A constitutive framework for micro-to-macroplasticity of crystalline materials under monotonic and cyclic deformation

Microplasticity, a behavior lies between elasticity and macroplasticity, is not only closely related to the origin of plasticity in crystalline materials, but also profoundly affects the service life of materials under small deformation, e.g., high cycle fatigue. Here a constitutive framework considering the micro- and macroplasticity is established for modeling the elastic-plastic transition of metallic materials. It reveals the origin of macro-yielding as the instability of microplasticity and demonstrates the intrinsic characteristics of the macro-yield point, including its dependence on dislocation density and offset strain. This model formulates the microplastic strain based on short-range dislocation motion, which stems from the dislocation network reconfiguration or dislocation pile-up against grain boundaries, depending on the characteristic length scales. By incorporating the grain anisotropy, a new crystal plasticity framework is developed and applied to examine the mechanical behaviors of lath martensitic steels under various loading modes, temperatures, and irradiation effects. The analysis of microplasticity encompasses aspects such as microstructural sensitivity, links to macro-yielding, and active slip systems involved. The significance of microplasticity is exemplified by its role in cyclic softening behavior, particularly in the irradiated case, which successfully captures the transition from nearly perfect elastic to elastoplastic cycling. This framework provides a quantitative understanding of microplasticity in crystalline materials, sheds light on the mechanisms underlying elastic-plastic transitions, and has potential to inform predictions of material damage and lifetime.