Analysis of dislocation microstructure characteristics of surface grains under cyclic loading by discrete dislocation dynamics

The dislocation structure formation under low-amplitude fatigue in fcc metals for multislip loading conditions is investigated using three-dimensional discrete dislocation dynamics. Tools based on graph analysis, a statistical description of stable dislocation arrangements such as dislocation dipoles and prismatic loops are developed and applied. Upon decreasing the loading amplitude one order of magnitude below the persistent slip band threshold, although qualitative microstructural differences are seen, the elementary features of the investigated defects are the same. A critical number of cycles is required to produce sessile Lomer junctions that stabilize the structure and result in dislocation clustering around them. The crystallographic orientation of the crystal with respect to the loading axis results in different patterns strongly linked to sessile junctions, which are analyzed using spatial correlation functions. The increase in irreversible bulk dislocation arrangements results in roughening of the free surface and increase in surface step heights. Furthermore the crystallographic orientation with respect to the free surface is shown to control the dislocation density evolution combined with the macroscopic Schmid factor.