Structural aspects of alpha-omega transformation in group IV transition metals and alloys

Abstract

The three most common crystal structures encountered in group IV transition metals Ti, Zr, and Hf and alloys based on them, under different temperature, pressure and alloy concentration conditions, are hcp(α), bcc (β) and simple hexagonal (ω ). Although the structural relations of α⇌β andβ⇌ω transformations are well understood, the same is not true for α→ω phase change, which occurs at high pressures. We have done high pressure experiments on Ti-V alloys, followed by electron diffraction to study this. These patterns from pressure treated foils of alloys Ti₉₅ V₅ and Ti₉₁ V₉ showed the presence ofβ-phase with fourω variants. Some of them showed the existence of all three phases, α,β andω , with the number of variants given by the lattice correspondence matrix, derived through the orientation relations of α→β andβ→ ω . This is a clear evidence that the α→ ω transformation proceedsvia theβ-phase. The atomic rearrangements required for α→ ω are found to be much smaller if the path is via theβ-phase, rather than the earlier model of Silcock.