High-temperature low-cycle fatigue of a nickel-based MAR-M200 + Hf alloy in AR and AR + 20% O₂ environment

A systematic study was made on the environmental influences of inert gas and oxygen on the low-cycle fatigue failure of MAR-M200 + Hf unidirectionally solidified nickel-based alloy at 975 ‡C. The cyclic load was constant and comprised creep tension and plastic compression. Under an inert argon atmosphere, a pure transgranular fracture free of environmental influences was obtained. Under an Ar + 20% O₂ atmosphere the fracture was intergranular along a path through the interdendritic microsegregation zone. In the area close to the propagating crack tip, a uniphase layer was detected, resulting from a process of internal oxidation which developed a preferred oxide-type bonding with the elements making up the principle precipitating phase, γ′, namely titanium and aluminium. The internal oxidation led to changes in the boundary layer between the matrix and γ′ phase, as a result of which the original coherence between them was lost. In order to reduce the surface energy between the precipitate and the matrix, a process of growth and coarsening of γ′ phase took place in these conditions. The internal oxidation and the formation of the uniphase layer increased the brittleness over a comparatively wide area adjacent to the crack tip. As a result, crack branching and blunting took place during propagation in the oxygen-containing atmosphere. The branching and the blunting were instrumental in lengthening the active life of the alloy in the oxygen-containing atmosphere compared with that in the inert atmosphere.