Study On The Preparation, Microstructure And Property Of Electrodeposite Nanocrystalline Nickel

The electrodeposition is one of the most promising methods for preparingfull dense nanocrystalline materials. In the present work, the pulseelectrodeposition was adopted to prepare bulk nanocrystalline nickel on thecopper base. The effect of pulse current density and saccharin concentration onthe microstructure as well as the grain size of nanocrystalline nickel wasinvestigated and the orthogonal method was employed to find the optimumresult of parameter for nanocrystalline nickel. The differential thermal analysis(DTA and DSC), X-ray diffraction (XRD), transmission electron microscopy(TEM) and so on, were applied to investigate the thermal stability ofnanocrystalline nickel. The corrosion resistance of nanocrystalline nickel wascompared with that of Watt nickel and polycrystalline nickel plate in thesolution of NaCl and HCl by the methods of static soak and polar anodicpolarization curve and the mechanism of corrosion was analyzed. Themechanical property of nano- crystalline nickel was evaluated by macroscopictension and the microscopic process of deformation and fracture was observedby in-situ tension test under TEM. The experimental results were shown asfollows. 1. The nanocrystalline nickel can be obtained by using pulseelectrodeposition with the addiction of saccharin into Watt-type solution. Theincrement of saccharin concentration and pulse current density can cause thegrain size of nanocrystalline nickel decrease. In the nanocrystalline nickel, theincreasing of saccharin concentration is beneficial to the preferential growth ofthe (111) planes, however, preferential growth of the (222) planes are enhancedwith the increasing of pulse current density. It was found by orthogonalexperiment that the optimum pulse parameter for the maximum micro-hardnessof nanocrystalline nickel can be 20% for duty ratio, 30 A/dm2 for averagecurrent density and 100ms for period. The optimum pulse parameter for currentefficiency can be 60% for duty ratio, 30A/dm2 for average current density and50ms for period. II2. The growing process of grains for nanocrystalline nickel during heatingcan be clarified as three stages: the stage ?, grains kept stabilizing (roomtemperature-200℃), the stage ? , grains growing abnormally at low temperature(200℃-300℃) and the stage ? , grains growing normally(300℃-500℃). Thetemperature of grain starting to grow significantly is about 270℃ and the activeenergy about 208 KJ/mol. The original (200) plate textures are kept in the stage? , howerer, the plane textures turn into (111) and (200) double texture in thestage ? , and again the plane textures turn into weak (111) ones in the stage ofgrains growing normally.The micro-hardness increases slightly in the stage ?and decreased with the increment of temperature after stage ?. 3. In the solution of NaCl and HCl, corrosion resistance of nanocrystallinenickel with varied grain sizes are lower than that of Watt nickel andpolycrystalline nickel plate. There are passivating areas in the curve of anodepolarization. In the HCl solusion, the corrosion potential decreases whilecorrosion current density increases as the grain size decreases. The local spotcorrosion occurs after etching in the NaCl and HCl solution and the density ofcorrosion pit in the HCl solution was much more than that in the NaCl solution. 4. The nanocrystalline nickel displays very high strength but very poorductility. The strength of fracture with grain size 27.8nm is about 986MPa, theyield strength about 624MPa, the elongation about 1.3%, deformation hardeningexponent n about 0.18. In the process of fracture in nanocrystalline nickel, thedislocations were emitted from the crack tip and moved forward, and then adislocation free zone (DFZ) formed in the front of crack tip. The dislocation canbe absorbed by the grain boundary or emitted from the grain boundary. Themicro-crack propagates from crack tip continually or discontinually.