Evolution of fatigue damage and dislocation structures in micron-sized nickel single crystals under low cyclic loading

Abstract

In single crystal metals, the threshold for fatigue crack initiation generally exceeds the critical resolved shear stress (τCRSS) determined from unidirectional loading tests. In contrast, at the micron-scale, it has been reported that fatigue cracks initiate at stress amplitudes below the τCRSS, and this reversal phenomenon might be attributed to differences in testing methodologies employed in various research groups. In this study, tensile tests and tension-compression cyclic loading tests were performed on 2 μm-wide nickel (Ni) single crystals using the same experimental setup. The τCRSS obtained from the tensile tests was approximately 20 times higher than that of the bulk counterpart. During the initial stages of cyclic loading at a stress amplitude approximately half of the τCRSS, the dislocation density increased progressively with the number of cycles without accompanying macroscopic plastic deformation, indicating that τCRSS provides a threshold at which dislocations multiply rapidly over wide regions under unidirectional loading, and that dislocations move at lower stresses and can multiply under cyclic loading. Subsequently, localized slip and crack-like intrusion formed on the surface in association with development of self-organized ladder-like dislocation structures. This sequence of processes was comparable to the early stage of fatigue crack initiation observed in bulk Ni. Unlike bulk materials, ladder-like dislocation structures and intrusion/extrusion appeared without the formation of veins, which are stable self-organized dislocation structures. The findings of this study represent a significant advancement in our understanding of the fatigue behavior of micron-sized single crystal metals