Microstructures And Mechanical Properties Of Electrodeposited Nanocrystalline Nickel

Nanocrystalline materials are single-or multi-phase polycrystals with grain sizes of nanometer region (typically less than 100nm in at least one dimension). Nanostructured (NS) materials are usually reported to have high strength and hardness, elevated strain rate sensitivity, low temperature superplasticity and limited plastic strain at room temperature. As the development of the microelectromechanical systems (MEMS), nanoelectromechanical systems (NEMS), integrated circuits, and micro- and nanoscale devices, it is essential to fabricate novel nanostructured (NS) materials by simple methods and undertake extensive studies on their microstructures and mechanical properties. Recently, interests on the NS materials are focused on the fabrication and improving and optimizing their mechanical properties of these materials both domesticly and externally. Bulk nanocrystalline (NC) materials which are composed of equiaxed grains, 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, electrodeposition, 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, electrodeposition has its own distinct advantages: (1) the sorts of nanocrystalline metas, alloys and composite materials which can be prepared by electrodeposition are larger; (2) the electric potential, the main motivation of electrodeposition crystallization, can be operated artificially and the whole process can be monitored by computer easily. Therefore, it has smaller difficulties in techniques and is prone to realize the transition from lab to factory; (3) it is operated at room temperature and normal pressure, which avoids the thermal stress in the heart of materials at high temperature; (4) the epitaxial growth of deposited atomics at unit crystal substratum is easier and the epitaxial growth layer can be obtained at the components with large acreage and complex shape. Therefore, it has prospective future to prepare nano-materials by electrodeposition and many researchers pay more and more attention to it.In this work, a surfactant-assistant electrodeposition technique was proposed and used to fabricate NS and NC Ni. Microstructures and mechanical properties were extensively studied by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction, MTS tensile testing system, etc. The main results are shown as follows:The layered NS Ni with the modulated hardness on the cross-section is fabricated by adjusting the content and supplement period of the electroplating additive-1,4-butenediol during the electrodeposition process. TEM observations show that the layers consists of the nano-sized crystalline grains (with an average grain size of 18 nm) and the ultra-fine crystalline grains (with the grain size ranging of 100-500 nm,average of 350nm) alternately. The layers of high and low hardness values correspond to the small and large grains layers, respectively. The mechanical properties of the layered NS Ni are located between that of the 18 nm grain sized Ni and that of the ultra-fine grained Ni (with an average grain size of 350 nm), where the strength was about 1130 MPa and the plastic strain was about 5.7%. It is believed that the hard layers in the layered NS Ni impart high strength and the soft layers stabilize the plasticity. However, it is found that the mechanical properties of the layered NS Ni composed of orderly mixture of large and small grains is worse than that of the reported bimodal grain sized Ni composed of randomly mixture of large and small grains. The reason for this might be that the incompatible strains between hard and soft layers lead to the early fracture of the layered Ni.Electrodeposited NS Ni has an average grain size of 37 nm from the respect of numbers and 65nm from volume friction with a wide grain size distribution of 10-160nm. This special microstructure with broad grain size distribution imparts the NC Ni the optimal mechanical properties. It is proved by the tensile experiment that the as-deposited NC Ni exhibits both high ultimate strength of 1440-1916 MPa and enhanced ductility of 5.6-11.3% in comparison with the previous reported NC and NS Ni. It shows remarkable high strain rate sensitivity (m=0.045) and small flow stress activation volume (11 b3) when deformed at low strain rate range. The resulted high m value can delay he early onset of necking and an enhanced ductility is obtained. The fracture surface morphologies of the NC Ni are different at different strain rates. At high strain rate, the typical ductile fracture morphology with evident dimples was shown. In addition, we can see clearly some large and deep dimples embedded in the flat surface with many small dimples, which is corresponding to the broad grain size distribution of the present NC Ni. However, at low strain rate, the fracture morphology evidently changes. Many smooth hunches are clearly seen and the dimples morphology appears to be indistinct. Such strain rate dependence of the fracture surface morphology was firstly observed in the NC Ni.