Study On Electrochemical Fabrication, Microstructures And Properties Of Fe-Ni Alloy Foils

Due to their magnetic properties and dimensional stability, Ni-Fe binary alloys have been fabricated by researchers using different techniques, such as sputtering, ball milling and electroplating. Compared with other methods, the electrodeposition method allows for mass production of Ni-Fe alloys at a low cost. Moreover, it provides the possibility of preparing Ni-Fe alloys with different structures. For example, Ni-Fe alloys with nanosized grains can be easily prepared using this method. Also, this method enhances the properties of the alloys, such as improved electrical resistivity and hardness, as well as corrosion resistance. Recently, the study of electrodeposition of nanocrystalline Fe-Ni alloys and their properties is focused on. But, proposed plating baths are not appropriate for a continuous deposition of Fe-Ni foils. In this dissertation, two baths being suitable for the continuous electrodeposition system are proposed to fabricate Fe-Ni alloy foils.1. Electrodeposition of Fe-Ni alloy foils from a citrate bathIn this dissertation, the effects of operating parameters on Fe-Ni alloy deposition in a citrate bath are studies in details. The results show that it belongs to an anomalous codeposition category, that is, the less noble metal Fe is deposited preferentially to the more noble Ni. During Fe-Ni alloy codeposition, the bulk concentration of Fe2+and the rotating speed of wrok electrode all affects remarkably the deposition rate of Fe. Increasing these two parmeter values will result in the increase of the iron content in Fe-Ni alloy, becase of increasing the Fe2+ion amount in the surface of work electrode and consequently accelerating the electrodeposition rate of Fe.Citric acid plays the role of pH buffering in solutions, hence, the hydrogen evolution reaction (HER) increases and the current efficiency decreases. Also, it is a complexing reagent of Fe3+, preventing Fe(OH)3from depositing. So, the concentration of citric acid selected is comparative with the Fe (including Fe2+and Fe3+), taking current efficiency and electrolyte stability into account. Moreover, citric acid and M2+(M=Ni, Fe) can also form complexs, which should discharge to metal atoms by a step process. Boric acid does not take a role pH buffering druing Fe-Ni codeposition. However, it can not only inhibit Fe-Ni deposition by means of absorbing on the elctrode surface, but also suppress HER. Electrochemical impedance spectroscopy and polarization curves proved this point.When we studied the electrodeposition of Ni on the Fe substrate, a catalyzed deposition process of Ni was accidentally found. This phenomenon implies that during Fe-Ni codeposition Ni2+ions are ready to reduce on the Fe atoms, accordingly forming Fe-Ni alloys.Throught varying the plating parmeters, bright Fe-Ni alloy foils with different iron contents (Fe%) are fabricated in the citrate bath. If the Fe%exceed60%, Fe-Ni alloy deposits will burst and even become powder. Fe-Ni alloy foils (Fe%<47) exhibit a fcc phase region,47<Fe%<60a mixed phase with fcc and bcc, and60<Fe%a bcc phase region. The deposits with fcc phase present a preferred orientation of (111) reflection and (110) for bcc alloy. At Fe%<47%, grain size of Fe-Ni alloy decreases with Fe%, from11.4nm to5.3nm, however, over47%increases rapidly with Fe%due to alloy phase transition.2. Electrodeposition of Fe-Ni alloy foils from a fluorborate bathCompared with the above citrate bath, the fluorborate bath is more appropriate for the continuous deposition of Fe-Ni alloy foils, because not only it can dissolve nickel and iron powder so fast as to to compensate the electrolyte during Fe-Ni electrodeposition, but also fast deposition rate (200um/h) was obtained in the bath.In the bath, the anomalous codeposition behavior of Fe-Ni alloy is aslo observed. Just like in the citrate bath, the bulk concentration of Fe2+and the rotating speed of wrok electrode are still key important parameters. Due to causticity of the bath, the anodic linear sweep voltammetry (ALSV) technology can utilize to analyze the composition of Fe-Ni crystals, and to further understand Fe-Ni alloy codeposition. At the same time, other conventional electrochemical technologies are used to study the process of Fe-Ni alloy electrodeposition. The results show that these processes occurred under mass transfer control, associated with nucleation and growth process. The nucleation and growth of Fe-Ni alloy is different from that of the single metal (Ni or Fe).Two nucleation and growth processes occurred during Ni-Fe alloy codeposition. That is, there was a nucleation and growth process of Ni-Fe alloy on Ni-Fe clusters, due to a change of the Ni-Fe alloy composition and phase. When adding sodium saccharin into the bath, the obtained Fe-Ni alloys have different ALSV curves, likely due to Fe-Ni-S codeposition.The obtained Fe-Ni alloy foils were smooth, bright and flexible, whose iron content can be up to75%. The Fe-Ni alloy phase also changes gradually from FCC to BCC with the increase of iron content, but the Fe%range for mixed phase is wider than that of Fe-Ni alloys obtained in the citrate bath.3. Properties of Fe-Ni alloy foilsMagnetic and mechanical properties of Fe-Ni alloy foils are studied, and the results indicate that they have better character than that prepared using other methods. Tensile strength is up to1.68GPa The best magnetic properties are:maximum permeability (μm)22468Gs/Oe, coercivity (He)0.2Oe, saturation flux density (Bs)1.5T. All Fe-Ni alloys all undergo a ductile-brittle-ductile transformation process as the annealing temperature increases. The brittlement temperature range for these alloy become narrow along with the increase of Fe%, and vanishs after ductile transformation at high temperature, indicating that the brittle transformation is not reversible.Ni-Fe foils with micro-porous or surface-relief-grating structures are electrochemically fabricated using a template method. They can be used in catalyst carriers, anticounterfeiting and solar applications.