Single crystal superalloys: The transition from primary to secondary creep

Abstract

In this paper we compare the effect of stress, orientation and alloy composition on primary and secondary creep rate in a number of single crystal alloys. The activation energies and stress dependence of primary and secondary creep, measured in the alloy CMSX-4, show a distinctive low activation energy associated with stacking fault shear during primary creep which disappears at low stress. Two-stage creep tests show that a fully developed dislocation network, introduced into the microstructure by limited deformation at 950°C, results in a substantial drop in primary creep strain during subsequent testing at 750°C. Comparison with identically-oriented samples without prestrain confirms the role of dislocation networks in the γ channels as the major hardening mechanism and shows that secondary creep is unaffected by the pre-strain. These observations are inconsistent with existing understanding of the transition between primary and secondary creep in some important respects. We argue that the mechanisms producing primary and secondary creep strain are similar, but that the secondary creep rate is determined by the rate of formation of defects at the γ/γ' interface. This enables many aspects of the dependence of creep rate on orientation, alloy composition and micro-structure to be rationalised.