Synthesis And Researches On Mechanical Properties And Corrosion Behavior Of Electrodeposited Nanocrystalline Ni-Co Alloys

As a new branch of the twenty-first century, Nano-science has been widely focused by researchers in various fields. Nano-materials have excellent macroscopic properties and extensive applications because of their special microstructure. Nanocrystalline (nc) materials with high grain boundary volume fraction and very fine grains show mechanical and corrosion properties different from the coarse-grained (CG) materials. The research of mechanical behavior is mostly on the discussion of grain size and stain rate effects and changes in deformation mechanism. Computer simulation has been adopted to study the changes of material mechanical parameters and deformation mechanism with dislocation-based deformation to grain boundary deformation. However, due to the lack of modified details and adequate visual experiments, simulation results only serve as valuable guidance. Corrosion-resistance research was mainly concentrated in the electrochemical corrosion behaviors. Presently, It is an disputable problem that nano-materals in different corrosive media show the different corrosion mechanism. Many results have been got in the research of pure nano-metals, but for binary and multiple nanocrystalline alloys, the results are not sufficient. The difficulty of preparation of ideal nanocrystalline alloys largely limited the progress of nano-science research.In the literature, perfect bulk nanocrystalline Ni-Co alloys were prepared by electrochemical deposition. The whole experiments consist of three stages: Electro-deposition of nano-materials is the first stage. We synthesized a series of Ni-Co alloys with different cobalt content through appropriate deposition process in optimal Watt electrolyte. The average grain size of these alloys are between 15³0nm,which are true nano-structured materials. The second stage is preparation of alloy samples, microstructure observation and performance testing. We used X-ray diffraction and transmission electron microscopy to detect the microstructure, and tested tensile and corrosion properties were tested by MTS, potentiodynamic polarization test, electrochemical impedance spectroscopy(EIS). The third stage consists of treatment of data and performance analysis. In order to seek the changes of deformation, we have analyzed tensile performance and fracture morphology. At the same time, results of electrochemical corrosion testing of Ni-Co alloys with different cobalt content in different solution were adopted to analyze the corrosion mechanism. The main conclusions are shown as follows:(1). We synthesize a kind of high density Ni-13%Co alloy with average grain size of 17nm by direct current electrodeposition., and test tensile properties at different strain rates. Another four kinds of Ni-Co nano-alloys were fabricated by controlling the content of CoSO4 in electrolyte and their corrosion resistance behavior were researched according to potentiodynamic polarization and EIS measurements.(2). As a kind of ideal bulk nanostructured materials, Ni-13%Co alloy shows a strong (200) plane texture of F.C.C. structure with a narrow grain size distribution and little impurity. The addition of element Co increases the tensile strength because of solid solution hardening effect and decreases the stacking fault energy (SFE) of pure nickel , which improves the dislocation activity to increase strain hardening capacity. Finally, material ductility becomes better. With the increase of strain rate (from 1.04×10^-5s^-1 to 1.04s^-1), the tensile strength increases from 1819MPa to 2247MPa, and ductility is from 9.2% to 8%. In general, the alloy shows good mechanical properties and rate sensitivity. The fracture surface morphology shows dimple structure. At high strain rate, the clear dimple structure and tearing trace of fracture surface indicate that dislocation deformation mechanism exists. At low strain rate, grain boundary deformation participates in the slow plastic deformation to increase ductility. In addition, the high m value and the small activation volume v also prove that materials deformation is accommodated by dislocation movement and grain boundary mechanism.(3). Ni-Co alloys with 0⁶4% Co were synthesized In the modified Watts bath. The average grain size of alloys are between 15-30nm. The introduction of Co can diminish the grain size to some extent. When the content of Co increases to 64%, dual phase F.C.C.+H.C.P. appears. Passive phenomenon and low corrosion current density of the alloy sample in 3% NaCl and 10% NaOH solution reveal good corrosion resistance. There is obviously no passive process of the samples in H₂SO₄ solution. It is precisely that alloys with different composition show contrary corrosion trends in different solution because of passive film. In the Nyquist impedance plots of samples in H₂SO₄ solution, we obtain the inductive loop in low frequency region, which maybe relates to the adsorption of middle products of anode dissolution reaction.