A thermally activated dislocation-based constitutive flow model of nanostructured FCC metals involving microstructural evolution

Abstract

Due to their interface and nanoscale effects associated with structural peculiarities of nanostructured, face-centered-cubic (FCC) ultrafine-grained/nanocrystalline (UFG/NC) metals, in particular nanotwinned (NT) metals exhibit unexpected deformation behaviours fundamentally different from their coarse-grained (CG) counterparts. These internal boundaries, including grain boundaries and twin boundaries in UFG/NC metals, strongly interact with dislocations as deformation barriers to enhance the strength and strain rate sensitivity (SRS) of materials on the one hand, and play critical roles in their microstructural evolution as dislocation sources/sinks to sustain plastic deformation on the other. In this work, building on the findings of twin softening and (de)twinning-mediated grain growth/refinement in stretched free-standing NT–Ni foils, a constitutive model based on the thermally activated depinning process of dislocations residing in boundaries has been proposed to predict the steady-state grain size and simulate the plastic flow of NT–Ni, by considering the blocking effects of nanotwins on the absorption of dislocations emitted from boundaries. It is uncovered that the stress ratio (η_stress) of effective-to-internal stress can be taken as a signature to estimate the stability of microstructures during plastic deformation. This model not only reproduces well the plastic flow of the stretched NT–Ni foils as well as reported NT–Cu and the steady-state grain size, but also sheds light on the size-dependent SRS and failure of FCC UFG/NC metals. This theoretical framework offers the opportunity to tune the microstructures in the polycrystalline materials to synthesise high performance engineering materials with high strength and great ductility.