Synthesis And Tensile Behavior Of A Bull Electrodeposited Nanocrystalline Ni-Fe Alloys

Nanocrystalline (nc) materials are characterized by a microstructural length or grain size in the range of 1~100 nm. Nanocrystalline materials have been the subject of widespread over the past couple of decades with significant advancement in their understanding especially in the last few years. As the grain size is decreased, an increasing fraction of atoms can be ascribed to the grain boundaries. Nanocrystalline materials exhibit various unique chemical, physical or mechanical properties such as increased strength/hardness, improved toughness, reduced elastic modulus and ductility, enhanced diffusivity, higher specific heat, enhanced thermal expansion coefficient, and superior soft magnetic properties in comparison with conventional polycrystalline materials. Nanostructured materials provide us not only with an excellent opportunity to study the nature of solid interfaces and to extend our understanding of the structure-property relationship in solid materials down to the nanometer regime, but also present an attractive potential for technological applications with their novel properties. Therefore, it is essential to fabricate novel nanocrystalline materials by simple methods and undertake extensive studies on their microstructures and mechanical properties.The mechanical properties of nanocrystalline metals have been a main research topic in materials science communities, which involve quantificational descriptions of macro-mechanical behavior and qualitative analyses of micro-deformation mechanisms of nc metals. Nanocrystalline materials usually are reported to have high strength and hardness, elevated strain rate sensitivity, low temperature superplasticity and limited plastic strain at room temperature. At present, interests on the nanocrystalline materials are focused on improving and optimizing their mechanical properties and revealing the strain rate sensitivity and mechanical mechanism of these materials.Based on the above points, the following research work was conducted in this doctoral dissertation and the resultant conclusions are presented as the following:1. A bull nanocrystalline Ni-Fe alloys were synthesized successfully by direct current electrodeposition technique on the mild steel samples with optimal parameters of the composition and pH value of plating solution, electrodeposition temperature, current density and additions etc.2. the microstructure of this nanocrystalline material can be observed and analyzed by EDS, XRD and TEM. It was demonstrated that the Ni-20.78wt.%Fe alloy has no detectable porosities or voids and the grains of this material are fine with the average grain size of about 16nm for number fraction and 27nm for volume fraction.3. Tensile test performed at wide strain rate range (1×10-5s-1~1s-1) and room temperature on MTS-810 indicated that high ultimate strength of 1939MPa and good ductility (δEF) of 9.3% with a relatively unapparent strain rate sensitivity. As the strain rate was raised from 1×10-5s-1 to 1s-1, the strength increased not so much, while the tensile elongation was almost invariable. Both at high and low strain rate, the typical ductile fracture morphology with uniform dimples and several smooth hunches (600nm) was shown.4. The relatively complex fracture morphology of the nanocrystalline Ni-Fe alloy is relative with the mixed distortion mechanism comparing with nanocrystalline Ni. Effectively grain boundary activity involving atomic shuffling and diffusion could stabilize the ability of continuous deformation result in relatively good and almost invariable ductility.