Microstructures and mechanical properties of pure V and Mo processed by high-pressure torsion

Two body centered cubic (bcc) metals, V and Mo, were processed by high pressure torsion (HPT) at ambient temperature. Hardness variation as well as microstructural evolution was examined with strain under a pressure of 2 to 6 GPa. It was shown that the hardness increases with straining and saturates to a constant level with the grain size of 330-400 nm in V irrespective of the applied pressures. Although the hardness variation with strain is the same for Mo with the grain size of ∼350 nm at the saturation level when the applied pressure is 6 GPa, the hardness level lowers below the saturation level and the grain size becomes coarser as the pressure is lowered. Tensile tests show that the strength significantly increases with some ductility for V after processing under any pressure and for Mo under lower pressures, but brittle fracture occurs in the Mo specimen processed at 6 GPa. The slower evolution of microstructure as well as the lower hardness levels observed in Mo is due to the applied pressure which is lower than the yield stress and thus due to the insufficient generation of dislocations for grain refinement.