Environmental Effects on LCF Failure of Unidirectionally Solidified MAR-M₂₀₀+Hf Alloy at 975°C

Unidirectional solidified MAR-M200+Hf nickel base superalloy failed under low-cycle-fatigue loading regime at 975°C. The cycle comprised creep by tension, and plasticity compression loadings. Different environmental gas influence were studied: oxygen (20% in argon), nitrogen (pure) and carbon-dioxide (pure). The crack initiation and propagation under the influence of the different environment was studied by scanning electron microscopy (Incl. chemical microprobe analysis). Auger spectroscopy, and X-ray diffraction. It was found that the chemical reaction between the gas and the alloy at the propagating crack tip controls the LCF failure. Under oxygen, ahead of the crack tip uniphase layer develops, composed of γ′-phase material, internally oxidised (oxide-type ligands develop with active elements atoms within otherwise metallic solid-solution), loosely packed oxide layers develop later. Under CO2 layers of oxycarbides and graphite-like develop. In both cases crack propagates through inter dendritic segregation zone. Under nitrogen propagation is transdendritic with coarse nitrides develop ahead of cracktip.