The role of Mn on twinning behavior and tensile properties of coarse- and fine-grained Fe-Mn-C twinning-induced plasticity steels

The twinning behavior, strain hardening and tensile properties of coarse- and fine-grained Fe-13/18/22Mn-1.0C twinning-induced plasticity (TWIP) steels were investigated by microstructural observations and mechanical tests. These steels have stacking fault energy ranging from similar to 27 to similar to 37 mJ/m(2) with increasing Mn content, and thus deformation twinning is the dominant mechanism. Tensile test results show that, with increasing Mn content, the yield and tensile strengths decrease slightly for both coarse- and fine-grained steels while the ductility increases significantly, especially for coarse-grained steels. Microstructural observations indicate that, at lower strains, an increase in Mn content suppresses the formation of twins, resulting in a lower twinning rate; however, at intermediate and higher strains, even higher twinning rate is observed in higher-Mn steels. In addition, increasing Mn content enables deformation twins to form in a wider range of strain. Such a varying twinning behavior with Mn content is even more apparent for coarse-grained steels. Furthermore, the sizes of twins vary with Mn content and also with grain size: both the thickness and spacing of twins or twin bundlers increase with increasing Mn content but decrease with refining grain sizes. Moreover, increasing Mn content decreases strain-hardening rate, but allows persistent strain hardening to higher strains. These strain-hardening behaviors and tensile properties observed in the twinning-dominated Fe-Mn-C steels with different Mn contents are correlated well their microstructural evolutions.