Factors influencing dwell fatigue life in notches of a powder metallurgy superalloy

The influences of heat treatment and cyclic dwells on the notch fatigue resistance of powder metallurgy disk superalloys were investigated for the LSHR disk superalloy. Disks were processed to produce material conditions with varied microstructures and associated mechanical properties. Notched specimens were subjected to fatigue cycles having a dwell at maximum load, as well as tensile, stress relaxation, creep rupture, and dwell fatigue crack growth tests at 704 °C. Two material conditions displayed a wide distribution of fatigue lives, with low lives reduced by two orders of magnitude, while others had more consistent fatigue lives. The occurrence of these very low lives was compared to other mechanical properties, in search of correlations. The low life failures for dwell fatigue life were correlated with material conditions giving highest ultimate tensile strengths and stress relaxation resistance. These results were analyzed by modeling relaxation of peak stresses in the notch with continued dwell fatigue cycling.