Nanostructured Ni-Co Alloys with Tailorable Grain Size and Twin Density

We present an experimental approach to systematically produce nanostructures with various grain sizes and twin densities in the Ni-Co binary system. Using electrodeposition with various applied current densities and organic additive contents in the deposition bath, we synthesize nanostructured fcc and hcp solid solutions with a range of compositions. Due to the low stacking fault energy (SFE) of these alloys, growth twins are readily formed during deposition, and by adjusting the deposition conditions, a range of twin boundary densities is possible. The resulting nanostructured alloys cannot be described by a single characteristic length scale, but instead must be characterized in terms of (1) a true grain size pertaining to general high-angle grain boundaries and (2) an effective grain size that incorporates twin boundaries. Analysis of Hall–Petch strength scaling for these materials is complicated by their dual length scales, but the hardness trends found in Ni-80Co are found to be roughly in line with those seen in pure nanocrystalline nickel. I