High temperature strength retention of Cu/Nb nanolaminates through dynamic strain ageing

TL;DR

This study investigates how Cu/Nb nanolaminates maintain high strength at elevated temperatures through dynamic strain ageing, revealing the role of oxygen diffusion in Nb layers at around 200°C.

Contribution

It uncovers the mechanism of dynamic strain ageing in Cu/Nb nanolaminates and demonstrates a new approach to improve high-temperature strength retention.

Findings

Dynamic strain ageing occurs at 200°C in nanolaminates with 7, 16, and 34 nm layers.

Oxygen atoms in Nb layers cause dynamic strain ageing by interacting with dislocations.

High strength retention is achieved due to this ageing mechanism, unlike at higher temperatures.

Abstract

The mechanical properties of Cu/Nb metallic nanolaminates with different layer thickness (7, 16, 34 and 63 nm) were studied by means of micropillar compression tests from room temperature to 400 C. Both strain-rate jump and constant strain rate tests were carried out and they showed evidence of dynamic strain ageing in the nanolaminates with 7, 16 and 34 nm layer thickness deformed at 200 C. Dynamic strain ageing was accompanied by a reduction of the strain rate sensitivity to 0, high strength retention at 200 C and the development of shear localization of the deformation at low strains (5%-6%) that took place along the Nb layers in the nanolaminates. Atom probe tomography of the deformed specimens revealed the presence of O in solid solution in the Nb layers but not in the Cu layers. Thus, diffusion of O atoms to the mobile dislocations in Nb was found to be the origin of the dynamic strain ageing in the Cu/Nb nanolaminates around 200 C. This mechanism was not found at higher temperatures (400 C) because deformation was mainly controlled by stress-assisted diffusion in the Cu layers. This discovery shows a novel strategy to enhance the strength retention at high temperature of metallic nanolaminates through dynamic strain ageing of one the phases.