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

摘要

A quantitative analysis of micro structure and strength as a function of strain is presented for polycrystallme nickel (99.5 percent ) deformed by high-pressure torsion in the strain range 1-300 (epsilon_(VM), 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 equi-axed subgrains and deformation twins. The evolution of micro structure and microstructural parameters falls in stages: (i) the first stage at epsilon_(VM) = 1-12; (ii) a transition stage at epsilon_(VM) = 12-34; and (iii) a saturation stage at epsilon_(VM) >= 34. A scaling analysis of spacing between boundaries shows a universal behavior up to epsilon_(VM) = 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.