Grain size and reversible beta-to-omega phase transformation in a Ti alloy

We report a significant grain size effect on the phase transformation in a Ti–36Nb–2.2Ta–3.7Zr–0.3O alloy during severe plastic deformation. For grain sizes larger than the submicrometer range, the body-centered cubic β phase transformed to hexagonal ω phase when deformed by high-pressure torsion under a relatively low pressure of 3 GPa and at room temperature. However, a reverse ω to β transformation was observed under the same deformation conditions when the grain sizes were less than 100 nm.