Thermomechanical and isothermal fatigue behavior of bare and coated superalloys

Thermomechanical fatigue (TMF) and isothermal fatigue (IF) experiments were performed on bare Mar-M246 as well as bare and coated Mar-M247 nickel based superalloys at strain ranges from 0.335 to 1 percent. The experiments were conducted in air, through a temperature range of 500 to 1038 degrees C at a constant strain rate of 5.0 x 10(-5) s(-1). Due to the coarse-grained structure of Mar-M246, TMF lives scattered considerably under low strain range conditions. Electron microscopy studies show that significant surface oxidation and gamma prime, gamma', depleted zones occur for Mar-M246 under these conditions. In the surface grain, where the change in gamma' morphology is most pronounced gamma' rafted along axes approximate to +/-45 deg from the loading axis. The influence of a protective coating on the IF and TMF lives of Mar-M247 was also examined. Results indicate that the coating does nor significantly affect the fatigue lives of Mar-M247. The oxidation behavior of bare and coated Mar-M247 was investigated via X-ray microprobe and Auger Spectroscopy. Through these analyses, it is evident that the coating was completely degraded when exposed to the test environment for a long time. In both Mar-M246 and Mar-M247, in-phase thermomechanical fatigue (TMF IF) loading resulted in intergranular cracking. TMF IP loading promoted multiple cracks at the coating/substrate interface of coated Mar-M247. Finally, Eshelby techniques were used to calculate stress distributions in the vicinity of a surface oxide and second-phase particle at the coating/substrate interface.