The effect of cavitation and microstructural damage on the intergranular creep fracture of nickel-base superalloys

Abstract

The creep behaviour of nickel-base superalloys in unusual in the sense that they rarely exhibit a well-established steady state creep regime. Rather, the creep rate progressively increases from a minimum value. The early onset of the tertiary regime can be explained either by assuming that the time-dependent coarsening of the γ' precipitates reduces the back stress and thus increases the creep rate or through the concept of strain-induced loss of coherency at the γ-γ' interface and the resulting higher rates of recovery (and hence creep) of the matrix dislocations. In this paper the combined effect of precipitate coarsening, strain-induced loss of coherency and cavitation (which is inevitable) on intergranular creep fracture is considered. In particular, the effect of cavitation and microstructural damage mechanisms on time and strain to fracture and the Monkman-Grant constant is analysed in detail. It is shown that both the time to fracture and the Monkman- Grant constant are reduced significantly in the presence of various forms of damage. In contrast, the strain to fracture is largely unaffected by these damage mechanisms.