Investigation of low temperature thermal stability in bulk nanocrystalline Ni

Grain growth behavior of bulk nanocrystalline Ni, prepared by an electrodeposition technique with average grain sizes of 20 and 15 nm was investigated in the homologous temperature (T/Tm) range of 0.20–0.40. In studying grain growth, the techniques of X-ray diffraction and transmission electron microscopy were used. The results show that in the temperature range of 0.20–0.30Tm, there is no appreciable grain growth, even after long annealing times. However, in the temperature range of 0.3–0.4Tm, the rate of grain growth was rapid during the initial period of annealing, which decreases with increase in time. The value of time exponent, n  , deduced from the grain growth equation of the general form D1/n−D01/n=Kt was found to be approximately 0.1 for both grain sizes of Ni. At temperatures higher than 0.3Tm, an approximate activation energy of 105 ± 3 kJ/mol, which is close to the activation energy for grain boundary diffusion in polycrystalline Ni, was measured. At temperatures lower than 0.3Tm, an approximate activation energy of 11 ± 3 kJ/mol was found. It is suggested that this low activation energy represents the energy for the re-ordering of the nanocrystalline grain boundaries.