Environmental, Stress-State and Section-Size Synergisms During Creep

The time-dependent deformation and fracture characteristics of Inconel alloy X-750 have been studied at 700 ^circC under uniaxial and biaxial stressing. Several unusual sets of results were obtained, which led to the discovery of synergistic interactions between the material and the environment employed during heat treatment. For example, when solid tension and thin-wall tubular torsion specimens were given the normal commercial heat treatment, lifetimes in torsion were found to lie outside the region delineated by the plane-stress isochronous rupture loci generated when fracture is controlled respectively by the maximum principal stress and the von-Mises effective stress. This is contrary to predictions of creep fracture models and at variance with published experimental work. The isochronous rupture behaviour only became `normal' when torsion life-times were compared with those obtained from tension tests on tubular specimens for which lifetimes were less than 5 % of those for solid ones. This unusually large effect of section size on lifetime was shown to be due to a corresponding change in creep resistance, rather than to a reduced fracture ductility. Detailed metallography identified the responsible micromechanism as being a very fine cavity dispersion in the near-surface region, which caused progressively more weakening as the section size was reduced. This profuse and spatially inhomogeneous cavitation was demonstrated as not being an inherent characteristic of the material but the result of some interaction with the environment-speculated as being the formation of metastable carbon dioxide gas bubbles - during heat treatment of testpieces.