FORMATION OF WIDMANSTATTEN ALPHA-STRUCTURE IN A TI-6AL-4V ALLOY

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 beta-transus and 700-degrees-C, the nucleation of primary alpha-particles occurs at the grain boundaries with forming a single C-curve. Acicular structures grow subsequently after the primary alpha-nucleation, but they exhibit two different characteristics, i.e., the alpha-plates with rather flat surface markings due to the volume change at temperatures above about 930-degrees-C and Widmanstatten 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 Widmanstatten 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 Widmanstatten laths form via a diffusional lattice transition with relaxing the transformation strain by the lattice invariant shear is proposed.