M. A. Dayananda

Zero-flux planes and flux reversals in Cu−Ni−Zn diffusion couples

Concentration profiles of isothermal diffusion couples in binary as well as multicomponent systems can be analyzed directly for interdiffusion fluxes without the need for a prior knowledge of interdiffusion coefficients. Such an analysis is presented and applied for the calculation of interdiffusion fluxes of each component at various sections of several diffusion couples in the Cu−Ni−Zn system investigated at 775°C. A major outcome of these calculations is the identification of “zero-flux planes” for the individual components within the diffusion zones of ternary couples. At a zero-flux plane the interdiffusion flux of a component goes to zero and on either side of the plane occurs a change or reversal in the direction of the interdiffusion flux of the component. The formation as well as the number of zero-flux planes of the components is dictated by the terminal alloys of the diffusion comple. The compositions of zero-flux planes for Ni and Cu identified in several Cu−Ni−Zn couples are found to correspond to composition points of intersection of diffusion paths and isoactivity lines drawn through the terminal alloys of the couples on a ternary isotherm.

An Analysis of Concentration Profiles for Fluxes, Diffusion Depths, and Zero-Flux Planes in Multicomponent Diffusion

Concentration profiles developed during isothermal, multicomponent diffusion for a single-phase, solid-solid diffusion couple are expressed on the basis of a relative concentration variable for each component and analyzed for the determination of interdiffusion fluxes. The individual concentration profiles intersect at a common cross-over composition where the relative concentrations of all components are identical. New relations are developed for describing internal consistency among the concentration profiles of the various components. A link is made between the cross-over composition and the depths of the diffusion zone on either side of the Matano plane for a diffusion couple. The cross-over composition is interpreted as the average relative concentration of each component over the diffusion zone. The identification of a zero-flux plane from concentration profiles is also described. The analysis offers several advantages in presenting as well as checking the self-consistency of results as illustrated with a single phase Cu-Ni-Zn diffusion couple annealed at 775 °C.

Average effective interdiffusion coefficients and the Matano plane composition

From an integration of the interdiffusion flux of a component over distance in the diffusion zone of an isothermal solid-solid diffusion couple, average effective interdiffusion coefficients can be evaluated over selected ranges of composition. An analysis is developed to inter-relate, in general, the average effective interdiffusion coefficients calculated for the composition ranges on either side of the Matano plane to the composition at the Matano plane. Characteristic depths for concentration profiles on either side of the Matano plane are employed to characterize the concentration profiles and their deviations from error function solutions. The analysis is applied to the concentration profiles of a binary Fe-C as well as a ternary Cu-Ni-Zn single-phase diffusion couple. From a knowledge of concentration profiles on one side of the Matano plane, profiles on the other side can be generated from an error function model based on the analysis.

Interdiffusion between U-Zr fuel and selected Fe-Ni-Cr alloys

As part of studies relevant to fuel-cladding compatibility in the Integral Fast Reactors, isothermal interdiffusion experiments were carried out at 700°C with solid-solid diffusion couples assembled with a U-23 at% Zr alloy fuel and a series of cladding alloys of selected compositions in the Fe-Ni-Cr system. Besides pure Fe and pure Ni, the alloys included binary Fe-20.1Cr, Ni-16.4Cr, and Fe-10.1Ni and a ternary Fe-16.4Ni-9.4Cr alloys (composition in at%). The diffusion structures developed in the various couples were examined metallographically and by SEM-EDS analysis. The development of diffusion layers and their variation with compositional changes of the cladding alloys are discussed in the light of phase diagrams, intermetallic formation, the relative diffusion behavior of the various elements and the experimental diffusion paths. From the composition profiles, average effective interdiffusion coefficients are determined for specific regions in the diffusion structures of selected diffusion couples. Intrinsic diffusion coefficients are also calculated at the composition of a marker plane in a (U,Zr)Ni2 phase layer.

Interdiffusion and diffusion structure development in selected refractory metal silicides

Abstract Solid–solid diffusion couples set up with disks of Mo, W, Re, Nb, or Ta in contact with disks of a single crystal of MoSi 2 were annealed at selected temperatures between 1300° and 1700°C for diffusion structure, determination of interdiffusion coefficients and energies of activation for interdiffusion in various silicides developed in the couples. The couples were analyzed and characterized by SEM and optical microscopy, microprobe analysis, X-ray diffraction and orientation imaging microscopy. From the interdiffusion fluxes determined directly from the concentration profiles, integrated and average effective interdiffusion coefficients were calculated for the components in the various binary and ternary silicide layers. The Mo vs. MoSi 2 couples developed Mo 5 Si 3 and Mo 3 Si layers with non-planar interface morphologies; the Mo 5 Si 3 layer exhibited oriented growth and microcracking. The W vs. MoSi 2 couples developed W 5 Si 3 and (W,Mo) 5 Si 3 layers with little microcracking. The diffusion structure of Re vs. MoSi 2 diffusion couples consisted of layers of Re 2 Si, (Re,Mo)Si and (Re,Mo) 5 Si 3 phases and cracks were blunted in the (Re,Mo) 5 Si 3 layer. The Nb vs. MoSi 2 couples developed the Nb 5 Si 3 and (Nb,Mo) 5 Si 3 phase layers with porosity in the diffusion zone. Layers of Ta 2 Si, Ta 5 Si 3 and (Ta,Mo) 5 Si 3 phases were observed in the Ta vs. MoSi 2 couples. The activation energies ( Q ) for the interdiffusion of both Si and W are calculated to be about 360 and 450 kJ mol −1 in the W 5 Si 3 and (W,Mo) 5 Si 3 layers, respectively. Values of Q for the interdiffusion of both Re and Si are about 190 kJ mol −1 in the Re 2 Si phase, 325 kJ mol −1 in the (Re,Mo)Si phase, and 270 kJ mol −1 in the (Re,Mo) 5 Si 3 phase. For the interdiffusion of Si and Nb in the (Nb,Mo) 5 Si 3 phase, Q values are about 300 and 240 kJ mol −1 , respectively; Q is about 265 kJ mol −1 in the binary Nb 5 Si 3 phase. Zero-flux planes (ZFP) and uphill diffusion are observed for Mo in the (Re,Mo)Si and (Me,Mo) 5 Si 3 layers where Me=W, Re, Nb or Ta; in these layers, the ternary cross coefficient D MoMe Si is about as large as the main ternary interdiffusion coefficient D MoMo Si and the interdiffusion of Mo is enhanced down the Me concentration gradient.

