Phase transformation mechanisms involved in two-phase TiAl-based alloys—II. Discontinuous coarsening and massive-type transformation

Abstract

This paper is directed towards examination of the mechanisms involved in two of the three major transformation modes appearing in two-phase TiAl-based alloys: discontinuous coarsening and massive-type transformation. The discontinuous coarsening implying grain boundary migrations is essentially promoted by highly stable interfaces in the primary lamellar structure, which render the normal lamellae coarsening of continuous type extremely difficult. During rapid cooling, the massive-type transformation with a direct transition α → γ replaces the primary γ lamellae precipitation. The nucleation takes place most likely on grain boundaries through the formation of either an ordered nucleus or a disordered f.c.c. nucleus followed by ordering, and the nucleus is coherent with one of the two grains. Since such a coherency renders diffusion across the nucleus/matrix interface a difficult process, the growth of the massive phase occurs into the opposite grain through thermally activated short-range jumps of individual atoms across an incoherent αγ interface.