Extreme structural inhomogeneities in high-pressure torsion samples along the axial direction

Structural inhomogeneities along the axial, and not only the radial, direction are shown to occur in Zr3Al deformed by high-pressure torsion (HPT) at room temperature. Scanning and transmission electron microscopy reveal a structure of nanocrystalline and coarse crystalline banded regions parallel to the shear plane. This contradicts the accepted equation describing microstructural evolution that is derived from the geometry of HPT. Surface treatment of one set of samples prior to deformation allows a systematic study as it leads to the localization of a refined nanocrystalline structure at the sample surface. Once a ∼50 μm wide nanocrystalline banded layer is formed (10 turns), the residual structure remains coarse grained up to the highest deformations studied (80 turns). In the narrow interface region between coarse structure and nanocrystalline structure, the grain size decreases by three orders of magnitude within less than 10 μm. This is accompanied by a change in texture, indicating the transition from a dislocation-mediated deformation mechanism to a grain-boundary-mediated one. The results of the structural study are explained in terms of the occurrence of work softening, as confirmed by microhardness measurements. Once a nanocrystalline layer—softer than the coarse structure—has formed, the vast majority of strain is accommodated by that layer: this leads to extreme inhomogeneities, a large strain gradient along the axial direction and huge deviations of the actual local shear strain from the calculated strain.