DIFFUSION STRUCTURES IN Mo VS. Si SOLID-SOLID DIFFUSION COUPLES

Silicides of refractory metals have been employed in a wide variety of industrial applications ranging from electronics to aerospace technology. Silicide films are employed extensively in the electronics industry as contacts and interconnects in integrated silicon devices. Interest in silicides and silicide-based composites for high temperature applications has been renewed in recent years. MoSi{sub 2} possesses many of the favorable properties that are required for a high temperature material. MoSi{sub 2} has a high melting point, low density, excellent high temperature oxidation and corrosion resistance, and it exhibits metallic-like thermal and electrical conductivity. A few studies have been carried out on the growth of silicides in the Mo vs. Si system but have reported different results on layer growth kinetics. The objective of the present study was to investigate diffusion structures in Mo vs. Si solid-solid diffusion couples annealed at selected temperatures between 900--1,350 C and to explore the stoichiometry and texture of the MoSi{sub 2} diffusion layer.

Zero-flux planes and flux reversals in the Cu- Ni- Zn System at 775 °C

Ternary diffusion in the Cu-Ni-Zn system was investigated at 775 °C for the development of zero-flux planes (ZFP) and flux reversals of the individual components. ZFP’s, where the interdiffusion flux of either Cu, Ni, or Zn goes to zero, were identified in several series of single phase and multiphase solid-solid diffusion couples assembled with a (fcc),β (bcc), or γ (cubic) Cu-Ni-Zn alloys and characterized by terminal alloys of similar thermodynamic activity for one of the components. Profiles of interdiffusion fluxes were directly determined from concentration profiles. The diffusion path for a single phase couple with a ZFP was experimentally found to be invariant with diffusion time. The locations of ZFP’s within the diffusion zone of a couple corresponded to sections where the activity of a component was the same as its activity in either of the terminal alloys of the couple. Couples developing ZFP’s showed regions where a component diffused up its own activity gradient. The diffusional interactions among the components described by the ratios of cross to main ternary interdiffusion coefficients were determined directly from the slopes of the diffusion paths at various ZFP compositions. In several multiphase couples, discontinuous flux reversals for the components were also identified at theβ/a and γ/β interfaces. A discontinuous flux reversal for a component was observed at a planar interface, when the activity of the component at the interface corresponded to its activity in one of the terminal alloys of the couple.

Analysis of constituent redistribution in the γ (bcc) U-Pu-Zr alloys under gradients of temperature and concentrations

Abstract Rods of a ternary alloy (71U–19Pu–10Zr by weight percent) were annealed under a temperature gradient of 220°C/cm for 41 days and examined for micro-structural development and compositional redistribution. An enrichment of Zr with concurrent depletion of U was observed within the γ (bcc) phase region on the hot-end side (T≅740°C). The experimental redistribution of the elements in the γ (bcc) phase was analyzed in the framework of multicomponent mass transport with due consideration of thermotransport and ternary diffusional interactions. Based on a new analysis involving an integration of interdiffusion fluxes in the diffusion zone, kinetic parameters related to the thermotransport and ternary interdiffusion were calculated for each component i over selected ranges of composition. The thermotransport coefficients of U, Pu, and Zr were in the approximate ratio of 1:2:−4.5 in the hot-end region. In addition, the interdiffusion flux contributions arising from the gradients of temperature and concentrations of U and Zr were estimated.

Average effective interdiffusion coefficients and their applications for isothermal multicomponent diffusion couples

Abstract Concentration profiles on either side of the Matano plane in a solid-solid, single phase, ternary diffusion couple can be characterized by average effective interdiffusion coefficients and characteristic depth parameters. The composition at the Matano plane is related to these coefficients and parameters. The present analysis allows the generation of concentration profiles on the basis of error function solutions that exactly reproduce the experimental interdiffusion fluxes at the Matano plane. Furthermore, from a knowledge of the average effective interdiffusion coefficients and the α parameters for one alloy side of the couple, the concentration profiles on the other alloy side may be generated. Therefore, the characterization of concentration profiles by average effective interdiffusion coefficients provides a simple and useful method for representing and generating concentration profiles in single phase multicomponent diffusion couples.

Identification of Zero-Flux Planes and Flux Reversals in Several Studies of Ternary Diffusion

Several past studies of isothermal diffusion in ternary systems have been examined for the identification of zero-flux planes (ZFP) and flux reversals of the individual components. The phenomenon of ZFP initially recognized in the Cu-Ni-Zn diffusion couples has now been identified in several ternary systems including Cu-Ag-Au, Cu-Zn-Sn, Fe-Ni-Co, Fe-Ni-Al, and Co-Ni-Cr. The development of ZFP is dictated by the terminal alloys of the diffusion couples and can occur in both single phase and multiphase assemblies. Discontinuous flux reversals can also occur at interfaces in multiphase systems. The compositions of ZFP’s developed for a couple correspond to the intersections of its diffusion path and the isoactivity lines for the components drawn through the terminal alloy compositions on a ternary isotherm.