Abstract
Transmission electron microscopic study on a α2 + γ two-phases Ti-40 at.% Al alloy demonstrates that (011)γ/(101)γ twin does occur in TiAl. Differently-orientated γ regions are observed to coexist within the same γ lamella with the (011)γ/(101)γ twin plane as their boundary. The variants pair corresponding to these differently-orientated γ regions should come from variants which have the same stacking sequence. The mechanism for the formation of (011)γ/(101)γ twin plane is suggested to grow from the ‘APB-induced fault’ formed in the γ lamella during the precipitation transformation of α2→α2 + γ. Total strain induced by the structure change (h.c.p.→f.c.c.) and by the tetragonality of γ phase during the precipitation transformation can be minimized by a composite of {111}γ and {011}γ twins coexisting in γ lamellae.