Strain-induced grain evolution of pure nickel under warm power torsional rolling process

Since the existing severe plastic deformation (SPD) technology cannot be used to prepare bulk ultrafine-grained material (BUGM) with industry size, a novel method entitled power torsional rolling (PTR), is proposed. The material flow, strain components, strain rate, and temperature were explored by finite element simulation, and the strain-induced grain evolution of pure nickel processed by warm PTR was systematically discussed. The simulation results reveal that the combination of radial compression deformation, longitudinal shear deformation and circumferential shear deformation is beneficial to refine grain size, and the continue local loading characteristic requires less forming load, which confirms that the PTR process has the potential to prepare BUGM with industry size. The experiment results indicate that the grain refinement caused by dynamic recrystallization is remarkable within the temperature range of 450–800 ℃, and the average grain size of pure nickel is refined from 110 μm to 10 μm after two passes PTR. The microstructure distribution of obtained bar is uniform and the isotropic microstructure is observed due to the three-dimensional compression-shear deformation. The tensile test results indicate that the yield strength and ultimate tensile strength increase simultaneously due to the grain-boundary strengthening and twin boundary strengthening.