The effect of N₂ and CO₂ environments on the fatigue failure of MAR-M200 + Hf superalloy at 975°C

High temperature, low cycle fatigue (HTLCF) failure mechanisms were studied in unidirectionally solidified MAR-M200 + Hf nickel-based superalloy in pure N₂ and CO₂ environments at 975°C. The loading conditions were constant and consisted of creep tension and plastic compression according to the cp mode of the strain-range-partitioning method. The crack initiation and propagation were investigated using scanning electron microscopy, X-ray diffraction and Auger electron spectroscopy. The results obtained indicated that N₂ and CO₂ atmospheres cannot be treated as passive gaseous environments. On the contrary, these environments have a significant influence on the HTLCF crack growth behaviour. In an N₂ environment the fracture was transgranular. This was a result of an intensive chemical interaction between nitrogen and the main alloying elements making up the γ′ phase Ni₃(Al, Ti) located inside the dendritic grain regions. In a CO₂ atmosphere a partial catalytic decomposition of this environment into its basic components oxygen and carbon was obtained. The fracture was intergranular owing to the preferential chemical interaction of oxygen with elements of the interdendritic microsegregation zones. In both CO₂ and N₂ environments a uniphase layer was formed ahead of the propagating crack tip. This layer was produced via internal oxidation or nitridation caused by interstitial penetration of oxygen or nitrogen respectively. It was evident that the nature of formation and the properties of the uniphase layer exert a decisive effect on the HTLCF crack growth characteristics, which enables identification of the mechanism of failure under the various tested environments.