Solute and grain boundary strengthening effects in nanostructured Ni-Co alloys

Abstract Nanostructured Nickel and Nickel-Cobalt coatings across the Co composition range of 0–83 wt% were synthesized by electrodeposition. Surface morphology, microstructure, and phase formation were investigated in conjunction with alloying level using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Rietveld refinement. Fine surface morphologies with average roughness of 6–20 nm and estimated grain sizes of 4–30 nm were achieved. The deposit hardness was found to increase with Co addition and reached a maximum value of 584 HV at 43 wt% Co. Further Co addition resulted in phase change and a decrease in hardness despite finer grain sizes of 15 nm and smaller. Although the HCP phase is expected in Co-rich alloys (>55 wt% Co), our results showed that the underlying phase gradually changed from FCC to HCP starting 43 wt% Co. Solid solution strengthening, phase formation, internal stress, and grain boundary strengthening were considered for their roles in Ni-Co alloys. Ni-rich, FCC-based deposits showed improved hardness from solid solution and Hall-Petch strengthening, while Co-rich, HCP-containing deposits did not benefit from the heavily refined grain size.