Thermal behavior of Ni (99.967% and 99.5% purity) deformed to an ultra-high strain by high pressure torsion

Polycrystalline Ni of two purities (99.967% (4N) and 99.5% (2N)) was deformed to an ultra-high strain of εvM = 100 (εvM, von Mises strain) by high pressure torsion at room temperature. The 4N and 2N samples at this strain are nanostructured with an average boundary spacing of ∼100 nm, a high density of dislocations and a large fraction of high angle boundaries (>15o) of 0.68-0.74, as determined by transmission electron microscopy, and 0.8-0.83, as determined by electron backscattering diffraction. The deformed samples were annealed isochronally for 1 h at temperatures from 100 to 600 deg. C, and the evolution of the structural parameters (boundary spacing, average boundary misorientation angle and the fraction of high angle boundaries), crystallographic texture and hardness were determined. Based on microstructural parameters the energy stored in the deformed state was estimated to be 14 MPa and 24 MPa for 4N Ni and 2N Ni, respectively. The isochronal annealing leads to a drop in hardness in three stages: a relatively small decrease at low temperatures (recovery), followed by a rapid decrease at intermediate temperatures (recrystallization) and a slow decrease at high temperature (grain growth). Both recovery and recrystallization of the 2N Ni are strongly retarded by the presence of impurities reducing the mobility of boundaries. In the recrystallization stage, changes in hardness, microstructure and texture show that the 4N Ni recrystallizes discontinuously, in spite of a large fraction of high angle boundaries in the deformed state. This finding contradicts previous experiments and theory, which suggest that recrystallization is continuous when the fraction of high angle boundaries is high. In the 2N Ni, the observations suggest that some structural coarsening (continuous recrystallization) may take place simultaneously with discontinuous recrystallization. The findings emphasize the importance of alloying, which can delay the process of recovery and recrystallization and thereby enable tailoring of the microstructure and properties through an optimized annealing treatment.