Abstract
Isothermal transformation characteristics of Ti-6 mass%Al-4 mass%V alloy has been investigated by means of microstructural and surface relief observations. At temperatures between β-transus and 700°C, the nucleation of primary α-particles occurs at the grain boundaries with forming a single C-curve. Acicular structures grow subsequently after the primary α-nucleation, but they exhibit two different characteristics, i.e., the α-plates with rather flat surface markings due to the volume change at temperatures above about 930°C and Widmanstätten laths with invariant plane strain type surface reliefs. It is likely that the partition of alloying elements is considerably small in the growth stage of Widmanstätten laths and proceeds mainly during the subsequent isothermal holding after the lath-formation. Considering the results with a crystallographic analysis, a model, the shear-assisted diffusional transformation, where Widmanstätten laths form via a diffusional lattice transition with relaxing the transformation strain by the lattice invariant shear is proposed.
