Structural transformations activated during the formation of the lamellar microstructure of TiAl alloys

The structural transformations activated during the formation of the lamellar microstructure have been studied in a Ti49Al47Cr2Nb2 alloy. By performing statistical analysis of the microstructure, a special attention has been paid to the orientations of the lamellae and to the interface relationships. Experiments were conducted on specimens in an as-HIPed condition as well in an isothermal forged plus annealed condition. In the annealed alloy, three mechanisms of the lamellar formation were evidenced depending upon the nucleation temperature: a heterogeneous transformation occurring at grain boundaries, a homogeneous nucleation in the α2 matrix and an interfacial nucleation of twin-related lamellae. The microstructural analysis of the as-HIPed alloy was aimed at determining the activated mechanisms among those evidenced in the annealed alloy.