HIGH TEMPERATURE LOW CYCLE FATIGUE OF NICKEL BASE SUPERALLOYS RENE' 95 AND INCONEL 718

Nickel-base superalloys Rene 95, in the As-HIP and the HIP + Forged forms, and Inconel 718, in forged form, were studied for their low cycle fatigue behavior up to 649(DEGREES)C (1200(DEGREES)F).; In the first phase of this study, continuous cycling and hold time LCF specimens tested at 649(DEGREES)C (1200(DEGREES)F) were analyzed for both Rene 95 (in As-HIP and HIP + Forged forms) and In 718. For the continuously cycled As-HIP Rene 95 cracks initiated at surface pores in all cases. Deformation was planar at low strains and became homogeneous as the strain level increased. The hold time specimens exhibited an extremely high dislocation density and surface connected initiation yet, without a significant life reduction. The observations were essentially similar for the HIP + Forged material except that deformation was planar even at high strains. At a unique combination of crack length and plastic strain, crack propagation changed from transgranular to intergranular. Environment played a major role in determining the fatigue life of Rene 95. Results of the continuous and hold time tests on In 718 showed that the LCF life was dractically reduced by a 15-minute dwell cycle. Cracks initiated crystallographically and propagation was intergranular. Microcracks formed at the grain boundary (delta) phase. Deformation was always along slip bands.; In the second phase of the study, three heat treatments were used for the As-HIP Rene 95 to obtain different grain size and (gamma)' size. Microstructure with the smallest grain size (5-10(mu)) and largest (gamma)' size (0.5(mu)) showed best LCF life over the temperature range of 25 to 649(DEGREES)C (70 to 1200(DEGREES)F). Cracks started by a classical Stage I mechanism at low temperatures, and deformation was found to occur along definite slip bands, with a precipitate shearing mechanism. At high temperatures, cracks always initiated at surface pores and deformation was homogeneous. Better LCF behavior was attributed to the combination of finer grain size and larger precipitate size. The results of this study showed the low cycle fatigue behavior is affected by the combination of deformation mechanism and environment and that significant improvements can be made in the LCF life of Rene 95 by controlling the microstructure.