Thermal Stability of Pulsed Electrodeposition Nanocrystalline Ni-Co Alloy Coating

Microstructures and composition of nanocrystalline Ni-Co alloy produced by pulse electrodeposition were studied by XRD and EDS.The microhardness of nanocrystalline Ni-Co alloy annealed at various temperatures was measured and the thermal stability of Ni-23.5%Co(mass fraction) alloy was studied extensively.The results indicated that microhardness of Ni-Co alloys increased slightly after annealing at low temperatures,and reached the maximum at 250℃,then decreased with the increasing of the annealing temperatures.The grain size of Ni-23.5%Co alloy increased from 13.5nm to 98.5nm when the annealing temperature rose to 300℃.The DSC curve obtained at 20 K·min-1 heating rate showed low energy exotherm beginning at 300℃ and continuing to 350℃ at which point the main thermal release occurred,reaching peak differential heat flow at 372℃,the release ΔH was 14.22J·g-1.According to the Kissinger equation,the activation energy for grain growth of Ni-23.5%Co alloy was calculated to be 212.5kJ/mol.