Size effects in bimetallic nickel–gold nanowires: Insight from atomic force microscopy nanoindentation

The crucial role that slip events emitted from free surfaces play in the overall plasticity and strength of low-dimensional crystals such as metallic nanowires (NWs) is well documented; however, the influences of stacking fault energy (SFE) and sample diameter on these local deformation processes are not clearly established. Experimental characterization by nanomechanical bending or tensile testing of NWs, in particular, may not be applicable to NWs made of different metals or exhibiting non-uniform dimensions. In this study, atomic force microscopy nanoindentation is used to probe the local plastic behavior and hardness properties of electrodeposited bimetallic Ni–Au NWs ranging from 60 to 358 nm in diameter and fixed on functionalized-glass substrates. Hardness measurements in individual NW segments are found to be larger in Ni than in Au owing to the difference in SFE and shear modulus between these two metals. However, the characteristic length scale associated with indentation size effects is shown to be material independent and directly linked to the NW diameter. Atomistic study of deformation mechanisms in single-crystalline NWs by molecular dynamics simulations further confirms that the interaction mechanisms between newly emitted dislocations and free surfaces are fundamentally different between Ni NWs and Au NWs during nanoindentation. By decoupling the intrinsic diameter dependence from indentation size effects in the hardness of bimetallic Ni–Au NWs, we find a marked reduction in size effects with a power-law scaling exponent of n = 0.18 during the incipient yielding of pristine NWs, in contrast to n = 0.8 in plastically pre-strained NWs.