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

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 under an applied pressure, P, of 6.0 GPa. 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 c-HPT. 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.