PNNL

Abstract

For multiphase advanced high-strength steels, the different mechanical behavior of constituent phases may lead to a non-uniform partitioning of stresses among different phases. In addition, the grain-orientation-dependent elastic/plastic anisotropy in each phase may cause the grain-to-grain interactions, which further modify further the microscopic load partitioning. In the current work, the micromechanical behavior of high-strength steels with multiple phases was characterized by using the in-situ high-energy X-ray diffraction (HEXRD) technique. For the investigated materials, the {200} lattice strains of the constituent phases (the ferrite, bainite, and martensite) with similar crystal structures were determined through separating their overlapped diffraction peaks and, then, examining the respective changes of peak positions during deformation. Based on those experimental data, the anisotropic elastic and plastic properties of the steels were simulated using a Self-Consistent (SC) model for predicting the grain-to-grain and phase-to-phase interactions. The constitutive laws for describing the elastic and plastic behavior of each constituent phase were directly obtained through comparing the predicted lattice- strain distributions with the measured ones. The transmission-electron-microscopy (TEM) observations of the microstructure developments of microstructures also verified the partitioning of plastic strains among different phases. The present investigations provide a good fundamental understanding of the micromechanical behavior, with an emphasis on the stress partitioning of soft and hard phases, and different work-hardening rates, of the studied steels during tensile loading.