EFFECTS OF GRAIN-BOUNDARY CARBIDES ON THE CREEP AND BACK STRESS OF A NICKEL-BASE SUPER-ALLOY

The beneficial effect of grain boundary carbides on high temperature alloy creep resistance was investigated for a nickel-base superalloy in which carbides were selectively dissolved or precipitated along grain boundaries. The creep data obtained are analyzed both in a classical manner for apparent temperature and stress sensitivities of steady state creep and according to the more recent average creep back stress concept. In terms of the more traditional apparent parameters the ability of grain boundary carbides to limit grain boundary sliding is reflected mainly in the change in the apparent stress exponent from a low value indicative of grain boundary sliding to a high value reflecting predominantly intragranular creep. Recent analyses of intragranular creep in terms of effective stress, which is simply applied stress minus an opposing average back stress, are also tested for applicability to grain-boundary-related creep phenomena. The presence of carbides along grain boundaries is found to increase this opposing back stress, again indicating that grain boundary sliding in this alloy system is either hindered or completely prevented by the presence of these precipitates. © 1979.