Study On The Microstructures And Properties Of Electrodeposited Nanocrystalline Ni And Ni-Co Alloy

Nanotechnology is the technology referring to the research of the structure, properties and interactions of materials at the range of 1nm-100nm, including atoms and molecule manipulations, which is the science of cutting-edge and cross-cutting fields, developing progressively in the last 20 years. Nanocrystalline materials are an important direction of the development of nanotechnology. Nanocrystalline materials are single-or multi-phase polycrystals with grain sizes of nanometer region (typically less than 100nm in at least one dimension), which are characterized by small grain size, high defect density, a larger volume percentage of grain boundary.With the development of science and technology, the demand for materials is becoming higher. Nanocrystalline materials, especially high-quality bulk nanocrystalline materials, are increasingly being used in microelectronics, biotechnology, optoelectronics and other fields because of their excellent performance, becoming top research of the 21st century. In recent years, the fabrication methods of bulk nanocrystalline materials have been an important area of nanocrystalline materials research. More and more fabrication methods of bulk nanocrystalline materials have been developed since H. Gleiter, etc, fabricated samples with nanocrystalline by the method of inert gas condensation in the 80's of the 20th century. For example, mechanical milling, sever plastic deformation, crystallization of amorphous phases, electrodeposition, and so on. Compared with other fabrication methods of bulk nanocrystalline materials, electrodeposition has its own advantages,: (1) The sorts of nanocrystalline metals, alloys and composite materials which can be fabricated by electrodeposition are more; (2) The main motivation of electrodeposition crystallization is electric potential, which can be operated artificially, and the whole process can be monitored by computer easily. Therefore, there is smaller difficulties in techniques and it is prone to realize the transition from lab to factory; (3) Electrodeposition is operated at room temperature and normal pressure, avoiding the thermal stress in the heart of materials at high temperature; (4) the epitaxial growth of deposited atomics at unit crystal substratum is easy and the epitaxial growth layer can be obtained at the components with large size and complex shape. Therefore, it has prospective future to fabricated nano-materials by electrodeposition and many researchers pay more attention to it.So far, lots of studies have shown that, compared with the traditional coarse-grained structure materials, nanocrystalline metals own higher strength and hardness, higher wear resistance and other excellent properties, but lower ductility. What is more, nanocrystalline materials also show higher strain rate sensitivity. How to improve the ductility of nanostructured materials and revealing the mechanism of their plastic deformation becomes a hot issue for the researchers at home and abroad.In this experiment, we used improved Watts solution as the electrolyte, fabricated a number of nanostructured Ni with different grain size and an Ni-Co alloy with 1.7%wt Co by the direct-current electrodeposition technique. Microstructures and mechanical properties were extensively studied by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction, MTS-810 tensile testing system, etc. The main results are shown as follows:1. Six pieces of bulk Ni with with different grain size are fabricated by controlling the additive content of electrolyte and regulating the current density during the electrodeposition process. Microstructure analysis show that the grain size of Ni fabricated by electrodeposition decreased with saccharin content in the electrolyte increasing. Their average grain size decreases gradually from 257 nm to 16nm and the grain size distribution changed from a bimodal distribution to a wide distribution, and eventually to a narrow distribution. XRD analysis show that the growth orientation of the electrodeposited nc Ni gradually changed from a strong preferred (200) crystallographic texture to the isotropic structure (random growth) with their mean grain size decreasing. Tensile tests at RT show that there is an optimal grain size region for the electrodeposited nanocrystalline Ni (28-41nm) with a good combination of high ultimate tensile strength(1542?1788MPa) and enhanced elongation to failure(7.6?7.9%). In this grain size region, the natural dislocation-based deformation and their inhomogeneous microstructure induced an extra work-hardening plastic to maintain a greater plastic strain. However, when the grain size decreased to 22nm or 16nm, the strength and the ductility of nanocrystalline Ni show an obviously downward trend, which can be attributed to the transition in deformation mechanism sourced from the grain size-dependent competition among multi-deformation behaviors including the GB dislocation-based deformation, the partial dislocation based deformation and the GB deformation accomplished by the partial dislocation.2. Bulk electrodeposited nc Ni-24.7%Co alloy has an average grain size of about 18 nm and a narrow grain size distribution of 5-45 nm. XRD analysis reveals that the Ni-24.7%Co alloy possesses the single f.c.c phase structure. Tensile tests at RT show that the nc Ni-24.7%Co alloy has a good combination of high ultimate tensile strength (1813?2232 MPa) and enhanced plastic strains (6%?9.6%) over a wide strain rate range of 1.35×10-5-4.17×10-1s-1. The good ductility may be due to the increase in the strain hardening rate by adding the alloying element, Co, which would lead to a reduction in the stacking fault energy. The strain rate sensitivity, m value, and the average activation volume, V, of the electrodeposited Ni-24.7%Co alloy are about 0.029 and 13b3, respectively, which reveals that the dislocation motion should be the main deformation mechanisms for the nc Ni-24.7%Co alloy.