Phase transformation and structure evolution of a Ti-45Al-7.5Nb alloy processed by high-pressure torsion

Intermetallic γ-based titanium aluminides of Ti-45Al-7.5Nb have been subjected to high-pressure torsion (HPT) processing. Significant grain refinement has been achieved from ∼10 μm to ∼30 nm, leading to the improvement in both physical and mechanical properties. Complementary studies correlated the microstructure, phase transformation behavior and the enhancement of mechanical properties. Neutron and X-ray diffraction revealed that an ongoing order-disorder transformation occurs by HPT processing, resulting in large heterogeneous behavior between the surface-near and the median layers of the disk. While the γ-phase almost disappeared underneath the surface region, such order/disorder phase transformation consistently decreases towards the middle-thickness section of the samples. A low bulk texture index is consistent with grain rolling and swirling rather than slip deformation. Vickers micro-hardness indentation confirms the improvement of hardness from 308 H_v to 605 H_v. For the first time, the present work demonstrates heterogeneity in structural transformation, such as displacive transformation and order/disorder transformation, which can be compared to earlier reported inhomogeneity in mechanical properties and microstructure within bulk nanostructured materials that were processed by HPT.