Aspects of twinning and grain growth in high purity and commercially pure nickel

Grain growth has been studied in 99.5% and 99.999% nickel, termed ‘impure’ and ‘pure’ respectively, by isochronal annealing at temperatures 0.64Tm, 0.68Tm, 0.72Tm, 0.76Tm. Both grain size measurements and grain orientation measurements, which allowed grain boundary parameters to be calculated, were made. Extensive anomalous grain growth was observed in the pure specimen annealed at 0.68Tm. The most significant results related to the evolution of the twin (Σ = 3) fraction with annealing temperature, which was evident from both the microtexture and the proportions of Σ = 3 and Σ = 3 related boundaries in each dataset. Twinning was influenced markedly by the specimen purity, from which it was inferred that small levels of impurities hinder the mechanism of twin nucleation in the wake of a migrating boundary. Furthermore, a fast boundary migration rate does not nucleate twins so readily as a slower migration rate. It is proposed that this is because twinning is of minor importance as an energy reduction mechanism for which reduction in grain boundary area, i.e. fast grain growth, is strongly operative.