Effects of strain amplitude, cycle number and orientation on low cycle fatigue microstructures in austenitic stainless steel and aluminum

The combined influence of grain orientation, cycle number and local strain amplitude on the dislocation structures during low cycle fatigue of austenitic stainless steel (AISI 316L) and aluminum single- and bi-crystals is studied. To this aim a novel approach which combines electron backscatter diffraction, digital image correlation and electron channeling contrast imaging is applied. This approach enables to systematically probe a large matrix of different parameters. Polycrystalline steel samples with three different double-slip orientations (〈1 ̅12〉 // LD, 〈1 ̅22〉 // LD, 〈012〉 // LD) and three different multiple-slip orientations (〈1 ̅11〉 // LD, 〈011〉 // LD, 〈001〉 // LD) are cyclically deformed for 5, 30, 50 and 100 cycles. The dislocation structures are investigated for three local strain amplitudes of 0.35%, 0.65% and 0.95%. Three double-slip oriented aluminum single crystals (〈2 ̅32〉 // LD, 〈3 ̅3 ̅2〉 // LD, 〈130〉 // LD) are cycled for 50 and 100 cycles with a constant strain amplitude of 0.3-0.4%. The dislocation structures formed after 50 cycles in two aluminum bi-crystals are studied. It is observed that the local strain amplitude differs significantly from the average global strain amplitude in polycrystalline stainless steel samples, having a high influence on the formation of dislocation structures. A significant orientation-dependence of the dislocation structure evolution in cyclically deformed polycrystalline stainless steel samples is observed. Aluminum single- and bi-crystals form dislocation cell structures during LCF. Analysis of dislocation structures formed in cyclically deformed Al bi-crystals reveals that the respective grain orientations are of higher importance for dislocation structure formation than the grain boundary character.