Influence of the High Pressure Torsion Die Geometry on the Allotropic Phase Transformations in Pure Zr

The effect of the press geometry in the α to ω+β transformation in pure Zr by high pressure torsion (HPT) was investigated. Specimens were processed in constrained and unconstrained setups using a wide range of applied pressures and 5 anvil turns. The resulting microstructures were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM and HRTEM) and electron backscatter diffraction (EBSD). Microhardness distributions were measured for each condition. The transformation kinetics and the stability of the fabricated nanostructures are enhanced when using an unconstrained setup. In the samples processed by constrained HPT the full transformation does not take place even at the highest pressures applied. Additionally, post-processing room temperature grain growth of some of the remaining nanocrystalline α-Zr grains with (0001) orientations occurs, leading to a significant decrease in hardness. HRTEM allowed confirming the presence of pure β-Zr.