The crack propagation theory under rolling contact

Abstract

The effect of moisture on the high temperature rupture behavior of an Ni-base superalloy and an FeNiCo superalloy was investigated. The high temperature life and ductility of Inconel alloy 718 and Incoloy alloy 909 is severely reduced when specimens are exposed to moist air. The major role of the moist air is to accelerate the kinetics of the grain boundary embrittlement by oxygen. For alloy 718 it was found that heat treatments which produce fully recrystallized microstructures also lead to lifetime and ductility reduction in laboratory air environments. In both alloys the reduction in lifetime and ductility is accompanied by a transition from a transgranular ductile fracture where failure occurs in the smooth gage section to a brittle intergranular fracture which occurs in the notch of the specimens. The transition in fracture mode is attributed to the diffusion of atomic oxygen along the grain boundary resulting in a reduction in the grain boundary surface energy. This allows for intergranular fracture to be favored over transgranular fracture within the present rupture stress-temperature regime. The brittle intergranular fracture is described quantitatively using the principal facet stress approach as opposed to the SAGBO mechanism previously thought responsible. In accordance with the principal facet stress approach the presence of atomic oxygen reduces the frictional forces on the grain boundaries inclined to the applied stress direction. The reduction in frictional force is suggested to occur via the oxygen increasing the grain boundary diffusivity. This then allows for extensive grain boundary sliding on the inclined boundaries which redistributes the stresses to the grain boundary triple junctions. The high stresses acting at the triple junctions give rise to accelerated cavity nucleation and growth causing eventual failure of the specimen. Values for the principal facet stress are calculated and it is shown that this is a viable approach for the present rupture conditions. The unrecrystallized microstructure of alloy 718 is found to exhibit an increased resistance to grain boundary embrittlement by oxygen in moist air. Microstructural variables which may contribute to the increased resistance of uncrystallized microstructures to grain boundary oxygen embrittlement are discussed.