Microstructures and Mechanical Properties of Pure V and Mo Processed by High-Pressure Torsion

Abstract

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.

Achieving superior grain refinement and mechanical properties in vanadium through high-pressure torsion and subsequent short-term annealing

“…3 that the hardness increases from an initial value of Hv  85 to ~240 after 10 turns of HPT. This microstructural refinement and hardness improvement during HPT is consistent with the only other reports describing the HPT processing of vanadium [23,24].…”

“…These materials showed increases in the ultimate tensile strength (UTS) from ~750 MPa after ECAP to ~920 MPa after ECAP plus cryorolling but this was accompanied by a ductility loss so that the total elongations to failure for these two processing routes were reduced from ~14% to ~8.5% [11]. Very recently there have been two reports describing grain refinement of pure vanadium processed by HPT [23,24] and these results show that the material exhibits little or no ductility.…”

On the minimum grain size obtainable by high-pressure torsion

“…These values were taken from Refs. [29][30][31][32][33][34][35][36][37][38][39][40][41][42]. Also included in this table are physical parameters characterizing the metals listed in the table.…”

Review on Recent Advancements in Severe Plastic Deformation of Oxides by High‐Pressure Torsion (HPT)

“…After compression, the lower HPT anvil is rotated against the upper anvil under pressure and large shear strain is induced in the material ( γ = 2 πrN / h ; γ : shear strain; r : radial distance from disc center, N : number of HPT turns, h : thickness of disc). The high pressure in this method significantly suppresses the failure of material and provides a unique condition to process hard and/or brittle materials such as high‐melting‐temperature metals, intermetallics, glasses, ceramics, silicon, carbon polymorphs, and polymers …”