On the creep behaviour of grain boundary engineered nickel

Grain boundaries described by low-Sigma CSL relationships (i.e. Sigma less than or equal to 29) have previously been shown to be resistant to grain boundary sliding, cavitation and fracture. The present work reports on efforts to reduce creep rates in conventional polycrystalline nickel by increasing the frequency with which these 'special' interfaces occur in the microstructure. Suitable thermomechanical processing was employed to enhance the frequency of 'special' grain boundaries (Sigma less than or equal to 29) in 99.99% Ni from 13 to 66%, resulting mostly from the formation of twins (Sigma 3) and crystallographically-related Sigma 9 and Sigma 27 boundaries. This 53% increase in the fraction of low-Sigma boundaries produced reductions of 16-fold in the steady-state creep rate and six-fold in the primary creep strain. Microstructures having 'special' boundary frequencies of less than 50% exhibited significant cavitation almost exclusively along 'random' boundaries (i.e. Sigma > 29) at or near triple points. No gross cavitation was evident in microstructures containing 'special' boundary fractions of 66%. Such improvements in creep properties provide considerable promise for the application of a 'grain boundary engineering' approach to developing interfacial materials for structural applications. (C) 1997 Elsevier Science S.A.