In-situ TEM investigation of deformation mechanisms of twinning-induced plasticity steel

High Mn twinning-induced plasticity (TWIP) steels have been the focus of research in the past decade due to their superior combination of strength and ductility. In-situ TEM mechanical testing is an efficient tool to provide direct insights into fundamental deformation mechanisms of materials, and thus, key observations by in-situ TEM mechanical testing are required to unravel the deformation mechanisms of TWIP steels. The present work reports three deformation mechanisms of TWIP steels observed by in-situ TEM mechanical testing: (1) formation of deformation twins from grain boundaries, (2) interference of growth of a deformation twin by a stacking fault, and (3) motion of extended dislocations contributing to the plasticity. Firstly, the formation of deformation twins by direct emission of Shockley partial dislocations from grain boundary sources was observed. The different degree of the motion of twinning partial dislocations indicates a different degree of growth kinetics of deformation twins depending upon local stress states. Secondly, Shockley partial dislocations moved in an inclined twin confined between a precipitate and a stacking fault, thereby leading to the growth of the deformation twin. The sluggish motion of Shockley partial dislocations near the stacking fault indicates a strengthening effect of stacking faults. Thirdly, direct evidence of motion of extended dislocation across the entire grain was obtained, which suggests a contribution of extended dislocation slip on the plasticity of TWIP steels.