Design of Metallic Nanocomposites for High-Strength, Plasticity and Fracture Resistance

Abstract

In metallic materials, flow strength and plasticity are typically mutually exclusive and there is a pressing demand for the design of nanostructures that will solve the strength-deformability trade-off. Metallic nanocomposites (MNC) have been studied to exhibit high uniaxial strengths in the form of multilayers, with a general “smaller is stronger” trend in terms of the layer thickness. However, localized deformation in the form of shear bands results in limited deformability of these multilayer MNCs. It was hypothesized that, through high-temperature co-sputtering of two immiscible metals, novel morphological designs at nano-scale, including bicontinuous intertwined structure and heterogeneous structures where larger crystalline phases present, will be produced and they will promote plasticity while maintaining high strength of the nanocomposite. Cu-Mo nanocomposites with various morphologies have been prepared via high-temperature co-sputtering, including vertical concentration modulation (VCM), lateral concentration modulation (LCM), random concentration modulation (RCM) structures and a hierarchical “composite of composites” architecture where sub-micron scale Cu-rich islands containing Mo nano-precipitates are dispersed in a matrix of phase-separated Cu-Mo with nanoscale ligaments. Through advanced electron microscopy characterizations and in situ nanomechanical testing, deformation mechanisms of these nanocomposites have been studied and related to the nanostructures. High flow stress over 2 GPa were measured from all these nanocomposites. It has been discovered that the VCM and LCM structures lack uniform deformability due to the formation of localized shear bands or kink bands. On the other hand, uniform deformability was observed in the rest two structures: the RCM or bicontinuous intertwined structure and the hierarchical structures. The complexity in the RCM structure poses geometric constraint on the softer Cu phase, promoting strain hardening and thus increasing the strength of the material. The tortuous interfaces effectively block the propagation of local strain concentration, resulting in the uniform deformation. In the hierarchical structure, the stronger matrix provides strength while the larger grain-size Cu-rich island promote deformability. The hierarchical structure has also been shown to have significant higher fracture resistance compared to the multilayers. These findings will provide insights to the understanding of interface microstructure-induced plasticity and fracture toughness enhancement in metallic materials and thus facilitate the design of metallic nanocomposites for advanced structural applications.