Plastic deformation induced extremely fine nano-grains in nickel

Abstract

Plastic deformation induced grain refinement was studied in a gradient nanostructure of nickel prepared from the surface mechanical grinding treatment at 77K. With increasing strains and strain rates, the deformation microstructures evolve from dislocation cells to nanolaminated structures of about 20–100 nm thick. With further straining, the nanolaminated structures are fragmented into equiaxed grains with size below 20 nm. The average grain size in the topmost surface layer is about 8 nm and the hardness is as high as 8.5 GPa. Dislocation slip dominated the plastic deformation in grains larger than about 60 nm. Below 60 nm, large amounts of twins formed. The fraction of grains containing twins increases in smaller grains and peaks at about 20 nm, indicating a dominating partial dislocation activity in deformation. With grain size smaller than 20 nm, both dislocation slip and twinning are limited, partial dislocation activities may assist subdivision of grains into few nanometers, as well as facilitate face-centered-cube to hexagonal-close-packed phase transformation in these extremely fine nano-grains. No inverse Hall-Petch relationship is observed at such a small grain size, which may be attributed to grain boundary relaxation during formation of these extremely fine nano-grains by plastic deformation.