固态扩散过程中的Kirkendall效应(英文)

在二十世纪四十年代,柯肯达尔(Kirkendall)发现,在二元固溶体中,扩散过程不能简单地用一种扩散系数来描述它,人们必须考虑到两种物质的扩散系数的不同。他的发现改变了以往的扩散理论和数据的处理方式。基于他的这种扩散理论成功解释了Kirkendall平面的行为。但是,多相扩散和扩散过程中组织形貌的演变是很复杂的,需要从物理化学的途径才能解决。从理论和工艺的角度来看,二元或多元体系中的相互作用是关键问题。本文综述了Kirkendall效应的发现以及最近的进展,及其在材料科学中的应用前景。     

Substitutional diffusion in multicomponent solids with non-ideal sources and sinks for vacancies

Based on previous work by the authors the diffusion equations for a multicomponent solid are derived. Generation and annihilation of vacancies are described by an evolution law which is directly coupled with an eigenstrain rate. Manning’s correlation factor f is used in the kinetic factor for diffusion of vacancies. In addition to presentation of the diffusion equations, a rigorous treatment of the boundary conditions, assuming no or an ideal source and sink for vacancies at the surface, is presented. As an instructive example development of the site fractions of the components and the eigenstress state are demonstrated for a multicomponent chemically inhomogeneous layer on a substrate. Both the role of nonlinear terms and the correlation factor f in the diffusion equations are studied. Computational procedures to calculate to the proper boundary conditions are also outlined.     

Periodic pattern formation in solid state reactions related to the Kirkendall effect

The periodic layered morphology for solid state reactions is shown to occur in various ternary and higher order material systems including metal/metal and metal/ceramic diffusion couples. The experimental results on different systems where the periodic pattern formation has been observed are systematized and early explanations for this peculiar phenomenon are discussed. It is shown that a special class of structures periodic in time and in space can be considered as a manifestation of the Kirkendall effect accompanying reactive phase formation in the solid state.     

Modeling of kinetics of diffusive phase transformation in binary systems with multiple stoichiometric phases

The thermodynamic extremal principle is used for the treatment of the evolution of a binary system under the assumption that all phases in the system are nearly stoichiometric with no sources and sinks for vacancies in the bulk. The interfaces between the individual phases are assumed to act as ideal sources and sinks for vacancies, and to have an infinite mobility. Furthermore, it is assumed that several phases are nucleated in the contact plane of the diffusion couple at the beginning of the computer experiment. Then, it is shown that the number of newly nucleated phases determines the maximum number of polyfurcations (i.e., branching of a single configuration into several distinct configurations) of the initial contact (Kirkendall) plane. The model is demonstrated on a hypothetical binary system with four stoichiometric phases. The inverse problem, namely, the determination of the tracer diffusion coefficients in newly nucleated phases from the thicknesses of new phases and the positions of polyfurcated Kirkendall planes, is treated too. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, the 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Utilizing the SIMS technique in the study of grain boundary diffusion along twist grain boundaries in the Cu(Ni) system

The secondary ion mass spectrometry (SIMS) technique was used to study grain boundary diffusion along (100) twist grain boundaries in the Cu(Ni) system. Concentration profiles of Ni down Cu twist grain boundaries with nominal disorientation angles of 10°, Σ5 (36.87°), and 45°, were measured using the SIMS technique. The average activation energy for grain boundary diffusion, Q_b, was found to be 245±22, 140±10, and 102±15 kJ/mol, for the 10°, Σ5, and 45° twist grain boundaries, respectively. The average grain boundary diffusion pre-exponential term, sδD_bo, was found to be 9.6±1.24×10⁻⁹, 1.1±0.17×10⁻¹⁴, and 1.3±0.36×10⁻¹⁶ m³/s, for the 10°, Σ5, and 45° twist grain boundaries, respectively.     

