Interpreting the ductility of nanocrystalline metals

Nanocrystalline (NC) metals are known for having excellent strength but perceived to have poor ductility. Miniature tensile tests on NC Ni-Fe measured ultimate strengths of 2 GPa and elongations, by digital image correlation, of up to 10%. Detailed examination of the fracture surface revealed dimpled rupture and cross-section reduction up to 75%, suggesting an intrinsic ability for small grained Ni-Fe to accommodate plasticity. A survey of published studies on NC metals reveals that this behavior is quite common; despite low macroscopic elongation, NC metals often achieve extensive deformation suggesting good intrinsic ductility. Unfortunately, the common sheet-like configuration of NC tensile specimens muddies a simple evaluation of ductility based on elongation, since thin and wide geometries promote localized necking that expedites catastrophic failure. This paper presents a compact review of ductility concepts and literature to interpret the experimental ductility measurements of an electrodeposited nickel alloy.