Effect of secondary twins on strain hardening behavior of a high manganese austenitic steel at 77 K by quasi in situ EBSD

Two high manganese austenitic steels with different grain size were prepared to comprehensively investigate the size effect on secondary twinning and resultant strain hardening at 77 K. The microstructure was characterised by means of quasi in situ electron back-scattered diffraction, transmission electron microscopy and X-ray diffraction. An abnormal phenomenon was found that both the tensile strength and total elongation showed a decreasing trend with the increase of grain size, especially the strain hardening ability decreased significantly in the later stage of deformation. The microstructure indicates that the difference of the fractions of secondary twins may be the reason for this change. Twin-twin interactions are an important strain hardening mechanism. In addition, the dynamic grain refinement caused by the primary twins is a "one-dimensional" refinement, therefore, with the increase of the density of primary twins, the activation of secondary twins becomes difficult. A high density of primary twins are formed in the CG steel in the early stage of deformation, but these high-density primary twins prevent the activation of secondary twins, leading to a significant decrease in strain hardening capacity. However, in the FG steel, the secondary twins are sufficiently activated, leading to a significantly increase in yield strength and strain hardening capacity.