XRD, TEM, microhardness and thermal analysis were carried out on a series of Ni samples produced by high-pressure torsion (HPT) technique under a pressure of 7 GPa with rotation numbers of 0.5, 1, 3, 5, and 7. Evolution of microstructures and their inhomogeneity were investigated in detail. The local microstrain level showed dynamical oscillations as a function of the numbers of rotation of HPT, demonstrating dynamical evolution of lattice defects during the HPT procedure. Both XRD and TEM showed that there is sitll small diference in grain sizes even after 5 rotations of HPT, showing smaller grain sizes at the periphery region. The higher microhardness at the periphery region is the result of smaller grain sizes and higher density of lattice defects, comparing with the central region. Thermal treatment at a heating rate of 20K/min from room temperature to 473K did not result in decrease in microhardness, but increase by about 10 per cent for samples treated not more than 3 rotations of HPT. The increase in microhardness was contributed to further refinement of grain sizes, formation of more high-angle grain boundaries and more equilibrium state of grain boundaries during recovery. Recrystallization occurred by heating to higher temperatures, which resulted in decrease of microhardness. The nonequilibrium and high-energy grain boundaries were the main source of the stored deformation energy.

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