Microstructure instability of fully lamellar TiAl alloy containing high content of Nb after long-term thermal cycling

Microstructure instabilities of the fully lamellar Ti-45Al-8.5Nb-(W,B,Y) alloy were investigated by SEM and TEM after long-term thermal cycling (500 and 1000 thermal cycles) at 900 degrees C. Two major categories of microstructure instability were produced in the alloy after the thermal cycling: 1) The discontinuous coarsening implies that grain boundary migrations are inclined to occur in the Al-segregation region after the long-term thermal cycling, especially after 1000 thermal cycles. Al-segregation can be reduced during the process of long-term thermal cycling as a result of element diffusion; 2) The alpha(2) lamellae become thinner and are broken after 1000 thermal cycles caused by the dissolution of alpha(2) lamellae through phase transformation of alpha(2)->gamma. The gamma grains nucleate within the alpha(2) lamellae or (alpha(2)+gamma) lamellae in a random direction.