Phase transformation mechanisms involved in two-phase TiAl-based alloys—I. Lambellar structure formation

Abstract

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 γα2 lamellar structure. This microstructure, the most typical of the alloys of this category, results from precipitation of γ (Ll0) lamellae in the α (A3) or α2 (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 γ lamellar precipitate is explained by the encounter of separately nucleated order domains.