| Twinning-induced-plasticity (TWIP) steels exhibit good ductility at a very high strength level, and are therefore promising materials for making lightweight automotive parts with complex geometry. The enhanced formability of such steels is attributed to the formation of mechanical twins during plastic deformation. Most austenitic steels, such as austenitic stainless steels and high manganese Hadfield steels, have low-to-moderate stacking fault energy (SFE) and, therefore, tend to form the extended stacking faults, deformation twins, and planar dislocation structures. These different planar-type lattice defects affect not only the macroscopic mechanical properties such as yield strength, tensile strength, and ultimate elongation, but also the micromechanical behaviors, such as the grain rotation and micro-scale stress distribution.In this thesis, microstructures and micromechanical behaviors of the TWIP steel are are examined by the transmission electron microscope (TEM), X-ray diffraction (XRD) and the neutron diffraction technique, including the grain rotation, development of intergranular stress, and stress partitioning between the twin and the parent grain during plastic flow. A Self-consistent (SC) model are employed to study the micromechanical behavior evolution for the Fe-30Mn-3Al-4Si high manganese steel with low stacking fault energy (SFE) to get better understanding of the influence of mechanical twinning deformation on the redistribution of microstresses and the grain-to-grain interactions of polycrystalline materials.Analyzed by TEM microscopic structure, results indicate that TWIP steel is plastically deform through dislocation slip and twinning. Only sparsely-distributed dislocations can be observed in the as-received steel sheet (ε=0). Although high-density dislocations are the dominant microstructural features for the2%deformed sample, twins can also be found in some grains. The high density of deformation twins can be seen from the sample with ε30%deformation. To understand the twinning-related micromechanical behaviors, in situ time-of-flight (TOF) neutron diffraction experiments under tensile loading were performed to study the evolution of lattice strains for different hkl reflections. A redistribution of microstress due to twinning activities was evidenced from the anomalous response of lattice strain to the in situ tensile loading. An abrupt increase of the longitudinal {200} lattice strain was measured responding to a slight increase of loading just beyond the yield point, and further increment of stress leads to a monotonic decrease of the transverse {200} lattice strain.XRD texture results show that the initial texture components for the as-received sample consist mainly of {011}<100>(Goss),{112}<111>(Copper), and {123}<634>(S), with a maximum texture intensity3.41located at the Goss component. With strain increased to15%, the Goss and Copper components are strengthened significantly. And a new texture component with a dominant {011}<111> is developed. Meanwhile, the texture component {123}<634> is shifted from to {123}<111>. Further deformation to30%strain greatly enhances the components of {123}<111>,{011}<111> and {112}<111> but only slightly increases the intensity of the Goss component.SC model is applied to simulate the micromechanical behavior and texture evolution during tensile deformation of TWIP steel. The results indicate that only {111}<110> slip cannot capture the micromechanical evolution, however, a reliable prediction of changes in lattice strains is achieved for cubic alloys with a low stacking fault energy undergoing tensile deformation in the TWIP steel considering both {111}<110> slip and {111}<112> twinning modes, particularly, the anomalous response of {200} lattice strain along both longitudinal and transverse direction to the applied stress. What is more, the grain rotation during uniaxial loading can be well predicted from the present model when both slip and twinning modes are considered. |
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