In this paper, the low-cycle fatigue life and mechanisms governing the fracture behaviour of coated nickel-base superalloys are presented and discussed. Cylindrical solid specimens were cyclically deformed with fully reversed tension-compression loading total strain amplitude control at two elevated temperatures and a constant strain rate of 10-4 s-1 (6%/ min) in air atmosphere without any dwell time. Three nickel-base superalloys, IN792, CMSX-4 and SCB, were coated with three different coatings: an overlay coating AMDRY997, a diffusion coating RT22 and an innovative coating ICl. The cyclic stress response, low-cycle fatigue (LCF) life and final fracture behaviour at the two temperatures are observed and compared.
At 500oC the coatings reduced fatigue life relative to the uncoated specimens while at 900oC the coated specimens showed longer life at all strain ranges than the uncoated specimens except RT22 under certain test conditions. The decrease in the fatigue life was caused by brittle coating cracking under their ductile to brittle transition temperature (DBTT). Over DBTT, lower yield strength of the coated superalloys with subsequent increase in ductility could cause the improvement of the fatigue life. These cracks could be also slowed by oxidation on front of the crack tip.
All uncoated and coated superalloys exhibit hardening and higher stress levels at higher applied strain amplitudes and at 500°C. At 900oC softening occurred together with lower stress response level. The coatings lowered the stress level response of the superalloys from about 12% to 31 %. Higher hardening was observed for polycrystalline IN792 caused by dislocation pileups at the the grain boundaries.
Most of the observed cracks initiated at the coating surface and majority was arrested in the transition zone except for IN792 where internal pores served as initiation sites for most cracks. Some improvement in the fatigue life have also been seen in coated IN792. No cracks found initiated from TCP phases were found. Cracks initially grew more or less perpendicular to the load axis in Stage II manner. Crack propagation path in IN792 is governed by grain or dendrite boundaries while in single crystals crack growth path is determined by concentration of deformation and damage in γ and γ' phases. Surface roughness or rumpling was found in the overlay coating AMDRY997 with some cracks initiated from the rumples maybe due to cyclic straining and not thermal cycling.