Analysis and modelling of the creep-fatigue behaviour of oxide dispersion strengthened superalloys

Abstract

The creep-fatigue behaviour of two representative, commercial ODS (Oxide dispersion-Strengthened) Superalloys, Inconel MA 754 and MA 6000, both of which are produced by mechanical alloying, has been investigated. The mechanisms leading to failure are identified and discussed with particular emphasis on creep-fatigue interaction phenomena. Fine-grain regions are found to tbe the primary source of crack-initiation by the formation of creep damage. A finite difference approach to modelling of these damage processes is introduced. Results of the model are compared with experimentally determined crack initiation life. In order to explain waveform effects it is suggested that, in analogy with prestraining experiments, rapid plastic deformation can introduce residual stresses in the neighbourhood of hard particlds such as oxide dispersoids during asymmetric creep-fatigue ccycling. Subsequent creep damage rates are thus strongly influenced by the magnitude, sign and rate of prior plastic straining.