Papers by Keyword: Interdiffusion

Abstract: In this chapter, we review the Nernst-Planck equation describing the cation interdiffusion coefficient, the two tracer cation diffusion coefficients and the thermodynamic factor in ionic compounds and how it is related to the analogous Darken-Manning relationship in binary alloy systems. We make use of the Onsager flux equations of non-equilibrium thermodynamics to rigorously address the problem. The recently found correction factor to the Nernst-Planck equation is analyzed visually by means of computer simulation

Abstract: In this work, the oxidation behavior of Ti₂AlN coating deposited on nickel super alloy IN718 was investigated. The coating was obtained by DC-magnetron sputtering at 540°C and subsequent vacuum annealing at 800°C for 1h. The coating morphology as well as the chemical composition were analyzed using SEM, EDS and XRD, respectively. The XRD results revealed that the coating mainly composed of Ti₂AlN MAX phase. Cycling oxidation was performed at 700 °C and 800 °C in air. The XRD and SEM results proved the interaction between substrate and coating and the formation of the quaternary Ti₃NiAl₂N phase during oxidation at the interface. Due to the Ni diffusion towards the surface, the Ti₃NiAl₂N phase grew continuously and the Ti₂AlN phase decomposed gradually resulting in a coating failure. The results indicate that the oxidation behavior of the coating is essentially controlled by the interdiffusion of Ni from substrate into the coating.

Abstract: The self-or tracer diffusivity of one component in a binary alloy is often required when there is knowledge of the other component’s self-or tracer diffusivity and the interdiffusivity (and the thermodynamic factor). In the present paper, this problem is addressed for the random alloy model by applying three possible approximations having different levels of accuracy: Darken (low level of accuracy), Manning (medium level of accuracy) and Moleko, Allnatt and Allnatt (MAA) (high level of accuracy). There are unexpectedly large differences between the results of these approximations that sometimes are reflected in the high sensitivity of the vacancy-wind factor to the level of approximation. Generally, for the application of Manning and the MAA approximations, it is found that there is a difference in the number of self-diffusivity roots depending on whether the tracer diffusivity is available for the faster diffuser or for the slower diffuser and depending on how close the composition is to the forbidden (according to Manning’s description) region. Provided that the interdiffusion coefficient (divided by the thermodynamic factor) is greater than the available self-diffusion coefficient multiplied by its complementary composition, the application of the Darken approximation always results in one self-diffusivity root.

Abstract: Using solid-to-solid couples investigation, this study characterized the reaction products evolved and quantified the diffusion kinetics when pure Mg bonded to AA6061 is subjected to thermal treatment at 300°C for 720 hours, 350°C for 360 hours, and 400°C for 240 hours. Characterization techniques include optical microscopy, scanning electron microscopy with X-ray energy dispersive spectroscopy, and transmission electron microscopy. Parabolic growth constants were determined for γ-Mg₁₇Al₁₂, β-Mg₂Al₃, and the elusive ε-phase. Similarly, the average effective interdiffusion coefficients of major constituents were calculated for Mg (ss), γ-Mg₁₇Al₁₂, β-Mg₂Al₃, and AA6061. The activation energies and pre-exponential factors for both parabolic growth constant and average effective interdiffusion coefficients were computed using the Arrhenius relationship. The activation energy for growth of γ-Mg₁₇Al₁₂ was significantly higher than that for β-Mg₂Al₃ while the activation energy for interdiffusion of γ-Mg₁₇Al₁₂ was only slightly higher than that for β-Mg₂Al₃. Comparisons are made between the results of this study and those of diffusion studies between pure Mg and pure Al [1] to examine the influence of alloying additions in AA6061.

Abstract: The Nernst-Planck flux formula is used in Darken's method to obtain the interdiffusion fluxes. The effective interdiffusion potentials, derived for the independent components in the system, allow obtaining the symmetrical matrix of the interdiffusion coefficients. The transport coefficients for 2, 3 and r-component system are presented. Interpretation of obtained matrixes in the light of Onsager's theory of irreversible thermodynamics is shown. Equation for the entropy production in the interdiffusion process is displayed. The presented approach allows calculation of entropy production during interdiffusion, as well as formulating Onsager's phenomenological coefficients for the interdiffusion in an explicit form, a form which is directly correlated with the mobilities of the atoms present in the system.

