evolution of a beta-solidified gamma-TiAl alloy

Abstract

In this study, TiAl alloy with nominal composition of Ti-43.5Al-6V-1Cr was fabricated by inductive skull melting technology. Microstructure evolution and microstructure formation mechanism were invested by quenching experiments. The results show that Ti-43.5Al-6V-1Cr alloy consists of lamellar colony (α2/γ), isolated γ phase and B2 phase. Phase transition equations and reaction temperatures during heating from RT to 1350 ℃ were identified: α2→γ, 835 ℃; α2→B2 + γ, 990 ℃; α2 + B2 + γ→B2 + γ, 1132 ℃; B2 + γ→α + γ, 1144 ℃; γ→α, 1183 ℃; α + γ→α + β + γ, 1278 ℃; α + β + γ→α + β, 1290 ℃; α + β→β, 1313 ℃. It’s found that adding of β-stabilizing elements makes β phase field of Ti43.5Al6V1Cr alloy expand to Al rich side, decreases temperature of β→α transition by 127 ℃ and causes β + α + γ phase field appearing in the phase diagram. Orientation relation (111)B2//(110)γ was found between B2 and isolated γ phase, besides (110)B2//(111)γ(K-S relation), the match of the two phases declares that β phase is parent phase of isolated γ phase. Above all, the phase transformation process during heating and cooling was deduced. During heating it’s: α2/γ + B2 + γ→B2 + γ→α + γ→β + α + γ→α + β→β, while during cooling it’s L→L + β→β→α + β→α + β + γ →α + B2 + γ→α2/γ + B2 + γ. Besides, the microstructure evolution mechanism was explained and depicted in schematic.