Microstructure evolution and phase transformation mechanism of Ti–46Al–8Nb–2.5V alloy based on phase transformation geometric models

Abstract

The microstructure of conventionally solidified and rapidly solidified Ti–46Al–8Nb-2.5V alloys was analyzed with phase transformation geometric models, revealing the phase transformation mechanisms. The β→α→γ phase transformation processes follow Burgers and Blackburn orientation relationships (ORs), respectively, from which the calculated transformation between the β/β0 and γ phases follow K–S OR. Whlie, the direct β/β0→γ phase transformation can be directly observed in the microstructure, which follows a variety of ORs, mainly following the K–S OR but also following the N–W or Bain ORs, most of which deviate from the strict ORs. In cases where N–W and Bain ORs are followed, there is a large dislocation spacing between the habit planes of the β/β0 and γ phases, resulting in low static energy at the interface and narrow elongated morphology of precipitated phases. Conversely, when following K–S OR for β/β0 → γ phase transformation, interfacial migration energy is relatively lower and grain precipitated phases is larger. The rapidly solidified microstructure of Ti–46Al–8Nb-2.5V alloy consists of the massive α2 phase, massive γ phase, and basket-weave β0 phase closely resembling those found in β-type titanium alloys. After annealing at service temperature, the rapidly solidified alloy forms a microstructure highly similar to that of conventionally solidified TiAl alloy. Various metastable phases, such as the Ti2Al, L12 and Ti1.4Al phases, precipitate in the blocky γ phase, which provides a theoretical basis for studying the phase transformations of TiAl alloys in service.