Crystallography of phase transformation during quenching from β phase field of a V-rich TiAl alloy

Well-developed α2′-martensitic laths were formed in a Ti–40Al–10V (at.%) alloy after a homogenization treatment within the β phase field followed by brine quenching. The morphology and crystallography of martensite were examined by electron backscatter diffraction. It was found that the martensitic transformation was incomplete and about 35 vol% β phase was retained to room temperature. No diffusional or massive transformation has been detected in the β grain interior. All the 12 martensite variants were formed according to the Burgers orientation relationship (BOR) with the parent β grain. While global variant selection has not been detected, local variant selection occurred so that three self-accommodant variants which shared a common 〈11.0〉 axis were predominant in local regions. Based on the traces of the variants, the habit plane was determined to be close to {679}_β by using a simply geometrical method. Besides, α2′ plates were frequently observed along the β grain boundaries, which accorded to a strict BOR with one of the adjacent β grains, but tended to grow into the other β grain and hence resulted in serrated interface. Such features were an indication of massive transformation. Moreover, an apparent variant selection criterion for the grain boundary α2′ plates was noted. That is, if a α2′ variant has, in addition to an exact BOR with one of the two adjacent β grains, a near BOR with the other β grain, this α2′ plate would be very large and even extends along the entire grain boundary.