Micromechanical Behavior Of TRIP Steels And Dual Phase Steels Studied By In-situ Diffraction Technique And Self-consistent Model

The micromechanical behavior of transformation induced plastic (TRIP) steel and dual phase (DP) steel was studied by the in-situ high-energy X-ray diffraction technique based on the synchrotron source. The microstructure of the studied TRIP steel consists of the ferrite, bainite and the residual austenite; while the DP steel is composed of ferrite and martensite. For TRIP steels, due to the obvious difference of the positions of diffraction peaks of the face-centred cubic phases (ferrite and bainite) and the body-centred cubic phase (austenite), it is easy to separate them from each other by the different positions of the respective diffraction peaks. Then, the lattice strains of the austenitic phase as a function of applied stress can be obtained.However, as for ferrite and bainite in the TRIP steel as well as the ferrite and martensite in the DP steel, it is difficult to separate them from each other by the single peak fitting. Although the ferrite/bainite and ferrite/martensite exhibit the same crystal structure, the (200) lattice strains of each phase can be determined by separating their overlapping diffraction peaks. This is achieved through fitting the lattice strain vs.20 curve by adopting two Gaussian functions with the different peak widths and positions. In this way, specific lattice strains of respective phases are determined for all the phases in both TRIP steel and DP steel.We also used an Elastic-Plastic Self-Consistent (EPSC) model to construct the respective constitutive laws for both phases from the experimental lattice strains and macrostress-strain curve. It is confirmed that the EPSC model can capture well the micromechanical behaviour of two-phase materials.Based on the work mentioned above, the following conclusions could be drawn:1. The microstresses in the materials mainly come from the phase stress caused by the phase-to-phaseinteraction when the macro-stress is low. With increasing the applied stress, grain-orientation-dependent stress is developed and dominate the microstresses in the 2. In the two-phase materials, such as TRIP800 and DP980 steels, the average stress in each phase and grain-orientation-dependent stress subjected by austenite, bainite and martensite are larger than the corresponding stresses by ferrite under the applied loading.3. The critical shear stress of TRIP800 composed by 14% austenite is approximately 300MPa in a phase and the corresponding stress of y phase is 600MPa. The critical shear stress of TRIP800 composed by 5% austenite is 300MPa in a phase and the corresponding stress of y phase is 750MPa. Similarly, the shear stress of ferrite and martensite in DP980 is 830 and 900MPa, respectively.4. The EPSC model can capture well the micromechanical behaviour in the two-phase materials. The EPSC model provides useful information on the phase-to-phase and grain-to-grain interations in the materials, which could not be obtained by simply performing experiments. It is proved to be a powerful tool in predicting the mechanical properties of the materials.