Deformation mechanism of high-Mn austenitic steel at extremely low temperatures

This study elucidates the exceptional low-temperature impact toughness and atomic-scale deformation mechanisms during Charpy impact testing of Fe-24.5Mn-0.42C-3.15Cr (wt%) high-Mn austenitic steel at extreme temperatures (0 °C to -269 °C). Charpy impact testing reveals outstanding toughness, with values of 116 J at liquid nitrogen temperature (-196 °C) and 80 J at liquid helium temperature (-269 °C), accompanied by ductile fracture morphology. The impact fracture morphology and microstructures near fracture were characterized using scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD) techniques. Particular emphasis was placed on analyzing the deformation mechanisms at the atomic scale in the experimental steel at liquid nitrogen and liquid hydrogen temperatures. Above -196 °C, deformation twins nucleate at grain boundaries via Shockley partial dislocation emission, with peak twin density (1.4 μm -1 ) at -165 °C enhancing toughness through twinning. At -196 °C, intersecting twins undergo path deflection without orientation change, while stress concentration at intersections triggers ε-martensite transformation. At -269 °C, suppressed dislocation glide promotes extensive stacking faults and nanoscale ε-martensite. Twinning, stacking faults, and ε-martensite phase transition synergistically sustain cryogenic toughness: twin boundaries impede crack propagation, while stacking fault-induced phase transformation accommodates plastic strain. These atomic-scale mechanisms establish a deformation framework for next-generation cryogenic structural materials.