Retardation and Acceleration of Dwell-Fatigue Crack Propagation in Ni-Base Superalloys: Experimental and Numerical Investigations on CMSX-4 and IN718

Effect of tensile dwell on crack propagationCrack propagation under subsequent fatigueFatigue loading during dwell-fatigueDwell-fatigue crack propagationCrack propagation in Ni-base superalloysSingle crystal Ni-base superalloy was investigated using a single crystal superalloySingle crystal superalloy, CMSX-4CMSX-4, and a wroughtWrought superalloySuperalloys, IN718. Crack propagationCrack propagation tests with single tensile dwell introduced during pure fatigueFatigue loading under various conditions of stress intensity and dwell time were conducted at 900 °C and 650 °C for CMSX-4 and IN718, respectively. In CMSX-4CMSX-4, fatigueFatigue crack retardation occurred after the tensile dwell, which was attributed to stress relaxationStress relaxation induced by creepCreep deformation during the dwell and the resultant residual compressive stress during the subsequent fatigueFatigue loading. FatigueFatigue crack propagationCrack propagation rate after the tensile dwell was quantified by evaluating effective stress intensity factors based on the residual compressive stress field obtained by finite element analysisFinite element analysis. In IN718, acceleration of the fatigueFatigue crack propagationCrack propagation occurred after the tensile dwell at high Kmax values whereas the retardation occurred following temporary acceleration after the dwell at low Kmax values. The acceleration was attributed to grain boundaryGrain boundary (GB) damage caused by oxygen diffusionDiffusion along the GBs induced by high stress near the crack tip during the tensile dwell. The transition from the crack retardation to the acceleration in IN718 was rationally explained based on a size relationship between the stress relaxationStress relaxation area and the GB damage zone around the crack tip caused by the tensile dwell.