In situ synchrotron X-ray diffraction and crystal plasticity studies of the deformation and fatigue crack growth behavior in a TRIP-assisted advanced high strength steel

Traditional studies on the phase-transformation-induced plasticity (TRIP) are focused on the phase stability, transformation kinetics, texture evolution, and others, which can be further compared and validated by the crystal plasticity finite element method (CPFEM). However, quantitative information such as the lattice strains over multiple phases and various grain families with different orientations have not been fully utilized with these micromechanical constitutive models. In this work, in situ synchrotron X-ray diffraction (S-XRD) experiments have been performed for a third generation advanced high strength steel under both tensile and fatigue-crack-growth conditions. The quantitative comparison of the lattice strain evolution to CPFEM simulations reveals multiple stages of the deformation over which the load partitioning mechanisms change, thus helping understand material microstructural design for delaying necking and improving tensile ductility. Besides the fatigue crack resistance from the surrounding plasticity, the S-XRD results find a thin strip of phase transformation zone in the vicinity and in the wake of the fatigue crack tip, suggesting the validity of the classic scenario of transformation toughening and the favorable agreement between quantitative estimates and experimental measurements. Consequently, it is the synergy between S-XRD and CPFEM approaches that unveils the microstructure-scale, micromechanical knowledge and enables the fundamental understanding of both ductilization and toughening from TRIP effects.