Kinetics of recrystallization and microstructure distribution during isothermal annealing of cold rolled nickel

The recrystallization kinetics of the cold rolled nickel during isothermal heating was investigated by means of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. The samples were cold rolled over a range of deformation strains (0.25 to 1.2) and isothermally annealed at the temperature range of 623 K to 823 K. It was found that the JMAK exponent varied between 1.1 and 3.8 in the deformation and temperature range of the investigation, indicating the time dependence of the nucleation and growth rate in the recrystallization process. The finite element simulation showed that, due to the redundant shear near the sample surface, the fraction recrystallized increased from center to surface and the microstructure was inhomogeneous through the sample thickness. The microstructure distribution was more homogenous at lower deformation strains and at higher annealing temperatures. The metallographic examinations as well as the hardness measurements confirmed the simulation results. The compression test results showed that the work hardening behavior of nickel is linear. Furthermore, the yield stress decreased while the work hardening slope increased with increasing the annealing time. (paper)