Phase transformation mechanisms involved in two-phase TiAl-based alloys .1. Lamellar structure formation

The present paper deals with the mechanism operating in one of the three major transformation modes appearing in two-phase TiAl-based alloys: the formation of the so-called gamma/alpha(2) lamellar structure. This microstructure, the most typical of the alloys of this category, results from precipitation of gamma (L1(0)) lamellae in the alpha (A3) or alpha(2) (DO19) matrix. This reaction is achieved through (i) pre-nucleation stage corresponding to the formation of a local f.c.c. type stacking sequence by the movement of Shockley partials; and (ii) nucleation and growth stages involving both ordering and ledge movement. The presence of numerous order domain boundaries in a given gamma lamellar precipitate is explained by the encounter of separately nucleated order domains.