The hardness and related deformation mechanisms in nanoscale crystalline-amorphous multilayers

Built upon recent findings that nanocrystalline-amorphous multilayers could possess both high strength and ductility due to nano-layer-limited structures, depth-sensing nanoindentation was employed to evaluate the hardness and shear band deformation behavior of nanoscale crystalline-Cu/amorphous-CuZr (C/A) multilayers among a wide range of amorphous layer thickness with constant crystalline Cu layer thickness. By varying individual layer thickness of amorphous layers, both weakening and strengthening effects on the hardness of C/A multilayers were observed, comparing with that derived from the rule of mixture, for which discontinuity of dislocation motion played a crucial role. A critical amorphous layer thickness of 20 nm was identified, above which interface-dislocation-absorption dominated plastic deformation, causing the weakening effect, below which, continuous-dislocation-impediment took over and was responsible for the strengthening effect. Mechanisms underlying the observed shear band morphologies under indentation were also extendedly discussed.