Deformation mechanisms, size effects, and strain hardening in nanoscale metallic multilayers under nanoindentation

The strain hardening and the related surface pile-up phenomena in CuNi, CuNb and CuNiNb nanoscale multilayered metallic (NMM) composites are investigated using atomistic simulations of nanoindentation on such multilayers with varying individual layer thickness. Using empirical load-stress and displacement-strain relations, the obtained load-depth curves were converted to hardness-strain curves which was then fitted using power law. It is found that the extent of surface pile-up is inversely related to the hardening exponent of the NMMs. Two deformations mechanisms which control the surface pile phenomenon are discovered and discussed. Furthermore, from the stress-strain data, it is found that interfaces and their types play a major role in strain hardening; the strain hardening rate increases with strain when incoherent interfaces are present. The relationship between the hardening parameters and the interfacial dislocation density as well as the relationship between interfacial density and length scales, such as layer thickness and indentation depth, are analyzed, and it is found that the hardness in these NMM has strong inverse power law dependence on the layer thickness. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4748149]