Micromechanisms of fatigue crack growth at grain boundaries in coarse-grained nickel

Abstract

1. External brittle intergranular failure of coarse-crystalline nickel in cyclic loading was not spontaneous and took place by the gradual cumulation of plastic strain along the grain boundaries to the critical level. Fatigue crack growth then started above this level. 2. Several mechanisms of FCG were superimposed. For example, the crack propagated simultaneously with intergranular macrobrittle failure and formation, on the fracture surface, of ductile fatigue striations or other relief elements. 3. The fatigue striations detected on the intergranular fracture surface at almost all levels of Δk indicated that FCG was of cyclic nature. In approach to the third section of KDFF, the fatigue striations grouped into blocks. 4. Correlation between the macro-and microrates of FCG measured on the block surface of the specimen and on the basis of the spacing of the fatigue striations was observed approximately in the second section of KDFF. Correlation between the macrorate and the spacing of the blocks of the striations was observed at the higher loads. 5. With the increase of the fraction of the shear component of the displacements at the crack tip, the tendency to the formation of ductile separation striations became less marked and shear striations of brittle appearance separated from each other by the extension of regular bands of intrusions and extrusions to the intergranular fracture surface formed. Reversed slip took place in the direction perpendicular to the plane of FCG.