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 'imp ure' and 'pure' respectively, by isochronal annealing at temperatures 0.64T(m), 0.68T(m), 0.72T(m), 0.76T(m). Both grain size measurements a nd grain orientation measurements, which allowed grain boundary parame ters to be calculated, were made. Extensive anomalous grain growth was observed in the pure specimen annealed at 0.68T(m). The most signific ant results related to the evolution of the twin (Sigma = 3) fraction with annealing temperature, which was evident from both the microtextu re and the proportions of Sigma = 3 and Sigma = 3 related boundaries i n each dataset. Twinning was influenced markedly by the specimen purit y, from which it was inferred that small levels of impurities hinder t he mechanism of twin nucleation in the wake of a migrating boundary. F urthermore, a fast boundary migration rate does not nucleate twins so readily as a slower migration rate. It is proposed that this is becaus e twinning is of minor importance as an energy reduction mechanism for which reduction in grain boundary area, i.e. fast gain growth, is str ongly operative.