Effect of volume fractions of gradient transition layer on mechanical behaviors of nanotwinned Cu

Abstract

Heterogeneous nanostructured (HNS) materials, such as gradient and laminated nanostructured metals, possess superior mechanical properties with respect to their homogeneous counterparts. The additional strengthening mechanism of HNS is mainly attributed to the non-uniform plastic deformation induced by gradient transition layers (GTLs) between components. Here, we designed three gradient nanotwinned (GNT) Cu samples with different volume fractions of GTLs (fg) of 10%, 50% and 100% while the rule-of-mixture strength and overall structural gradient are constant to quantitatively reveal its effect on the extra strengthening behaviors. As fg increases, the yield strength is improved and the elastic to plastic stage is prolonged at small strain. Moreover, larger fg suppresses the strain localization and reduces nucleation of cracks at larger strain, accompanied by more widely distributed geometrically necessary dislocations (GNDs) and more bundles of concentrated dislocations (BCDs), thus improving the elongation. Stress partitioning analysis and numerical simulations show that the more widely-distributed GNDs at larger fg result in higher overall back stresses but hardly affect effective stresses, indicative of the origin of the improved strengthening. Such GND-distribution dominated strengthening mechanism paves a fundamental way for developing higher-performance HNS materials.