Microstructure and microhardness of molybdenum processed by monotonic and cyclic high-pressure torsion.

Abstract

High-pressure torsion (HPT) was employed to investigate the effects of monotonic and cyclic shear deformation on the microstructural evolution and hardness of commercial-purity molybdenum processed at room temperature (RT). Disks were subjected to monotonic HPT (m-HPT) and cyclic HPT (c-HPT) with strain reversals for total numbers of turns ranging from 1/2 to 7. Electron backscatter diffraction analysis showed that m-HPT promotes progressive grain refinement and continuous transformation of low-angle boundaries (LABs) into high-angle boundaries (HABs) giving an equiaxed ultrafine-grained structure with an average grain size of ~0.19 µm and a high fraction of ~82% of HABs after 7 turns. In c-HPT there are elongated or lamellar grain morphologies, reduced grain refinement to ~0.25 µm and a suppressed development of HABs to ~67% after 7 turns. There was hardening and radial hardness homogeneity in m-HPT but slower hardening and persistent radial hardness gradients in cHPT. It is concluded that strain reversals during c-HPT enhance a stress-assisted dynamic recovery, promote dislocation annihilation and disrupt subgrain rotation, thereby suppressing the continuous dynamic recrystallization.