Researches On Microstructures And Properties Of Electrodeposited Nanocrystalline Ni-Co Alloys

Nano-materials are generally defined as the crystal or amorphous materials thesize of whose monomer structure is in1¹00nm in the direction of one or moredimensions. Due to the very small grain size of nano-materials, and compared withthe composition of coarse grain material, it usually shows special mechanical andphysical properties, such as the quantum effect, small size effect, surface and interfaceeffects, and so on. Because of such good properties of nano-materials, it has a lot ofapplications in computer electronics, magnetic storage materials and auto parts.Compared to traditional coarse grain material, nanocrystalline materials alwaysdisplay high strength and hardness, and have a high strain-rate sensitivity, but theplastic in general is poor. Even the research of nano-materials has made substantialprogress，there are many aspects not being known well and more researchs should becarried out on deep step，such as the phenomenon of anti-Hall-Petch in mechanicalproperties of nanometer material, deformation mechanism of nanocrystallinematerials, and so on. Currently people use molecular dynamics simulation study onmechanical properties and deformation mechanism of nanocrystalline materials, butthe results of the simulation results were not sufficient to support, so the simulationresults can only be seen as the theoretical basis of the experimental study. In practicalusing, nano-materials parts may be under the influence of wet, acid and alkalineconditions,so study on the corrosion resistance of nano-materials are necessary. Atpresent, there is little study on corrosion behavior of nanocrystalline materials, andmainly focusing on the electrochemical corrosion of pure metals, but the study on thecorrosion resistance of nanocrystalline alloy is very limited. In order to furtherimprove the performance of nano-materials, better meet the requirements ofmicroelectronics, biotechnology and other aspects, more research on properties ofnano-materials should be done. On the basis of predecessor study on electrodeposited nano-alloys, in this article,we used DC electrodeposition method to prepare Ni-9%Co and Ni-46.7%Co alloyswith compact structure by changing the solution components, optimization ofexperimental parameters. The average grain size is less than20nm, So they arenano-material. We analysis the composition and microstructure of alloys by x-raydiffraction （XRD） and scanning electron microscopy （SEM）. Doing nanoindentationexperiments at different strain rates to study the strain rate sensitivity ofnanocrystalline nickel-cobalt alloys and their deformation mechanism. Doingpolarization curves and electrochemical impedance spectroscopy experiments indifferent corrosion medium to research the corrosion resistance of nickel-cobalt alloysand analyse corrosion mechanism. The study results of the article are as follows:1. By changing CoSO4content in electroplating solutions with the appropriateparameters, we prepared Ni-9%Co and Ni-46.7%Co alloys with DC electrodepositionmethod. The average grain size is respectively19nm and12nm. Doingnanoindentation experiments at different strain rates to study the strain rate sensitivityof nanocrystalline nickel-cobalt alloys. Doing polarization curves and electrochemicalimpedance spectroscopy experiments in different corrosion medium to research thecorrosion resistance of nickel-cobalt alloys.2. By XRD analysis we found nanocrystalline Ni-9%Co alloy has a single FCCstructure and strong （111） texture. In nanoindentation experiments, indentationhardness increased from5.191GPa to6.497GPa as strain rate increased from2.0×10^-3s^-1to5.0×10^-2s^-1. It indicates that nanocrystalline Ni-9%Co alloy has highstrain-rate sensitivity; the characterization of strain rate sensitivity-m value is0.073,activate volume ν=5.35b3. The alloy’s deformation mechanism is special; when thestrain rate is high, the dislocation deformation mechanism is the main deformationform; when the strain rate is low, grain boundary adjustment gradually involved inmaterial deformation, deformation of alloys is a result of the coordination ofdislocation and grain boundary adjustment mechanism.3. By XRD analysis we also found nanocrystalline Ni-46.7%Co alloy has asingle FCC structure and strong but weaker than Ni-9%Co alloy （111） texture. Nanocrystalline electrodeposited Ni-46.7Co and Ni-9%Co alloys cannot form aprotective passivation film in the5wt.%H2SO4solution, and large number of grainboundary became active corrosion points, so as to accelerate the corrosion process.Alloys can form a passivating film in9wt.%NaOH and5wt.%NaCl solution, whichwill delay the corrosion process, and the alloys perform good corrosion resistance.Large amounts of grain boundaries will accelerate or delay the corrosion process, itmainly depends on if the alloy can form passivation film on the surface.