The effects of grain size on yielding, strain hardening, and mechanical twinning in Fe-18Mn-0.6C-1.5Al twinning-induced plasticity steel

The objective of the present study was to investigate the influences of grain refinement on yielding, strain hardening, and mechanical twinning during tensile deformation in Fe-high Mn twinning-induced plasticity (TWIP) steel. For this purpose, Fe-18Mn-0.6C-1.5Al TWIP steels with average grain sizes of 2, 10, and 50 μm were tensile tested at room temperature, and their stress-strain and strain hardening rate curves, dislocation densities, and microstructures were measured and analyzed by means of transmission electron microscopy and neutron diffractometry. The stress-strain curves showed a transition from continuous to discontinuous yielding with grain refinement, which was due to a lack of mobile dislocations, not due to mechanical twinning or martensitic transformation. The grain refinement increased the dislocation density, caused the planar to non-planar slip, and retarded primary and secondary mechanical twinning. The strain hardening rate-strain curves of TWIP steels used were able to be divided into five stages by the slope change. Until the stage III, dislocation hardening was predominant; at the stages IV and V mechanical twinning became more contributive to strain hardening. The suppression of both planar dislocation slip and mechanical twinning by grain refinement is most likely due to the increase in the back stress of dislocations on a slip plane, which was caused by the rapid accumulation of dislocations by plastic deformation in the fine-grained TWIP steel. A high level of back stress narrows the width of stacking faults, facilitates the cross slip of dislocations, and reduces the interactions between partial dislocations required for mechanical twinning.