Simultaneous twinning nucleation mechanisms in an Fe-Mn-Si-Al twinning induced plasticity steel

Two simultaneous twinning nucleation mechanisms were observed in transmission electron microscopy (TEM) during low strain rate tensile deformation of a Fe-27Mn-2.5Si-3.5Al twinning induced plasticity (TWIP) steel at −40 °C. Deformation twinning took place on the conjugate (11¯1) plane through the activation of pole and three-layer mechanisms producing Shockley twinning dislocations having the same Burgers vector. Secondary twinning mechanisms were not identified. The active twinning mechanisms were influenced by the stacking fault energy (SFE) of the deformed austenite, which as estimated using electron and X-ray diffraction was ∼15 mJ/m2 and 20 mJ/m2 for 2% and 5% strains, respectively. This low SFE is believed to hinder cross-slip of twinning Shockley partials so that the stair-rod cross-slip twinning mechanism was not favored. The selection of twinning mechanisms by the microstructure is explained through critical twinning stress, the incidence of intrinsic and extrinsic stacking faults, as well as proper a2[11¯0] dislocations having a screw component that serve as suitable pole dislocations. The active twinning mechanism is plausibly not contributing markedly to the strain hardening of TWIP steels but the SFE and other hardening mechanisms are seemingly more prevalent.

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