Papers by Keyword: Internal Oxidation

Abstract: The aim of this paper is to present a model that predicts the transition from internal to external oxidation. This variant is based on the simultaneous resolution of the diffusion equations and the equilibrium equation that stems from the assumption of the local instantaneous thermodynamic equilibrium. It accounts for the possible formation of large precipitates fractions that may act as diffusion barriers. This effect is modeled by introducing a dependence of the diffusion coefficients upon the mass fraction of precipitates. As a counterpart, it is generally impossible to solve the non-linear equations of the model analytically. Thus, a semi-analytical and a finite elements models are presented.

Abstract: Fe-33Ni-19Cr alloy is the Ni-based alloy used at high temperature condition due to its excellent ability to form protective oxide layer at high temperature. Fe-33Ni-19Cr alloy was undergo a series of solution treatment process to vary the grain size of the alloy. The Fe-33Ni-19Cr alloy was solution-treated at 3 different temperature, namely 1000°C, 1100°C and 1200°C, for 3 hours soaking time, followed by water quench. The solution-treated alloys were then experienced an isothermal oxidation test at 900°C for 500 hours in laboratory air. The oxidized Fe-33Ni-19Cr alloy were characterized in terms of phase analysis and cross-sectional analysis using XRD and SEM-EDX to investigated the effect of different grain size alloy to the oxidation behavior. The solution treatment process was produced varies grain size of Fe-33Ni-19Cr alloy. The solution-treated Fe-33Ni-19Cr alloy at 1000°C exhibited the fine grain size, while solution-treated Fe-33Ni-19Cr alloy at 1200°C produced a coarse grain size. The oxidized Fe-33Ni-19Cr alloy recorded a formation of several oxide phases consists of Cr-rich oxide, Fe-rich oxide, Ti-rich oxide and spinel oxides structure. The cross-sectional analysis displays a several layer of oxide scales formed on the alloy surface with evidence of internal oxide penetration through the grain boundary area.

Abstract: The Cu-Al₂O₃ composite materials applying for electrode materials were successfully synthesized by internal oxidation process. The process included steps: producing solid solution α-Cu[Al] and oxidizing completely solid solution α-Cu[Al] as powders, then adding pure Cu powders and mixing in planetary ball milling, the mixtures were compressed into cylindrical samples, reduced at 750°C in 3h and sintered at 950°C in 2h in reduction atmosphere. The XRD patterns showed that total Al in Cu-matrix were completely oxidized in range of 20¸25h. After reducing and sintering process, the final products were Cu-Al₂O₃ composite materials. The results showed that micro-hardness, porosity and compress strength of Cu-Al₂O₃ materials increased with increasing of Al₂O₃ content, which was the result of dispersion of the fined Al₂O₃ grains in Cu-matrix. When Al₂O₃ content was unchanged, the porosity of received materials decreased with increasing of compress pressure. Whereas, the electrical conductivity of Cu-Al₂O₃ composite materials decreased comparing with pure Cu and it decreased with increasing of Al₂O₃ content.

Abstract: The cellular automata method offers a promising approach to describe diffusion and diffusion-controlled precipitation processes at high temperatures. During high temperature exposure, technical components like gas-turbine blades, furnaces, or exhaust systems, are operating in corrosive atmospheres. The resulting material-degradation processes are diffusion‐controlled, and corrosive species penetrate into the material leading to the formation of embrittling precipitates. Cellular automata (CA) represent distributed dynamical systems whose structure is particularly well suited to determine the temporal evolution of the system. In this study, it is shown that the model is able to consider diffusion, nucleation and growth aspects, interdiffusion between scales, and high diffusivity paths like grain boundaries. This has been demonstrated by applying CA to (i) nitrogen diffusion, (ii) internal intergranular oxidation of nickel-based alloy, and (iii) interdiffusion of a binary diffusion couple.

Abstract: Stainless steels can form a protective oxide layer when exposed to a high temperature oxidising environment, this protective layer forms a diffusion barrier and slows the oxidation of the alloys in harsh environments. This characteristic has made stainless steels one of the most commonly used alloys for high temperature industrial applications. In this work, a systematic testing procedure has been used to investigate the high temperature oxidation of two commonly used grades of stainless steel, 316 and 310. Samples of each alloy have undergone isothermal testing in air at 1050°C, 1150°C and 1250°C for a range of time periods up to 8h. The oxidation kinetics were also investigated using thermo-gravimetric analysis in air at the same temperatures for 8h. The oxide layers formed on the samples were characterised using X-Ray diffraction, Scanning electron microscopy and energy dispersive spectroscopy. Information derived from oxide layer characterisation was used to explain any differences between the two alloys in terms of oxidation rate and overall alloy performance in the high temperature environment.

