Non-monotonically size-dependent shear instability in Cu/amorphous CuNb nanolayered composites

Crystalline/amorphous (C/A) nanolayered metallic composites usually exhibit a size effect of “smaller is stronger”, but the shear instability caused by shear bands is often “smaller is more severe”. This trade-off between strength and plasticity restricts the application of C/A nanolayered composites. The key problem is that the size with optimal plasticity (usually large) cannot match the size with optimal strength (usually small) from a few nanometers to several hundred nanometers. Herein, a new shear instability-layer thickness relationship is reported to circumvent the above problem. In this paper, Cu/amorphous CuNb (Cu/a-CuNb) composites with the layer thickness (h) ranging from 5 to 100 nm have been prepared by magnetron sputtering, and their size-dependent strength and shear instability have been systematically investigated using nano/micro-indentation tests. A 3-D stress state based theoretical model is proposed to quantify the shear instability behavior under micro-indentation test. The results show that as h decreases, the strength of the composites increases, while the shear instability shows a non-monotonic trend. Kinking-like shear bands are prevalent in Cu/a-CuNb composites, and the effective strain is low at large or small layer thicknesses (e.g., h = 100 nm or h = 10 nm) and reaches a maximum at h = 50 nm, with an inverted V-shape size-dependence of the shear instability behavior. The composite with h = 10 nm achieves a combination of high strength and high plasticity. It is revealed that the unique inverted V-shape size dependent shear instability is governed by the competition among the size dependent strengthening mechanisms in the constituent layers. This study gives a deeper understanding on the size dependent strengthening and plasticity of nanolayered metallic composites with crystalline/amorphous interfaces based on the quantitative analysis on the localized strains in the shear banding areas under micro-indentations.