Effects of grain size on the properties of bulk nanocrystalline Co–Ni alloys

Bulk nanocrystalline Co₇₈Ni₂₂ alloys with grain size ranging from 5 nm to 35 nm were prepared by high-speed jet electrodeposition (HSJED) and annealing. Microhardness and magnetic properties of these alloys were investigated by microhardness tester and vibrating sample magnetometer. Effects of grain size on these characteristics were also discussed. Results show that the microhardness of nanocrystalline Co₇₈Ni₂₂ alloys increases following a d ^−1/2-power law with decreasing grain size d . This phenomenon fits the Hall–Petch law when the grain size ranges from 5 nm to 35 nm. However, coercivity H _c increases following a 1/ d -power law with increasing grain size when the grain size ranges from 5 nm to 15.9 nm. Coercivity H _c decreases again for grain sizes above 16.6 nm according to the d ⁶-power law. (paper)