X-Ray Diffraction Studies of Forward and Reverse Plastic Flow in Nanoscale Layers During Thermal Cycling

The biaxial stress-strain response of Cu and Ni layers within Cu/Ni nanolaminates was determined from in-plane X-ray diffraction during heating/cooling. Thinner (11 nm) Cu and Ni layers with coherent, cube-on-cube interfaces reached similar to 1.8 GPa (Cu) and similar to 2.9 GPa (Ni) without yielding. Thicker (21 nm) layers with semi-coherent interfaces exhibited unusual plastic phenomena, including extraordinary increases in stress during early yielding, reverse plastic flow at modest (similar to 12%) unloading and evidence that plastic flow in Cu layers can reduce the flow strength of adjoining Ni layers. Estimates of dislocation line energy, pinning strength, net interfacial dislocation density and hardness are provided.