Mechanical Properties And Corrosion Resistance Of Electrodeposited Nanocrystalline Ni And Ni-Co Alloys

In this work, a surfactant-assistant direct-current electrodeposition technique wasproposed and exploited to synthesize bulk nanocrystalline Ni-Co alloys. Byreasonably controlling the deposition parameters (such as current density, platingtemperature, PH value), the content of additives and the content of CoSO47H2O inthe electrolyte, high-purity, fully-dense bulk NC Ni-Co sheets with different Cocontent were successfully fabricated. To explore the compositional and structuraleffects on the mechanical properties of NC metallic materials, nanoindentation testswere conducted on an Agilent Nanoindenter G200with a tip of Berkovich diamondindenter. Three kinds of NC Ni-Co samples, Ni-29.1%Co、 Ni-50.4%Co andNi-74.2%Co, were chosen for testing. Chemical composites, morphologies andmicrostructures of the samples before and after deformation and mechanicalproperties of NC Ni-Co were extensively studied by X-ray diffractmeter (XRD),transmission electron microscope (TEM), scanning electron microscope (SEM) andElectron Back Scattering Diffraction (EBSD-TKD). The corrosion-resistanceproperties of the Ni-Co alloys were also investigated. The main results are shown asfollows:1. NC Ni-Co alloys with varying Co content were fabricated via direct-currentelectrodeposition by adding different amount of CoSO47H2O in a modified Wattsbath. Three kinds of NC Ni-Co alloys with Co content of29.1%,50.4%and74.2%(weight percent) were obtained. NC Ni samples were also prepared for comparisontests. We find that the Co content significant affect the grain size and phase structureof Ni-Co alloys. Calculated by Scherrer’s formula and statistical TEM analysis, wefind Ni, Ni-29.1%Co, Ni-50.4%Co alloy have the average grain size of25nm,20nm, 15nm respectively. But the grain size of Ni-74.2%Co alloy increases sharply to70nm, and has a lot of twin structure. The XRD results reveal that Ni, Ni-29.1%Co,Ni-50.4%Co all have signal face-centered cubic (FCC) structure, and this is furtherverified by EBSD-TKD characterization. But it was found that Ni-74.2%Co alloy hasa face-centered cubic (FCC)+hexagonal close-packed (HCP) dual-phase structure.Compared with nanocrystalline Ni，Ni-29.1%Co alloy and Ni-50.4%Co alloy hasapparent (200) texture, illustrating that the addition of Co would promote thegeneration of (200) texture.2. Nanoindentation tests are performed on Ni-29.1%Co, Ni-50.4%Co andNi-74.2%Co alloys with loading rate from4×10-1s-1to1×10-2s-1. The obtainedhardness data exhibit an obvious Hall-Petch strength scaling, namely the finer grainsize, the higher hardness. In the loading regime, the load required for a given indenterdisplacement increases with increasing strain rate, indicating a pronounced strain ratesensitivity of hardness or flow stress of NC Ni-Co alloys, which can be evaluated bystrain rate sensitivity factor, m. It was also found that Ni-74.2%Co has larger m valuethat those of Ni-29.1%Co and Ni-50.4%Co alloys, despite the former has much largergrain size than the latter two. This can be rationalized by the existence of a largenumber of twin boundary and phase interface in the Ni-74.2%Co samples, whichmake dislocation movement concentrated near them in the process of plasticdeformation, rendering higher m value. By conducting nano-indentation creepexperiment, we found that during the holding regime, all these three kinds of Ni-Coalloy have very obvious creep deformation. The relaxation of unstable dislocationstructure in nanometer grain and nucleation of new dislocation is the main cause ofcreep. The Ni-74.2%Co alloy has a large number of twin boundary and HCP phase,which significantly enhance the creep deformation at the holding time.3. In the electrochemical corrosion test, nanocrystalline Ni and Ni-Co alloyscannot form passivation film in5wt.%H2SO4, but can form passivation film in5wt.%NaOH and3.5wt.%NaCl. Passivation membrane effectively reduces the Ni ions, Coions and electrons moving speed in the corrosive medium, slowing down the corrosion process. A continuous, compact passivation film is the key factor ofcorrosion resistant ability for nanocrystalline Ni and Ni-Co alloys. Grain size andmicrostructure of nanocrystalline Ni and Ni-Co alloys also has a certain influence onthe corrosion performance. The decrease of grain size leads to the increase of theproportion of the grain boundary defects, and thus the great increase in the corrosionprocess in5wt.%H2SO4. But it will present the opposite case in5wt.%NaOH and3.5wt.%NaCl. Ni-74.2%Co has a lot of twin boundary surface and the FCC, HCPmixed structure, make corrosion accelerated in acid medium. But in alkaline andneutral medium, the passivation film has more nucleation points, which slow downcorrosion process. Twin boundary has lower energy, which make grain boundary noteasy to lose electron.