Effect of grain size on the thermal stability of electrodeposited nanocrystalline nickel: X-ray diffraction studies

Nanocrystalline (NC) Ni coatings have widespread engineering applications, many of which are in high temperature environments. Therefore, it is important to study the thermal stability of NC Ni. The present study compares the thermal stability of electrodeposited (ED) NC Ni of varying grain sizes: 20 nm, 50 nm, and 100 nm. In-situ high-temperature X-ray diffraction (XRD) investigations have been employed to follow the grain growth behavior of (111) and (200) grains under isochronal annealing up to 550 ?. Based on the calculated activation energies, the anisotropic growth mechanisms of (111) and (200) grains are evaluated. The studies revealed that the onset of grain growth occurred at progressively higher temperatures with increasing initial grain sizes. While near uniform grain size distribution is observed for the 20 nm grain size variant, bimodal distributions are observed for its counterparts. Electron Backscatter Diffraction studies of NC Ni samples in the final-processing temperature of XRD showed that < 100 >//ED texture is preserved during the annealing process. While high fractions of sigma 3 coincidence site lattice (CSL) boundaries are observed in the 20 nm sample, reduced fractions with no particular trend is observed for the 50 nm and 100 nm grain-sized samples. It is shown here that the incomplete recrystallization arising from inhomogenous strain distribution is the reason for the observed bimodal distribution and deviation from an expected trend in the formation of sigma 3 CSL as a function of initial grain size.