Effect of thermal treatment on microstructure in high pressure torsion (HPT) deformed nickel

Polycrystalline 99.9% Ni has been deformed at room temperature to equivalent strains of about 55 by high pressure torsion (HPT) deformation under a hydrostatic pressure of about 7 GPa. After annealing at different temperatures the structural element (grain/subgrain, scattering domain) size was determined by using scanning electron microscopy and multiple X-ray Bragg profile analysis in order to draw a correlation to the strength evolution obtained by microhardness measurements. There has to be differentiated with respect to the occurring characters of the microstructure—the as-deformed and recovered microstructure, respectively, and the structure resulting after recrystallisation. The strength is directly correlated to the grain size and decreases when the grain size starts to increase at homologous temperatures above 0.4Tm. The recovery of the very high dislocation density is shifted to low temperatures starting even at 0.2Tm and has primarily no effect on the strength.