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5 However, the only parameters that can be determined via Fick's equation are the pre-exponential factor ( D 0) and the activation energy ( E). Traditionally, the concentration gradient was seen as the driving force behind segregation and diffusion equations such as that of Fick were used to derive segregation models. Segregation is defined as the exchange of atoms between the surface layer and bulk solid until equilibrium is reached. 4 With the advent of surface analysis techniques, these studies became experimentally viable and indispensable. 1≣ Segregation studies thus had their origin in finding solutions to temper embrittlement caused by grain boundary segregation and it has been extensively investigated by pioneers such as McLean. This phenomenon is known as segregation and it may be a significant hurdle for manufacturers, as it can cause failure of components that were heated, welded or operating at elevated temperatures, to name a few examples.
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At such high temperatures, elements of low bulk concentration (even a few ppm) have high mobilities and tend to diffuse to the surface and grain boundaries. The interaction energy between molybdenum and nitrogen was ΔΩ Mo-N = ≡9 ☓ kJ mol ≡.ĭuring the manufacturing process of automotive components, metallic alloys are often heated for prolonged periods. The segregation parameters for nitrogen in the ternary system were D 0,N = 2.8 ×10 ≠☓ m 2 s -1, E N = 323 ± 43 kJ mol ≡ and Δ G N = ≡9 ± 3 kJ mol ≡. The segregation parameters for molybdenum in the ternary system were determined as: D 0,Mo = 1.9 ×10 ≤☑ m 2 s ≡, E Mo =271 ± 11kJ mol ≡ and Δ G Mo= ≣2 ± 5 kJ mol ≡. These segregation parameters were then used as initial values to fit the experimental data of the ternary system. For the binary system the segregation parameters for molybdenum were: D 0,Mo = 2.4 ×10 0☑ m 2 s ≡, E Mo = 323 ± 16kJ mol ≡, and Δ G Mo = ≣8 ± 5 kJ mol ≡. The segregation profiles were fitted with the modified Darken model and the segregation parameters, namely, the pre-exponential factor ( D 0), activation energy ( E), segregation energy ( Δ G) and the interaction energies ( Ω Fe-Mo, Ω Fe-N, and Ω Mo-N) for molybdenum and nitrogen, were determined. The segregation profiles of the two systems were acquired at constant temperatures in the range 797≨88 K using Auger electron spectroscopy (AES).
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Exposing the binary crystal to nitrogen at high temperatures, the molybdenum segregated to the surface. A novel approach was undertaken to minimize the number of unknown variables and thus reduce the calculation time needed for the ternary system, by first analysing the segregation behaviour of molybdenum in a binary system that was exposed to a nitrogen ambient. Ternary segregation systems are considerably more complex than binary systems in the sense that there are seven unknown segregation parameters to determine, as opposed to three for a binary system. In this study, the co-segregation of molybdenum and nitrogen to the (100) surface of an iron single crystal was investigated by studying a Fe≣.5wt%MoN single crystal. IINational Metrology Institute of South Africa, Private Bag X34, Pretoria 0040, South Africa
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IDepartment of Physics, University of the Free State, P.O. It also reveals competitive deposition between the insulating film and metal ions.The effect of nitrogen on the co-segregation with molybdenum in a Fe≣.5wt%MoN(100) single crystal Impedance analysis specifically reveals presence of an adsorbed insulating film on cathode surface, contributed by MSA or water. It also indicates close deposition potentials for each metal ion. The CV and impedance analysis confirm chelation of Ag and Cu with TU and absence of such chelation with Sn ions. On the other hand, the microstructure is found to be increasingly refined with increasing TU content. The deposited films have close to eutectic composition with slightly higher Cu content for all the TU concentrations. A study of the bath behaviour at TU concentrations in the range 0.06–0.2 M is undertaken with the help of elemental and microstructure analysis, cyclic voltammetry (CV) and impedance analysis. The bath contains thiourea (TU), which functioned as an effective chelating agent in controlling the bath stability as well as the elemental and microstructural properties of the deposited film. We have developed a methane sulfonic acid (MSA) based ternary electrolytic bath for co-deposition of the eutectic Sn–Ag–Cu films.