Mechanical Behavior of Nanolayered Composites: Constituent Strength and Interface Properties

Nanolayered composites can exhibit unprecedented combinations of material properties. In principle, they also offer a superior system to understand how internal defects---such as grain boundaries or interfaces---control strength. However, in practice, only average composite properties are measured, while the underlying strengthening phenomena occur within individual layers. To address this missing capability, two novel methods have been developed to determine the constituent strengths of nanolayered composites. The first, an indentation-based method, couples finite element simulations with experimental nanoindentation and micropillar compression. The second, a diffraction-based method, uses heated x-ray diffraction and provides useful, external verification of the first method. Both methods are applied to the Cu/Ni system, and interestingly, layer constituent strengths are in good agreement with published strengths of pure, nanocrystalline Cu and Ni. The methods serve complimentary purposes. The diffraction-based method reveals previously unreported features of work hardening, reverse plasticity, and the role interfaces play as dislocation sources, but is limited to crystalline systems on substrates. The indentation-based method provides only individual constituent strengths but allows for more rapid, widespread adoption. It also exposes potential errors in the conversion of hardness to uniaxial strength simultaneously providing new routes to optimize the hardness of nanolayered composites.