Abstract: For the simulation of internal oxidation phenomena, different numerical approaches are proposed in the literature based on 1D finite differences or on explicit time integration schemes which need small time-steps leading to very long computation times. The aim of this paper is to detail a multi-dimentional finite element approach which is coupled with an efficient implicit time integration algorithm. The thermodynamic activities and the total mass fractions are both used as principal nodal variables. The use of finite elements rather than finite differences greatly facilitates the meshing of 2D and 3D bodies. Its implicit time-integration allows using much larger time-steps without any degradation of the results. An application is proposed for the modeling of internal oxidation of chromia for Ni-Xwt%Cr alloys at 950°C by considering the barrier effect of precipitates.

Abstract: A lot of technical processes require metallic materials which are able to withstand very high temperatures under extreme conditions. Examples are applications in glass industry, space technology and crystal growing. Application temperatures are in the range from 1100°C to 2300°C. Besides the extremely high temperature the materials are often influenced simultaneously by high mechanical loading and chemical attack. Due to their outstanding chemical stability, corrosion resistance and high mechanical strength the platinum group metals, in particular platinum, rhodium and iridium, are therefore ideal materials for high temperature use under extreme conditions. These metals are widely used in spite of their high prices. High temperature applications require high melting point metals, commonly strengthened by solid solution or oxide dispersion hardening. This paper reports e. g. on the development of oxide dispersion hardened platinum and platinum alloys manufactured by fusion technique. Furthermore the paper presents a comprehensive review of studies on platinum materials which facilitate the design of equipment used for high temperature applications under extreme conditions. Stress-rupture strength and creep behavior have been investigated in a temperature range between 1200°C and 2300°C. The results of the investigations can supply a basis to optimize materials selection for high temperature applications under extreme conditions.

Abstract: This paper prepared Cu-ZrO₂ composite by a modified internal oxidation in order to improve the high temperature strength of resistance Spot-welding electrode. The results showed that with the increasing of the cold deformation, Hardness increased while electrical conductivity decreases. Hardness reached 100HV and electrical conductivity was up to 86%IACS after cold drawn in 56%. With the increasing of annealing temperature and annealing time, Hardness of the composite decreased more slowly compared with the conventional Cu-Cr-Zr alloy and was higher than that of the Cu-Cr-Zr alloy after annealed above 700°C for 2h and at 600°C for 5h , which was attributed to the dispersion strengthening effect of the Zirconia particles. Thus, the Cu-ZrO₂ composite has good high-temperature stability and can be used for the electrode

Abstract: the behavior of internal oxidation of Ag-Cu-Zn alloy was studied at various internal oxidation temperatures for different internal oxidation time. The experimental results show that the short path diffusion, such as boundary diffusion and surface diffusion, is the main diffusion channel of oxygen in initial stage of oxidation, oxidation rate is fast, while the internal oxidation exponent n=1. Then the oxygen diffusion translated from short path diffusion into long path diffusion with the extending of internal oxidation time, and the oxidation rate was slowed down, while the internal oxidation exponent n=1.67. The action size is increased with the temperature increase. The diffusion activation energy of oxygen in initial stage of oxidation is lower than that in middle and late stage.

Abstract: A metallurgical method for preparing oxide nano-rod array structure was developed with internal oxidation of Ni(Al) solid solution obtained by aluminizing Ni with a pack cementation technique. The present method was applied in order to fabricate nano-rod array on the micro-channel surface of microreactor. A micro-channel with 1 mm in depth, 1mm in width and 10 mm in length was fabricated on the Ni disk by an electric discharge machining and electropolishing. The Ni disk with a micro-channel was annealed at 1300°C for 12 h in vacuum. The sample was aluminized by a pack cementation using pack powder mixture consisting of 10 mass% of Ni3Al, 88 mass% of Al2O3 and 2 mass% of NaCl at 1100°C for 12 h in a flow of argon gas. The sample was oxidized from 1000 to 1200°C for 6 h with the Co/CoO buffer. Ni matrix in internally oxidized zone was removed by electropolishing to expose nano-rods. Al2O3 nano-rod array located on the micro-channel surface was successfully fabricated with the proposed technique. Diameter and height of nano-rods were ranged from 100 to 300 nm and from 1.5 to 3 µm, respectively.