Formation of nano-laminated structure in nickel by means of surface mechanical grinding treatment

A polycrystalline Ni (99.882% purity) bar sample was subjected to surface mechanical grinding treatment (SMGT) at ambient temperature. Gradient microstructures along depth from the treated surface were generated owing to a graded variation of strain and strain rates, including dislocation structures, submicron-sized structures, and nanostructures, respectively. In the subsurface layer of 10-80 μm deep, 2-dimensional laminated structures with low angle boundaries and strong deformation textures were formed of which the average thickness is ∼20 nm, one order of magnitude smaller than that of the ultrafine structures in Ni induced by conventional severe plastic deformation. The extraordinary grain refinement was ascribed to the high strain rates and high strain gradients that enhance accumulation of geometric necessary dislocations with a suppressed recovery dynamics. Deformation of the nano-laminated structures is governed by dislocation slip, and supplemented by deformation twining at the nanoscale, eventually leading to fragmentation into nano-sized equiaxed grains.