Nano- and micro-scale free volume in ultrafine grained Cu–1 wt.%Pb alloy deformed by equal channel angular pressing

Nano- and micro-scale free volumes in ultrafine grained Cu–1 wt.%Pb alloy deformed by equal channel angular pressing (ECAP) were examined by a combination of radiotracer diffusion measurements, transmission electron microscopy, depth milling by focused ion beam, and positron annihilation spectroscopy. The positron lifetime spectroscopy revealed the existence of vacancy-type defects, presumably related to grain boundary states, and vacancy clusters. The positron annihilation sites probed by coincident Doppler broadening spectroscopy are surrounded by both Cu and Pb atoms, implying preferred nucleation of nanovoids near small Pb inclusions. Larger free volume defects, such as nano- and submicron-sized pores and submicrometer cracks, were directly observed by the focused ion beam technique. Ultra-fast penetration of liquid tracer solution in the sample bulk was established in the radiotracer diffusion experiments. The results provide an unambiguous proof of the existence of open porosity in ultrafine grained Cu–Pb alloy deformed by ECAP. Under specific deformation conditions, a connected network of open channels, which promotes long-range penetration of a liquid radiotracer solution, was shown to form during the severe deformation. The relative volume fraction of the percolating porosity was estimated at about 2 × 10⁻⁶.     

Effect of yttrium additions to nickel on the volume and grain boundary diffusion of Ni in the scale formed on the alloy

NiO scales were formed on pure Ni and Ni-0.1 wt.% Y alloy by oxidation in pure O₂ at 1200°C, and the grain boundary self-diffusion coefficients of Ni in the dense region of the scale were measured by the tracer-sectioning technique. The volume diffusion coefficient was also measured in the alloy scale. The grain boundary and volume diffusion coefficients and the grain boundary width were found to be the same for the two materials within the experimental uncertainty.Work supported by the Division of Basic Energy Sciences, U.S. Department of Energy.     

An insight into the role of the grain boundary in plastic deformation by means of a bicrystalline pillar compression test and atomistic simulation

A combination of experimental and molecular dynamics (MD) simulations is used to investigate the interaction of dislocations with a selected grain boundary (GB) in bicrystalline pillars (BCPs) with component crystals oriented for single slip and multiple slip. As a reference, single-crystalline pillars with the same orientations are also tested and compared with the BCPs. Orientations identical to the experiments are used to generate models in MD simulations. Further, electron backscatter diffraction (EBSD) measurements on the cross-section of the pillars are performed to investigate the crystal lattice rotation in correlation with the excess dislocation density. A clear change in mechanical behavior of the BCP was observed when the size of the component crystals reduced below 1 μm. The EBSD analyses of these small BCPs showed an increase in the misorientation in the vicinity of the GB. MD simulation provided atomistic insights into the dislocation nucleation process and the BCPs’ interaction with the GB. On the basis of these observations, it is concluded that in BCPs smaller than 1 μm the dislocation–GB interaction plays a more crucial role than the dislocation–dislocation interaction.     

Hydrogen diffusion in Sm2Fe17 and Sm2Fe14Ga3 compounds

The short-range and long-range diffusion of hydrogen in Sm₂Fe₁₇ and Sm₂Fe₁₄Ga₃ compounds was investigated by magnetic after-effect (MAE) and thermal desorption spectroscopy (TDS). The substitution of iron by gallium greatly reduces the uptake and diffusion of hydrogen in these compounds. After charging in a high-purity H₂ atmosphere of 0.13 MPa at 570 K for 1 h the H content was determined to be Sm₂Fe₁₇H₅ and Sm₂Fe₁₄Ga₃H_1.3. Sm₂Fe₁₇ shows a hydrogen-induced MAE relaxation maximum at 190 K with a mean activation enthalpy of Q=(0.48±0.02) eV and a pre-exponential factor of

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s. In the case of the Ga-substituted compound this maximum is replaced by a double maximum at 200 and 250 K corresponding to activation ethalpies of (0.52±0.02) eV and (0.64±0.02) eV with a pre-exponential factor of

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s. These relaxation maxima are attributed to the short-range diffusion of hydrogen, since they are absent in the degassed compounds. TDS measurements of Sm₂Fe₁₇H₅ and Sm₂Fe₁₄Ga₃H_1.3 were performed in the temperature range from 291 to 515 K yielding activation enthalpies of Q=(0.59±0.02) eV and Q=(0.75±0.03) eV, respectively. The results are explained by different interstitial sites for hydrogen and the impact on the hydrogen disproportionation process is discussed.