Effect of annealing twins on crack initiation under high cycle fatigue conditions

Several experimental studies have shown that fatigue cracks in Ni-based superalloys preferentially form in the vicinity of favorably oriented annealing twin boundaries. However, this increase in the probability of crack nucleation has not been quantified in detail since 3 material modeling has not typically considered twins. The present work employs finite element simulations and a continuum crystal plasticity constitutive model for RR1000 Ni-based superalloy to assess the potential enhancement of nonlocal fatigue indicator parameters (FIPs) incurred by the presence of I 3 pound twins. Two cases are analyzed: single crystals with twins of different widths subjected to single slip cyclic shear deformation aligned with the twin plane and polycrystals with 20 % of the grains half-twinned. Simulations with and without twins are compared in both cases. The results demonstrate the detrimental effect of twins in shifting the upper tail of the FIP distribution to higher values; this shift is more pronounced with an increase of twin thickness.