Behaviour of single crystal superalloys under cyclic loading at high temperatures

The Low-Cycle-Fatigue (LCF) behaviour of single crystal superalloys developed as blade materials for the use in aircraft engines was investigated. Interrupted LCF-tests at 980 degrees C were performed. Special attention was given to the influence of hold periods at either the maximum (tension phase) or the minimum (compression phase) strain level. After a certain number of cycles the specimen were extensively examined by SEM, to quantify the evolution of creep/fatigue damage on the surface. Furthermore metallographical investigations were carried out to determine and to quantify the evolution of the gamma/gamma'-microstructure. It was revealed that the formation of a complex Al-containing oxide layer after a few cycles depletes a surface near region, thus leading to a gamma'-free zone. Both, the layer and the gamma'-free zone grew steadily, but at homogeneously distributed sites on the specimen surface, the formation of nickel oxide speeds up. At these sites protrusions are observed, under which small cracks perpendicular to the specimen axis are hidden. The initial stages of damage under LCF loading with hold periods are indentical, but after the formation of the oxide layer the crack growth behaviour changes. With hold periods in the compressive phase, the surface cracks stop growing laterally and sharp inclined cracks are generated at their extremities, some of these grow more rapidly and lead to rupture. With hold time in the tensile phase, cracks originating from bulk pores grow and become predominant. Regarding the number of cycles to failure, hold periods in compression phase are more damaging than hold periods in the tension phase.