| TRIP/TWIP steels possess superior properties such as high strength and ductility due to the Transformation induced plasticity (TRIP effect) and Twinning induced plasticity (TWIP effect). The steels can meet the requirements for weight saving, fuel efficiency as well as the safety for the high energy absorption capacity, which can improve the security class of vehicles. Recently, this kind of steels has become a major concern on automobile manufacturing and attracts considerable interests.In TRIP/TWIP steels, HCP (s) martensite and FCC (y) deformation twin ae formed by the movement of Shockley patial dislocations and subsequent intrusion of stacking faults, on every other {111} close-packed plane for the former and on every {111} plane for the latter. It is well-accepted that work hardening is a key factor affecting mechanical properties of TRIP/TWIP steels, and numerous investigations have been carried out to reveal the effect of the ε martensite transformation and deformation twinning on work hardening. However, a deeper understanding is needed for the further development on TRIP/TWIP steels. In the present paper, the deformation microstructure evolution and detailed deformation mechanisms were studied at the early deformation stages in Fe-30Mn-4Si-2Al TRIP/TWIP steel and Fe-30Mn-3Si-3Al TWIP steel. Furthermore, the s martensite plate intersection mechanism was systematicly investigated. The orientation dependent and deformation temperature dependent of intersection structure of ε martensite were analysized. And the intersection structure of deformation twin was also investigated. The major experiments and results are shown as the following:(1) Three kinds of deformation structures (the planar dislocation band, the ε martensite and the deformation twin) are commonly formed on the {111} habit planes and exhibit plate-like morphology during tensile deformation. Therefore, in this study, the deformation microstructure and deformation mechanisms of the studyed alloy were investigated by the combined use of atomic force microscope (AFM), electron backscatter diffraction (EBSD) and transmission election microscopy (TEM). The results indicate that:The dominant deformation mechanism is the stress-induced γâ†'ε martensitic transformation for Fe-30Mn-4Si-2Al steel at the early deformation stage, accompanied with stacking fault; The dominant deformation mechanism is dislocation slip for Fe-30Mn-3Si-3Al steel at the early stage of plastic deformation, deformation twinning becomes the dominate deformation mode at 10%.(2) The microstructure at the intersection of ε plates on different {111} habit plane was observed in Fe-30Mn-4Si-2Al steel by combined use of EBSD and TEM. The intersection y 90° rotated from the matrix is actually not created by the lattice rotation, but by the combination of two intersecting half-twinning shear on two different conjugate {111} planes. The intersection y has a near <001> orientation where the slip deformation is more domiant than γâ†'ε martensitic transformation. As a consequence, the intersection y does not further transform into e, but grows continuously on subsequent tensile loading. Furthermore, A {1012} ε twin is another intersection product, when one of the intersection plates is a thin ε lamella or a stacking fault bundle. The intersection involves three kinds of lattice defects:the lattice mismatch between the intersection y and the initial ε, considerable basal slip on a {111} plane of the intersection y, and the lattice transient region between the intersection y and the e twin.(3) The orientation dependence of intersection reactions and variant selection of ε martensite was investigated. The results indicate that:variant selection of ε martensite at the early plastic deformation stage is well-predicted by the Schmid law. In grains with [001]-[1 01] orientations, two different ε plates are likely to intersect to form the intersection y and{1 012} twin. The grains with [001]-[111] orientations also have a trend to intersect each other where the structure of the intersecting part is complex.(4) Temperature dependence of ε intersection structure was investigated. The results indicate that:with decreasing deformation temperature, a part of the intersection is replaced by an ε phase, which is seemingly explained by considering subsequent γâ†'ε transformation from the intersection, as the thermodynamic stablility of ε phase increases.(5) A wide variety of intersection products (the intersection y, the intersection ε and ε twin) were observed in Fe-30Mn-4Si-2Al steel by systematical study. The variations can be explained by considering thermodynamic stability of ε phase, crystal orientation and shear amount.(6) The microstructure at intersection of deformation twinning was observed. The results indicate that:the stress concentration is induced by the intersection of deformation twinning, the second order twin will forms inside barrier twins when the stress concentration is large enough.Therefore, in the present study, we clearly reveal the deformation mechanism at early deformation stage in Fe-30Mn-4Si-2Al TRIP/TWIP steel and Fe-30Mn-3Si-3Al TWIP steel and the intersection mechanism of ε martensite.The intersection structure of deformation twinning is investigated. The theoretical study of TRIP/TWIP steels is further enriched. Especially we relate the intersection reaction with work hardening, which is aimed for providing a support for industrial applications of automotive TRIP/TWIP steels. |
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