Evolution of microstructural parameters and flow stresses toward limits in nickel deformed to ultra-high strains

A quantitative analysis of microstructure and strength as a function of strain is presented for polycrystalline nickel (99.5%) deformed by high-pressure torsion in the strain range 1–300 (εVM, von Mises strain). Typical lamellar structures consisting of extended boundaries and short interconnecting boundaries have been found, with additional features at large strains which are equiaxed regions, small equiaxed subgrains and deformation twins. The evolution of microstructure and microstructural parameters falls in stages: (i) the first stage at εVM = 1–12; (ii) a transition stage at εVM = 12–34; and (iii) a saturation stage at εVM ⩾ 34. A scaling analysis of spacing between boundaries shows a universal behavior up to εVM = 300, indicating that the predominant deformation mechanism is dislocation glide whereas twin formation is of minor importance. A clear link is observed between the evolution in structure and flow stress, which can guide the development of strong metals with a structural scale extending below 50–100 nm.