On the impact toughness of gradient-structured metals

Gradient-structured (GS) materials are capable of displaying high strength without compromising ductility, which can result in damage-tolerant structures. However, due to the difficulties in fabricating bulk GS materials, there has been only limited studies on the fracture behavior in GS metals. In the present work, the impact toughness of the macroscale GS pure Ni plates was investigated using instrumented Charpy impact testing. The gradient orientation was found to have a significant influence on the impact toughness of GS Ni. For gradient structures that transition from coarse grains (CG) to nano-grains (NG), termed CG→NG gradients (in the present study from ∼8 μm to ∼30 nm), the absorbed energy and the tensile strength were increased, respectively, by 1.6 and 2.3 times from those exhibited by uniform coarse-grained structures, demonstrating a simultaneous enhancement in strength and impact toughness. Analysis of load-displacement curves revealed that the resistance to both crack initiation and propagation were significantly enhanced as the crack penetrated through the CG→NG gradient structure, leading to markedly rising dynamic R-curve behavior estimated from nonlinear-elastic fracture mechanics J-based measurements. The superior fracture resistance in the CG→NG gradient structure was found to originate from sustained ductile fracture by microvoid coalescence, taking place not only in the initial CG zone, but also within the latter NG regions where adiabatic shear bands form during impact; in these latter regions, plasticity becomes enhanced due to grain coarsening induced by recrystallization under the dynamic loading. The present work not only reveals how the dynamic fracture resistance can be significantly enhanced in GS metals, but also provides structure-design strategies for developing superior metallic materials for impact engineering applications.