Inability of precipitation-hardening to improve the fatigue limit of Ni-based superalloy 718 through a perspective of shear-mode cracking threshold

Fatigue strength of precipitation-hardened materials is well known to be lower than that expected from their high yield and tensile strengths. To elucidate the reasons behind such anomaly, torsional fatigue tests, in addition to uniaxial fatigue tests, were conducted on samples of the Ni-based superalloy 718, subjected to two different heat treatments (solution-annealing and precipitation-hardening). Despite the significant differences in tensile and yield strengths between the two samples, the torsional fatigue strengths were mutually equivalent. This result was interpreted in light of the resistance against fatigue crack-initiation and -propagation in shear-mode, which cannot be enhanced by precipitation-hardening due to the depletion of precipitates by persistent dislocations motion within localized glide-bands.