Jet Electrodeposition Of Bulk Nanocrystalline Co-Ni Alloys And Study On Their Structure And Properties

How to prepare bulk compactive nanocrystalline materials (BCNM) is a key problemin the field of nanocrystalline materials research, since the characteristics of BCNM withresidual micro-pores and/or interfacial contamination deviate from their intrinsicproperties. Accordingly, it is hindered BCNM from fundamental investigations andengineering applications.Considering above problem, the main purpose of this work is to prepare BCNM ofCo-Ni alloys by a new method of electrolyte jet electrodeposition (JED). Through thiswork, not only can the BCNM samples be prepared for scientific studies, but also may anew applying area be exploited for JED. And the preparing problem of BCNM could besolved simultaneously in some metals field. Besides preparation, the microstructure andproperties of nanocrystalline Co-Ni alloys are also studied systematically.The influences of depositing parameters, such as electrolyte composition, jet speed,temperature and current density on the morphology, chemical composition, phasestructure, grain size and propertyes of Co-Ni alloys were investigated. The cathodicpolarization curves in jet electrodeposited bulk nanocrystalline Ni-Co alloys wasmeasured by a currentstat method. Behavior and principle of cathodic polarization anddeposition were studied in this paper. SEM and EDX were used for the alloys'morphology and compositional analysis. The microstructures of the deposits alloys werecharacterized using XRD and TEM. The alloy hardness and magnetic properties weremeasured by the microhardness tester, nanoindentaion and VSM. The thermal stability ofalloys was investigated by DSC and high-temperature XRD.Samples of 5.1 nm nanocrystalline Co-Ni alloys were produced by a double-refinedtechnique of jet deposition and additive. And then the grain was grown by annealingexperiments to investigate the effect of the grain size on the alloys properties.Experimental results indicate that the current density of JED as a high-speedelectroplating technique with special flow characteristics, compared with the conventionalelectroplating, are both enlarged largely owing to the promotion of plating solution. Thisis the mechanism of the refining grain size and increasing depositing rate. When thecurrent density was 477 A/dm2, the deposition rate can be up to 47.33 μm/min, which is90 times faster or more than that of conventional electrodeposition. And the currentefficiencies are all over 90%. Influences of depositing parameters and additive oncathodic polarization and depositing action are different. With increasing of theelectrolyte jet speed and the cathodic current density, cathodic overpotential increases.And with increasing of the Co2+ ions concentration in the electrolyte and the electrolytetemperature, cathodic overpotential decreases. Effect of few additives on cathodicpolarization are obvious, it results in the increasing of cathodic overpotential. The aboveparameters have effect on the microstructure and properties of deposits. Increasing theCo2+ ions concentration in the bath and the electrolyte jet speed all result in decrease ofgrain size and increase of cobalt content and microhardness. Coercivity and remanenceincrease with the increasing of Co2+ ions concentration and decrease with the increasingof electrolyte jet speed. The current density and electrolyte temperature has little effect onthe chemical composition of deposition layer, but they fine grain size obviously and resultin an increase of microhardness and decreases of coercivity and remanence in the alloy.Few additives such as saccharin in the electrolyte also favor the formation of the finergrains and an increase of microhardness in the alloy deposits, its highest microhardnesscan be over HV600. But it results in decrease of coercivity and remanence in the alloydeposits. The Co-Ni alloys of different cobalt content from 0 to 80wt.%Co in the alloycan be prepared by the choice of right depositing parameter. The alloys with low Coconcentration exhibit single phase of face-centered cubic (fcc) structure;With the Coconcentration over 60.39%, the alloys are composed of face-centered cubic (fcc) phaseand hexagonal close-packed (hcp) phase. The thermal stability of electrodepositednanocrystalline Co-Ni alloys has been examined. The peak temperature and activationenergy of grain growth were found to be higher for nanocrystalling Co-Ni alloys than thatobserved for nanocrystalline Ni and Co. Its peak temperature, heat released enthalpy andactivation energy of grain growth were estimated to be 389.9 °C, 14.26 J·g-1 and 269.1kJ·mol-1 respectively. These indicate an increased thermal stability for 5.1 nmnanocrystalline Co78Ni22 alloys compared to nanocrystalline Ni and Co. The relation ofmicrohardness and grain size for 5.1nm nanocrystalline Co78Ni22 alloys annealed can beexpressed according to the relation Hall-Petch within 5~40 nm, whereas, Hc was found tohave a peak value at 15.9 nm. Hc increases with the increasing of grain size from 5.1 nmto 15.9 nm and decrease from 16.6 nm to 34.4 nm.