Ultrafine-grained nickel refined by dislocation activities at intermediate strain rate impact: deformation microstructure and mechanical properties

This article presents an application of the impact-induced deformation in effective grain refinement in polycrystalline nickel. Ultrafine-grained microstructure was processed by means of Dynamic Plastic Deformation at room temperature using a falling impactor with a maximum impact velocity of 10 m s⁻¹. The commercially pure (98.4 wt%) starting material was characterised by a coarse-grained (~25 μm) microstructure. Electron backscattering diffraction and transmission electron microscopy studies showed that the initial equiaxed grains evolved into a laminar structure of submicron size narrow domains delineated by high-angle grain boundaries. The texture after deformation exhibits preferential orientations including a strong 〈220〉 fibre texture. The mechanical behaviour under quasi-static compression at room temperature and at a strain rate of 2 × 10⁻³ s⁻¹ was investigated in directions parallel and perpendicular to the impact axis. Stress–strain responses showed an increased yield strength (440–520 MPa) compared with the initial state (90 MPa). The strain-hardening behaviour was found to strongly depend on the orientation of the compression axis.