Nonlocal crystal plasticity and damage modeling of dual-heterostructured steel for strengthening and failure analysis

Abstract

Ultrafine elongated grain (UFEG) steel, characterized by its unique multi-level and multi-scale laminated heterogeneous structure, shows considerable promise in addressing the challenge of balancing high strength and toughness in metallic materials. In this work, we develop a coupled nonlocal crystal plasticity and damage phase field model. We derived the dislocation flux term from this model to introduce geometrically necessary dislocation (GND) and back stress to reflect the heterogeneous deformation of the material, and corrected the critical plastic work density term based on the relationship between grain boundary misorientation and grain boundary energy to investigate the strengthening and softening mechanisms of medium carbon steel with UFEG structure under uniaxial tensile deformation. Simulation results indicate that the strengthening effects of GNDs and back stress are closely linked to the material's initial dislocation density and grain size. Higher initial dislocation densities and larger grain sizes limit these effects. Moreover, a higher grain aspect ratio enhances the strengthening effect of GNDs. Different textures significantly affect the tensile properties of the material. The experimentally obtained <110>//RD fiber texture provides some strengthening effect, but there remains a gap compared to the ideal fiber texture. Damage initiates in the elongated grains, but the equiaxed grains help slow its progression. High-angle grain boundaries promote intergranular damage, which restricts the spread of intragranular damage. These boundaries are also critical in the formation of delamination cracks within the BCC material. These insights provide a foundation for understanding the role of grain morphology and GND density in the deformation and failure mechanisms of dual-heterostructured medium carbon steels, offering potential guidance for optimizing microstructure design in these specific material systems.