Constitutive modeling of the elastoplastic and fatigue behaviors of gradient-nanostructured 316L stainless steels with hierarchical structures

The gradient nanostructured metallic materials possess the excellent mechanical properties, including the high strength, good elongation and fatigue performance. In this work, a microstructure and mechanism-based constitutive model is established to explore the strength-ductility relation and fatigue properties of the gradient nanostructured 316L stainless steel (316LSS) through considering the various microstructure distributions. Inspired by the experimental observation of distinguish distribution of nanograined austenite and martensite phase, nanotwinned austenite grains and coarse grains, the micromechanical constitutive model is developed to describe the axial tensile deformation behaviors of the gradient-nanostructured 316LSS, involving the flow stress for different phases and the contribution of microcracks on plastic deformation. The simulation results demonstrate that the proposed constitutive model enables to describe the experimental results successfully, including the yield strength, strain hardening and ductility. Additionally, with considering the gradient distribution evolution of microstructures and the damage evolution during cyclic deformation behavior, the fatigue constitutive model for gradient nanostructured metals is developed to describe the uniaxial tensile cycle characteristics of gradient nanostructured 316LSS. The numerical results show that the strain-controlled cyclic deformation behavior of gradient nanostructured stainless steel can be well described, including the cyclic softening and secondary hardening behaviors. The proposed fatigue constitutive model is also applied to forecast the fatigue behavior of the various amplitudes of the stress and the strain, and the various distribution of the fine-grained martensitic phase, nanotwinned austenite grains and coarse grains. These findings could provide the theoretical basis for regulating the strength-ductility relation and fatigue properties of gradient nanostructured metals.