Multiple mechanism based constitutive modeling of gradient nanograined material

Gradient nano-grained (GNG) materials, inside which grain size increases gradually from nanoscale in the surface to micro-scale in the substrate, have shown synergetic strength and ductility. The extra strain hardening of GNG materials is considered to result from both geometrically necessary dislocations (GNDs) accommodating nonuniform plastic deformation and superior kinematic hardening characterized by back stress. However, few quantitative investigations were performed to evaluate the contribution of various strengthening mechanisms to the mechanical response of GNG materials. In this work, we develop a multiple-mechanism-based constitutive model, in which constitutive laws for GNDs and back stress at both grain level and sample level are established. Microstructure-based finite element simulation successfully predicts the uniaxial tensile behavior of a GNG interstitial-free (IF) steel sheet. The simulation results demonstrate that GNDs and back stress at sample level have little influence on the strengthening of the GNG IF-steel, while the back stress induced by pileup GNDs contributes about 35% to the flow stress. The uniform elongation of the GNG sample is improved by the constraint of coarsegrained core on GNG layer. This work helps to understand the contributions of deformation mechanisms to the synergetic strength and ductility of GNG materials and to guide the microstructure design and optimization for improved strength-ductility combination

Strengthening effects of various grain boundaries with nano-spacing as barriers of dislocation motion from molecular dynamics simulations

Strengthening in metals is traditionally achieved through the controlled creation of various grain boundaries (GBs) such as low-angle GBs high-angle GBs and twin boundaries (TBs). In the present study a series of large-scale molecular dynamics simulations with spherical nanoindentation and carefully designed model were conducted to investigate and compare the strengthening effects of various GBs with nano-spacing as barriers of dislocation motion. Simulation results showed that high-angle twist GBs and TBs are similar barriers and low-angle twist GBs are less effective in obstructing dislocation motion. Corresponding atomistic mechanisms were also given. At a certain indentation depth dislocation transmission and dislocation nucleation from the other side of boundaries were observed for low-angle twist GBs whereas dislocations were completely blocked by high-angle twist GBs and TBs at the same indentation depth. The current findings should provide insights for comprehensive understanding of the strengthening effects of various GBs at nanoscale