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Abstract: The effects of Al addition on dynamic flow response of the fully austenitic high Mn steel were investigated by conducting high strain rate compression tests on Fe–22Mn–xAl–0.6C steels (x =0, 3, 6 in wt.%). While dynamic yield strength of the 0 Al steel and the 3 Al steel were comparable, the 6 Al steel exhibited the highest one. Meanwhile, strain hardenability of the 0 Al steel was the highest and that of other two steels was nearly same. Under the present dynamic loading, no obvious dynamic recrystallization by adiabatic heating was observed in all steels. Fully compressed microstructures revealed (a) ɛ-martensite and mechanical twin bands for the 0 Al steel, (b) multi-layer deformation bands and mechanical twin bands for the 3 Al steel, and (c) a variety of dislocation configurations such as the directional slip traces, tangled dislocations, and incomplete dislocation cells for the 6 Al steel. These findings inform that dynamic flow of the 0 Al steel was associated with both TRIP and TWIP, and that of other two steels was dominated by dislocation gliding – mainly, planar glide for the 3 Al steel and the combination of both planar glide and wavy glide for the 6 Al steel. The dynamic flow response of the present steels was discussed in terms of the stacking fault energy affected by the Al content and adiabatic heating during dynamic loading and of the strain rate effect on the critical stress for mechanical twinning. [Copyright &y& Elsevier]

Abstract: The effects of nitrogen addition on the strain induced martensitic transformation (SIMT) behavior of duplex stainless steel (D-STS) with the low nickel content were examined in a wide range of strain by means of cold rolling. Nitrogen of 0.1, 0.2 and 0.3wt.% was added into Fe–20Cr–5Mn–0.2Ni D-STS (in wt.%) and cold rolling was conducted up to the effective strain of ∼1.45 after annealing at 1000°C for 30min. In the as-annealed state, the austenite fraction increased with increasing the N content. Regardless of the N content, the ferrite grain size was coarser than that of austenite. The stacking fault energy of austenite of the present D-STSs inferred by the element partitioning analysis was low enough so that SIM transformation is available. Accordingly, during cold rolling, SIMT occurred in austenite with a sequential manner of austenite→ ɛ martensite→ α′ martensite with increasing strain. By contrast, the deformed microstructure of ferrite was dominated by dislocation cells. The SIM fraction, which was normalized with reference to the initial austenite fraction in order to eliminate its effect, increased with increasing the N content. Along with the present results, the factors influencing the SIMT kinetics in the present D-STSs with the different N content were discussed. [Copyright &y& Elsevier]

Abstract: The mechanical response of two grades of the fully austenitic high Mn steel (Fe-22Mn-0.6C (0 Al steel) and Fe-22Mn-6Al-0.6C (6 Al steel)) was examined under quasi-static and dynamic loading, mainly focusing on the effects of Al addition. For bothloading condition, Al increased yield strength but decreased strain hardenability. The Al addition was found to suppress mechanical twinning by increasing the austenite stability (i.e. increasing the stacking fault energy). Therefore, twinning induced plasticity dominates plastic deformation of the 0 Al steel while that of the 6 Al steel is governed by planar glide. [Copyright &y& Elsevier]

Abstract: The room temperature deformation characteristics of a duplex Fe–20Mn–9Al–0.6C steel with the reduced specific weight of 6.84g/cm3 in the fully solutionized state were described in conjunction with the deformation mechanisms of its constituent phases. The phase fraction was insensitive to annealing temperature in the range of 800–1100°C. The ferrite grain size was also nearly unaltered but the austenite grain size slightly increased with increasing annealing temperature. This revealed that there is little window to control the microstructure of the steel by annealing. The steel exhibited a good combination of strength over 800MPa and ductility over 45% in the present annealing conditions. Ferrite was harder than austenite in this steel. Strain hardening of both phases was monotonic during tensile deformation, but the strain hardening exponent of austenite was higher than that of ferrite, indicating the better strain hardenability of austenite. In addition, the strain hardening exponent of austenite increased but that of ferrite remained unchanged with increasing annealing temperature. The overall strain hardening of the steel followed that of austenite. Considering element partitioning by annealing, the stacking fault energy of austenite of the steel was estimated as ∼70mJ/m2. Even with the relatively high stacking fault energy, planar glide dominantly occurred in austenite. Neither strain induced martensite nor mechanical twins formed in austenite during tensile deformation. Ferrite exhibited the deformed microstructures typically observed in the wavy glide materials, i.e. dislocation cells. The mechanical properties of the present duplex steel were compared to those of advance high strength automotive steels recently developed. [Copyright &y& Elsevier]