Fabrication And Research About The Mechanical And Corrosion-resistance Properties Of Electrodeposited Nanocrystalline Nickel

Nanocrystalline materials are single-or multi-phase polycrystals with grain sizes with nanometer region (typically less than 100nm in at least one dimension). NS materials usually are reported to have high strength and hardness, elevated strain rate sensitivity, low temperature superplasticity and limited plastic strain at room temperature. Nanocrystalline materials can be classified into several categories according to the nanostructure dimensionality.At present, interests on the NS materials are focused on the fabrication and improving and optimizing their mechanical properties of these materials. NS materials, which can be defined as materials with crystal size less than 100 nm in dimension, are synthesized by either"top-down"or"bottom-up"processes. The"top-down"methods for processing NS materials involve starting with a bulk solid and building nanostructure by structure decomposition. The typical processes of"top-down"are mechanical milling and sever plastic deformation. The"bottom-up"approach, such as inert gas condensation, electrodepostion, crystallization of amorphous phases, starts with atoms, ions, or molecules as"building blocks"and assemble bulk materials from them. Compared with other fabrication methods for bulk NS materials, the advantage of electrodeposition are low cost, industrial applicability, simple operation, and versatility. Therefore, many researchers pay more attention on the electrodeposition and use the electrodeposited NS materials as the model materials to study the real mechanical mechanism in nanoscale.In this paper, nanocrystalline nickel was obtained by a direct current electrodeposition method from the Watt electolye bath. Based on the eletrodeposition method, component of electolye bath, PH of electolye bath, temperature of plating bath, current density and content of additive are fitted together, we changed these parameters of these processes and obtained the optimal nanocrystalline nickel. The crystallographic structure was analyzed by using X-ray diffractometer. Microstructures of the nanocrystalline nickel were observed with a transmission electron microscope. The surface texture of deformed specimen was studied by general area detector diffraction system (GADDS).1. The fully dense (99.4±0.5% of theory density) of nc Ni was obtain in this paper. The electrodeposited nc Ni is characterized by a mean grain size of about 40nm, a strong preferential orientation along the {200} planes, and a microstrain 0.39%. There are many grain clusters with the size of about 150-300nm. The clusters are surrounded by nanograins with the size of about 10-20nm, or by fiber-like structures. The nc Ni has the stress about 1200MPa at a strain rate of 1.04×10-3s-1 and RT. This nc Ni exhibits prevalent high tensile ductility (7.5-8.3%). And there exists the obvious necking region. The other reported NS or nanocrystalline (nc) Ni usually had limited plastic strains less than 3%. The strain rate sensitivity, m value, of our electrodeposited Ni is about 0.012, which deduce that the dislocation movements should be responsible for the plastic deformation. Furthermore, The surface relief morphologies, like the superplasticity deformation, are observed on the deformation region surfaces by SEM. In addition, the XRD detections on the necking regions shown that the original (200) texture is weaken with increasing the strain, which means that the grains or grain clusters rotation would occur during the plasticity of the NS Ni.2. The nc Ni coating was direct-current electrodeposited on the AZ61D magnesium alloy substrate aimed to improve its corrosion resistance using a direct electroless plating of nickel as the protective layer. As comparison, two electroless Ni coatings on the magnesium alloy with different thinness were also presented in the paper. The nc Ni coating had an average grain size of about 40nm and an evident {200} preferred texture, too. And it showed more compact than the electroless Ni deposits. The electrochemical measurements showed that the nc Ni coating on the magnesium alloy had the lowest corrosion current density and most positive corrosion potential among the studied coatings on the magnesium alloy. Furthermore, the nc Ni coating on the AZ61D magnesium alloy exhibited very high corrosion resistance than the electroless Ni coatings with the same thickness in the rapid corrosion test illustrated in the paper. The reasons for an increase in the corrosion resistance of the nc Ni coating on the magnesium alloy should be attributable to its fine grain structure and the low porosity in the coating. The hardness of the nc Ni coating was about 580VHN, which was far higher than that (about 100VHN) of the AZ61D magnesium alloy substrate. Therefore, the nc Ni coating with a combination of high corrosion-resistance and high hardness would be expected to enlarge the application of the magnesium alloy.