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

摘要

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 (alpha(2)/gamma), isolated gamma phase and B2 phase. Phase transition equations and reaction temperatures during heating from RT to 1350 degrees C were identified: alpha(2)->gamma, 835 degrees C; alpha(2)-> B2 + gamma, 990 degrees C; alpha(2) + B2 + gamma -> B2 + gamma, 1132 degrees C; B2 + gamma -> a + gamma, 1144 degrees C; gamma ->alpha, 1183 degrees C; alpha + gamma ->alpha + beta + gamma, 1278 degrees C; alpha + beta + gamma -> a + beta, 1290 degrees C; alpha + beta ->beta, 1313 degrees C. It's found that adding of beta-stabilizing elements makes p phase field of Ti43.5Al6V1Cr alloy expand to Al rich side, decreases temperature of beta ->alpha transition by 127 degrees C and causes beta + alpha + gamma phase field appearing in the phase diagram. Orientation relation (111)(B2)//(110)(gamma) was found between B2 and isolated gamma phase, besides (110)(B2)//(111)(gamma)(K-S relation), the match of the two phases declares that beta phase is parent phase of isolated gamma phase. Above all, the phase transformation process during heating and cooling was deduced. During heating it's: alpha(2)/gamma + B2 + gamma -> B2 + gamma ->alpha + gamma ->beta + alpha + gamma ->alpha + beta-beta, while during cooling it's L -> L + beta ->beta ->alpha + beta ->alpha + beta + gamma -> alpha + B2 + gamma ->alpha(2)/gamma + B2 + gamma. Besides, the microstructure evolution mechanism was explained and depicted in schematic.

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