A Study On Effects Of Rare Earth Addition On Microstructure And Mechanical Properties Of TWIP Steels And Deformation Mechanisms Of TWIP Steels

Over the past few decades, twinning-induced plasticity(TWIP) steels have increasingly attracted interest owing to their excellent combination of high strength and large elongation associated with deformation twinning phenomenon. Although effects of C, Mn, Al and Si additions on microstructure and mechanical properties of TWIP steels have been widely reported, effects of rare earth(RE) addition on those of TWIP steels are not investigated, and deformation mechanisms of TWIP steels are not revealed sufficiently.In this study, the microstructure and mechanical properties of a Fe-15Mn-1.5Al-0.6C TWIP steel with and without rare earth addition were investigated. The RE-alloyed steel showed both improved yield strength, tensile strength and total elongation(TEL). Optical microscopy and electron back-scattered diffraction(EBSD) characterizations showed that the RE-alloyed steel possessed a more homogeneous microstructure and a larger fraction of high-angle grain boundaries, which increased the TEL. Meanwhile, EBSD and transmission electron microscope(TEM) analyses indicated that the RE-alloyed steel exhibited more uniform distributed annealing twins and secondary annealing twins with a thickness of 10-20 nm appeared in some grains, which resulted in the higher tensile strength. The increased yield strength of the RE-alloyed steel was associated with solid solution strengthening and dislocation pinning effect of RE atoms.Also, the room-temperature deformation mechanisms of a Fe-15Mn-1.5Al-0.6C TWIP steel were studied by means of X-ray diffraction and TEM. The results indicated that the microstructure of Fe-15Mn-1.5Al-0.6C TWIP steel was single phase austenitic during plastic deformation. At low strains, the deformation mechanism was dislocation slip. With the increase of strain, twinning was activated and twin density increased. When the strain reached a certain extent, microbands were activated. With the further increase of strain, the continuation of twinning was difficult, while the formation and intersection of microbands became the main deformation mechanism until the final fracture.