Grain boundary strengthening in copper/niobium multilayered foils and fine-grained niobium

Uniaxial tensile tests were performed on Cu/Nb multilayered foils to investigate yield strength and grain boundary strengthening in the layered foils at room temperature and in fine-grained Nb at 600 °C. At room temperature, yielding in Cu/Nb multilayered foils is controlled by deformation in both layers, and grain boundary strengthening is observed with a Hall–Petch slope (k_RT) of 198 ± 56 MPa·μm^1/2 at a strain rate of 10⁻⁴ s⁻¹. At 600 °C, yielding in Cu/Nb multilayered foils is controlled by deformation in just the Nb layers. Hall–Petch strengthening is observed over a range of strain rates, but the Hall–Petch slope decreases from 197 ± 71 MPa·μm^1/2 for a strain rate of 10⁻⁴ s⁻¹ to only 25 ± 40 MPa·μm^1/2 for a strain rate of 10⁻⁶ s⁻¹. The significant drop in the Hall–Petch slope for Nb with decreasing strain rate indicates a change in the controlling deformation mechanism from dislocation glide to dislocation creep.