Creep-Fatigue Failure in High Temperature Alloys

Abstract

Components operating at elevated temperature must withstand complex strain-time patterns which include cyclic deformation (fatigue) and steady state deformation (creep) and combinations of the two processes leading to a creep-fatigue interaction. In addition to the imposed deformation extended exposure to temperature can result in changes in material metallurgical structure by thermal ageing processes, together with the oxidation of freshly exposed surfaces.

Over recent years studies of the behaviour of high temperature alloys e.g. austenitic stainless steels, ferritic steels and nickel based alloys have indicated that a creep-fatigue interaction can significantly reduce fatigue life in certain circumstances and not in others. This has led to some confusion regarding the failure process and hence to difficulty in formulating life prediction methods.

Fatigue failure is essentially by a time independent process and takes place by the rapid nucleation and subsequent controlled propagation of a surface crack. Propagation is controlled to a large extent by continuum plasticity effects ahead of the advancing crack. Creep failure on the other hand is a time dependent process and occurs by the nucleation and growth of internal grain boundary cracks or cavities as a result of grain boundary sliding. The mechanism of creep-fatigue failure will therefore be dictated by the dominant fracture mode. This paper develops previously formulated models of fatigue crack propagation to take account of the influence of creep fracture damage and attempts to rationalise the creep-fatigue response of the type 300 series austenitic stainless steels and ferritic steels based on a mechanistic understanding of the prevailing failure process.