Static and dynamic mechanical behaviors of gradient-nanotwinned stainless steel with a composite structure : Experiments and modeling

The metals with gradient nanostructures possess exceptionally superior mechanical properties. Here, the gradient-nanotwinned 304 stainless steel wires are fabricated by surface mechanical attrition treatment (SMAT) with a range of process time. The quasi-static tensile tests and dynamic compressive tests are conducted to examine the constitutive response of gradient-nanotwinned 304 stainless steels under different loadings. The experimental measurements show that under static and dynamic loadings, their mechanical properties are closely related to the SMAT process time. With an increase in the process time, their yield strength is improved, while their ductility is weakened. Furthermore, a theoretical model is proposed to describe the static and dynamic constitutive relation of gradient nanotwinned 304 stainless steels. The micromechanical model of nanotwinned composite is developed to characterize the constitutive relation of the material with nanograins embedded in nanotwinned matrix in depth. For the constitutive relations of each phase in nanotwinned composite structure, the athermal behaviors of dislocations are only considered in describing the flow stress under the static loadings. The sizedependent athermal flow stress and rate-dependent thermal flow stress are both involved under the dynamic loadings. The theoretical simulations demonstrated that the mechanical response of gradient-nanotwinned 304 stainless steels under different loadings can be successfully characterized by the presented model. A good agreement is obtained between the numerical results and experimental measurements. Furthermore, the mechanical properties of gradient-nanotwinned 304 stainless steels are forecasted for the various distribution of twin spacing along the depth. The results in this work are helpful for optimizing the static and dynamic mechanical performance of the gradient-nanostructured metallic materials through controlling microstructural size and distribution.