Enhanced hardness via interface alloying in nanoscale Cu/Al multilayers

Ultrahigh hardness (yield strength) was achieved in magnetron sputtering nanoscale Cu/Al multilayers upon annealing. The microstructure and mechanical properties of the multilayers were systematically investigated by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and nanoindentation. Annealing promoted diffusion of Cu and Al atoms in the interfaces and the sharp interface turned to mix, resulting in the formation of Cu/Al intermetallic compounds and its deformation at nanoscale. The Cu/Al intermetallic compounds mainly including Al₂Cu grew toward to Al layers and would reducing the effective length between the reduced adjacent layers. As the annealing temperature was increased from 100 °C to 500 °C, various kinds and larger size Cu/Al intermetallic compounds emerged, causing the hardness to first increase, reaching an unusually high peak (never reached before in other thin metallic multilayer systems), and then remain nearly unchanged. The physical mechanisms underlying such remarkable enhancement were explored in terms of interface alloying, reduced layer thickness and grain size effects.