The combined and interactive effects of orientation, strain amplitude, cycle number, stacking fault energy and hydrogen doping on microstructure evolution of polycrystalline high-manganese steels under low-cycle fatigue

We studied the combined and interactive effects of crystallographic orientation, strain amplitude, cycle number, stacking fault energy (SFE) and hydrogen doping on the microstructure evolution of polycrystalline high-manganese steels (HMnSs) under low-cycle fatigue (LCF). An integrated experimental approach combining digital image correlation (DIC), electron backscatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) at interrupted cycles was performed in the same region of interest on the bulk shear samples, which enables us to systematically compare the dislocation patterns of grains with defined loading conditions at a much larger field of view and less artefacts compared to transmission electron microscopy (TEM). We found that Taylor factor (M) works well with describing the effect of crystallographic orientation, which was further proved by the crystal plasticity finite element method (CPFEM). In detail, grains with a medium M value (3.3-3.9) tend to form more complex dislocation pattern, which is defined as sensitive value. Generally, increasing strain amplitude and cycle number both promote the evolution of dislocation pattern while their efficiencies depend strongly on grain orientation. The promotion effect of SFE on dislocation evolution becomes obvious at larger strain amplitudes (>0.4%) and non-sensitive M value. Hydrogen can strongly assist the formation of epsilon-martensite and reduce its critical resolved shear stress (CRSS), while it retards the evolution of dislocation pattern. The number of activated martensite variants in individual grain can be well predicted by its M value. With increasing strain amplitude, the fraction of epsilon-martensite increases in a manner of thinner but denser plates.