Damage Behaviors at Twin and Grain Boundary in Alloy 690 Material in Very High Cycle Fatigue Regime

Recent results show that fatigue crack initiation in very high cycle fatigue regime can occur at a non-defect origin in the matrix. The mechanism is not well understood. This paper provides a study on the influence of cyclic loading on the damage behavior at grain and twin boundaries in Alloy 690 material. The results show that the strains in the fatigue-tested specimen were highly localized, which were mainly caused by the dislocation accumulation in the grains with high Schmid factors during each small cyclic loading. This has led to the formation of local fine grain area consisting of numbers of new twins. The study also shows that the impingement between slip bands and grain or twin boundary is one of the main fatigue damage mechanisms. The results in this paper indicate that the role of a twin or grain boundary to block dislocation slip transmission depends on crystal orientation, Schmid factor and boundary orientations.