Abstract: 800x600 Intermetallics are compounds of two metals or of metal(s) and semimetal(s). Their structures are usually different from those of the constituents. Some intermetallics are interesting functional materials, others have attracted attention as high-temperature structural materials. We remind the reader of some fundamentals of solid-state diffusion and to the major techniques for tracer diffusion measurements, interdiffusion studies and the growth kinetics of layers in solid diffusion couples. Starting from self-diffusion, which is the most basic diffusion phenomenon in any solid, the paper covers the main features of diffusion in binary intermetallics from the systems Cu-Zn, Ni-Al, Fe-Al, Mg-Al, Ni-Ge, Ni-Ga, Fe-Si, Ti-Al, Ni-Mn, Mo-Si, Co-Nb and Ni-Nb.. We illustrate the influence of phase transitions on diffusion and point out some common features of diffusion in intermetallics. We discuss in detail diffusion in silicides of iron, molybdenum and of silicides of refractory metals. We also consider aluminides of iron, nickel, and titanium and in the aluminium-magnesium system. We consider diffusion in intermetallics of the cobalt-niobium and nickel-niobium system and in in the Nb-Sn and V-Ga systems. We finish with some remarks about grain boundary diffusion in intermetallics. Normal 0 21 false false false UK X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Calibri","sans-serif";}

Abstract: The Kirkendall effect appears due to the unbalanced diffusion fluxes causing the vacancy flux. There are several numerical methods that allow to predict the position of Kirkendall plane after the diffusion couple annealing. In this work for the first time the entropy density distribution is used to estimate the trajectory of the Kirkendall plane. The entropy density distribution is calculated with use of the bi-velocity method, which combines: (1) the volume continuity, (2) the conservation of mass, (3) momentum and (4) entropy-density. The method is applied to simulate the diffusion in Ni-Pd diffusion couple.

Abstract: The objective of this work was to examine the microstructure, interdiffusion of elements, and hardness of joining sialon to AISI 420 martensitic stainless steel using diffusion bonding process. These materials were diffusion bonded at 1200^oC for one hour under 20 MPa in a vacuum of 2.1x10^-6 Torr. The microstructural analyses showed that joining sialon to nitrided steel produced thinner reaction layers and no gap or crack were formed on the sample. Gaps were produced in joining sialon to as-received steel. From the elemental analyses, alumina and iron silicides were formed at the interface layer of sialon/as-received steel joint. Alumina and smaller amount of silicides were detected at the interface layer of sialon/nitrided steel joint. Diffusion layer and parent steel of the sialon/nitrided steel joining contained nitrides. The hardness test across the joints indicated that reaction layers possessed intermediate hardness between sialon and steel. The layers contributed to ductility of the joint that help to attain the joint.

Abstract: In turbine engines, Ni or Co based alloys are used at high temperature, either as base materials, superalloys, or deposited on the surface of superalloys, as coatings. In the present study, two different MCrAlY overlay coatings, Ni and Co based, on a Ni based superalloy IN792 were aged for different times in air at three temperatures, 900°C, 1000°C and 1100°C. The aging processes were simulated by using DICTRA software by focusing on the interdiffusion behavior in the superalloy-coating systems. The results of simulation captured the main microstructural features observed and were used to analyze the diffusion behavior of alloying elements and the corresponding microstructure development. It was found that coating composition and temperature affected significantly the microstructure near the superalloy-coating interface, and their relations were mapped as a summary.

Abstract: Nanoshell formation has been studied experimentally in Ag/Au and Ag/Pd systems in a hemispherical geometry at different temperatures. The void formation in these systems is the result of pure Kirkendall-porosity formation, because it is caused mainly by the inequality of the intrinsic atomic fluxes and other effects (e.g. stresses), inevitably present during nanoshell formations in solid state reactions (oxides, sulphides), can be less important or can be neglected. The kinetics of the process was followed by Transmission Electron Microscopy. Both the growth and shrinkage regimes of the process were observed at the same temperature and even the temperature dependence of the characteristic time (t_cr) describing the crossover of the two different regimes was observed. We succeeded to show that t_cr shifts to smaller values with increasing temperature. This confirms the theoretical results:the growth and the shrinkage regimes are controlled by the faster as well as the slower diffusion coefficients (D_A as well as D_B), respectively. It is also illustrated that, confirming recent theoretical predictions, the pore radius linearly depends on the initial particle radius and the slope of this straight line increases with the average composition of the faster component.