What really governs the upper bound of uniform ductility in gradient or layered materials?

Gradient metallic materials, which can be made by surface mechanical attrition, non-equilibrium growth, or other thermomechanical means, have been deemed as an effective way to overcome the strength-ductility tradeoff as commonly seen in traditional metallic alloys. Although a large number of research works have been devoted to the unique strengthening mechanisms as resulting from the enhanced plastic-strain gradients and back stresses in these novel materials, the disproportionally fewer studies on ductility are rather restricted to the delay of the Considere necking limit due to the elevated work hardening. This view is incomplete, because the onset of necking (i.e., the uniform ductility) depends critically on both the amplitude and wavelength of initial perturbations. Even with a mere simplification of gradient materials by a film-on-substrate or a sandwich structure, this work shows that with random surface perturbations, the upper bound of uniform ductility is dictated by an extremely short-wavelength perturbation at the surface of the outer hard layer (thus denoted as Rayleigh mode). On the other hand, a pre-patterned surface with perturbations of varying periods is practicably unable to evolve into the Rayleigh asymptote and thus can achieve even higher uniform ductility. A revisit of literature experiments is suggested to be undertaken with a particular focus on the spectral growth kinetics of surface morphology and its connection to uniform ductility in these gradient materials. (C) 2021 Elsevier Ltd. All rights reserved.