Grain size dependence of twinning behaviors and resultant cryogenic impact toughness in high manganese austenitic steel

Abstract

Two high manganese austenitic steels with different grain size were prepared to comprehensively investigate size effect on twinning and resultant cryogenic impact toughness at −196 °C. The microstructure was characterized by means of electron back-scattered diffraction, transmission electron microscopy and X-ray diffraction. It is found that the Charpy impact absorbed energy can be enhanced by 36% via increasing grain size from ~11 to ~47 μm, exhibiting an inverse size effect of cryogenic impact toughness. The major difference in deformed microstructure is that the number fraction of type III grains (intense twinning in grain interior) increases from ~16% to ~56% as the grain size increases from ~11 to ~47 μm. Moreover, it has been demonstrated that the larger grain size, inducing better Charpy impact toughness at −196 °C, is due to the following aspects: (1) stronger dynamic grain refinement; (2) higher dislocation density and wider extended dislocations contributing to stronger strain hardening and better plasticity as well as (3) relatively homogeneous plastic deformation.