Factors controlling creep strength of cast Ni–Cr-base alloys

Abstract

The effects of microstructural variations arising from the casting conditions on the creep performance of alloy IN939 have been assessed in order to determine the structural features having the greatest influence on creep properties. The results of creep tests at 850°C show that while grain size has an effect, the dendrite spacing is a more significant parameter in influencing performance. In particular, the life-to-rupture was greatest in specimens with fine dendrites as a consequence of the improved ductility with this type of structure. Thus the optimum combination of creep properties was obtained for material with fairly coarse grain size (~1mm) and fine dendrites (~20 μm). The structural changes associated with the various stages of the commercial heat treatment have been determined by optical metallography and electron microscopy of thin foils, and evidence has been found of dislocation bowing and shearing of γ′ particles in fractured creep specimens. The creep results are discussed in the context of the microstructural changes observed and the relevance of mechanisms of deformation involving dislocation–particle interactions.

MST/537