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Tensile and high cycle fatigue behaviors of high-Mn austenitic steels, including 25Mn, 25Mn0.2Al, 25Mn0.5Cu, 24Mn4Cr, 22Mn3Cr and 16Mn2Al specimens, were investigated at 298 and 110 K. Depending on the alloying elements, tensile ductility of high-Mn steels either increased or decreased with decreasing temperature from 298 to 110 K. Reasonable correlation between the tendency for martensitic tranformation, the critical twinning stress and the percent change in tensile elongation suggested that tensile deformation of high-Mn steels was strongly influenced by SFE determining TRIP and TWIP effects. Tensile strength was the most important parameter in determining the resistance to high cycle fatigue of high-Mn steels with an exceptional work hardening capability at room and cryogenic temperatures. The fatigue crack nucleation mechanism in high-Mn steels did not vary with decreasing tempertature, except Cr-added specimens with grain boundary cracking at 298 K and slip band cracking at 110 K. The EBSD (electron backscatter diffraction) analyses suggested that the deformation mechanism under fatigue loading was significantly different from tensile deformation which could be affected by TRIP and TWIP effects.

High-Mn austenitic steels utilizing TWIP (twinning induced plasticity) effect have excellent combination of tensile strength and ductility. The near-threshold fatigue crack propagation (FCP) behavior, as represented by the ΔK th value, of high-Mn steels was examined with the emphasis on the effect of stacking fault energy (SFE), grain size, twinning and tensile properties. Even though no predominant parameter determining the near-threshold FCP behavior of high-Mn steels was found, the SFE showed the most reasonable correlation to the ΔK th values among the variables examined. It was also suggested that the slip reversibility as determined by SFE could not solely explain the near-threshold FCP behavior of high-Mn steels. The presence of twin boundaries appeared to be not beneficial in improving the resistance to FCP of high-Mn steels in low ΔK regime. The near-threshold FCP characteristics of high-Mn austenitic steels were discussed and correlated with the well-known parameters presumably affecting FCP.

The microstructural-mechanical correlative study has been conducted for characterization of selective laser melted H13 tool steel. Transformation behavior from austenite to martensite has been observed with partitioning of C in matrix with correlative atomic diffusivity during selective laser melting process. During solidification, columnar grain structures are formed due to epitaxial growth following the build direction of H13 tool steels. Columnar microstructures are mostly composed of martensite with small amount of retained austenite. Supercooling of H13 with high laser scan speed increased the nucleation sites, which reduced the diameter of columnar grain. During tensile test, deformation appeared in grain boundary while there was no significant martensitic phase transformation confirmed by X-ray diffraction (XRD) method and electron backscattered diffraction (EBSD) analysis. S2 (scan speed of 200 mm/s specimen had the better tensile property with tensile strength of 1700 MPa and elongation of 1.6% than the rest(