Mechanical Properties Evolution of γ′/γ" Nickel-Base Superalloys During Long-Term Thermal Over-Aging

The long-term stability of Inconel 718 and Waspaloy forgings in the 600 degrees C to 850 degrees C temperature range and up to 10,000 hours was studied by means of mechanical tests and microstructural analyses. Hardness and tensile properties were found to decrease with increasing over-aging time and temperature. The comparative analysis of tensile results of both alloys indicates that they exhibit an equivalent stability over time if the over-aging temperature of Waspaloy is increased by 50 degrees C to 100 degrees C compared to Inconel 718. No detrimental intermetallic precipitation at grain boundaries has been detected in both alloys. Microstructure characterizations performed by scanning electron microscopy indicate that the higher stability of Waspaloy can be attributed to the slower kinetics of coarsening and dissolution of gamma' precipitates with respect to gamma('') precipitates that account for the majority of hardening particles in Inconel 718. Additionally, fatigue results indicate that in both alloys over-aging promotes surface crack initiation, thus leading to shorter fatigue life.