Microstructural behavior and mechanical properties of nanocrystalline Ti-22Al-25Nb alloy processed by high-pressure torsion

Abstract

In the work reported in this paper, high-pressure torsion (HPT) was conducted on Ti-22Al-25Nb alloy under the applied pressure of 6 GPa at ambient temperature in 1, 5, and 10 turns. Investigation of the microstructural characteristics and microhardness evolution of the HPT-processed Ti-22Al-25Nb alloy showed that the lamella B2-phase could be transformed to O-phase at ambient temperature. This was due to severe shear transformation of the B2 lattice caused by the high density of dislocations generated during HPT. Significant grain refinement was achieved, from grain size of ~67 μm in the initial annealed specimen to ~53 nm after 10 turns in the HPT process. In addition to dislocations, nanotwins played an important role in the grain refining process. With increasing accumulation of equivalent strain, the microhardness value of this alloy increased and reached a saturated value of about ~600 HV when the equivalent strain reached ~20. This is higher than that obtained by conventional processes. The hardening mechanisms were primarily attributed to pronounced grain refinement and the high density of dislocations induced by the HPT process.