Interface reaction and a special form of grain boundary diffusion in the Cr-W systemReaction aux interfaces et diffusion de type particulier aux joints de grains dans le systeme Cr-WGrenzflächenreaktion und eine spezielle form der korngrenzen-diffusion im system Cr-W☆
Abstract
The two phase diffusion between chromium and tungsten, forming b.c.c. solid solutions, has been followed right from the beginning by means of a special X-ray diffraction technique, light microscopy and electron microprobe analysis. It appears that in a first stage only tungsten diffuses into the chromium, while the tungsten remains pure. The Kirkendall-effect however indicates that chromium is the much more rapidly diffusing component. It is shown that these two observations are not contradictory. In this stage the Cr-rich alloy can dissolve more tungsten than the equilibrium value; a thermodynamic explanation of this effect is given. A second stage consists of the formation of a W-rich alloy of a distinct composition in a discontinuous way at separate sites of the interface, presumably by an interface reaction. At the junction of the interface and a tungsten grain boundary a special form of grain boundary diffusion occurs, during which the grain boundary most likely moves away from its initial position, as in the case of discontinuous decomposition.
The experimental results are discussed in the form of an atomic description of the various phenomena, by taking into account the enormous difference in melting point of chromium and tungsten. This difference leads to a model of the atom and vacancy jumps at the interface right from the beginning of the diffusion; this model is based upon a much higher vacancy concentration in chromium compared to tungsten and a much greater mobility of the chromium atoms. From the considerations one might expect an “incubation time”, during which the motion of Kirkendall-markers is delayed. An analogous atomic model has been used to give a description of the mechanism, by which the special form of the diffusion of chromium along a tungsten grain boundary occurs. In this boundary, treated as a phase with a very high vacancy concentration, it is proposed that a reaction should take place, leading to the deposition of a tungsten-rich W-Cr alloy, epitaxially to the adjacent chromium crystallite or to one of the adjacent tungsten crystallites. As a result the initial grain boundary will split into two boundaries or will move away from its original position.
Résumé
La diffusion à deux phases entre le chrome et le tungstène formant des solutions solides c.c.c. a étéétudiée depuis son démarrage par une méthode spéciale de diffraction des rayons X, par microscopie optique et par analyse à la microsonde électronique. Il apparait que dans un premier stade, on assiste seulement à une diffusion du tungstène vers le chrome, alors que le tungstène reste pur. Cependant, l'effet Kirkendall montre que le chrome est le composant dont la diffusion est de beaucoup la plus rapide. L'auteur montre que ces deux observations ne sont pas contradictoires. Dans ce premier stade, l'alliage riche en Cr peut dissoudre plus de tungstène que la quantité correspondant à l'équilibre; l'auteur donne une interprétation thermodynamique de ce phénomène. Un second stade est constitué par la formation d'un alliage riche en tungstène de composition distincte, d'une manière discontinue et dans des régions de l'interface séparées les unes des autres, probablement par une réaction d'interface. Au jonction de l'interface, et sur un joint de grains de tungstène, il se produit un type spécial de diffusion aux joints de grams, au cours duquel le joint de grains se déplace le plus vraisemblablement loin de sa position initiale, comme dans le cas de la décomposition discontinue.
Les résultats expérimentaux sont discutés en envisageant une description atomique des différents phénomènes, tenant compte de l'écart très important entre les points de fusion du chrome et du tungstène. Cet écart conduit à imaginer, dès le début de la diffusion, un modèle de sauts des atomes et des dislocations à l'interface; il est basé sur une concentration en lacunes beaucoup plus élevée dans le chrome que dans le tungstène, et sur une mobilité beaucoup plus grande des atomes de chrome. A partir de ces considée dans le chrome que erations, on pourrait s'attendre à un “temps d'incubation” au cours duquel le mouvement des marqueurs de Kirkendall est retardé. Un modèle atomique analogue a été utilisé pour décrire le mécanisme par lequel se produit la diffusion spéciale du chrome le long d'un joint de grains de tungstène. Dans ce joint, traité comme une phase contenant une concentration en lacunes très élevée, on propose qu'il pourrait se produire une réaction conduisant au dépôt d'un alliage W-Cr riche en tungstène, en épitaxie sur le cristallite de chrome adjacent ou sur l'un des cristallites de tungstène adjacents. Il en résulterait, pour le joint de grains initial, un éclatement en deux joints ou un déplacement loin de sa position d'origine.
Zusammenfassung
Die Zweiphasendiffusion zwischen den eine kubisch-raumzentrierte Legierung bildenden Metallen Chrom und Wolfram wurde vom Beginn der Diffusion an mit Hilfe einer speziellen Röntgenbeugungsmethode, optischer Untersuchungsverfahren und der Elektronen-Mikroanalyse untersucht. Es ergibt sich, daβ im Anfangsstadium nur Wolfram in das Chrom diffundiert, während das Wolfram rein bleibt. Der Kirkendall-Effekt zeigt jedoch, daβ Chrom die viel schneller diffundierende Komponente ist. Es wird gezeigt, daβ sich diese beiden Beobachtungen nicht widersprechen. In diesem Stadium des Diffusionsprozesses kann die Cr-reiche Legierung mehr Wolfram lösen als der Gleichgewichtskonzentration entspricht; eine thermodynamische Erklärung dieses Effektes wird gegeben. Das zweite Stadium besteht in der Bildung einer W-reichen Legierung ganz bestimmter Zusammensetzung auf diskontinuierliche Weise und an verschiedenen Stellen der Grenzfläche; dies geschieht vermutlich durch eine Grenzflächene̊aktion. Am Schnitt einer Grenzfläche mit einer Wolframkorngrenze findet eine spezielle Art der Korngrenzendiffusion statt, wobei sich die Korngrenze sehr wahrscheinlich wie bei der diskontinuierlichen Entmischung von ihrer ursprünglichen Lage fortbewegt.
Die experimentellen Ergebnisse werden anhand einer atomistischen Beschreibung verschiedener Phänomene diskutiert; dabei wird der enorme Unterschied in den Schmelzpunkten von Chrom und Wolfram berücksichtigt. Dieser Untershied führt zu einem Modell, das Atom- und Leerstellensprünge in der Grenzfläche vom Beginn der Diffusion an voraussetzt. Dieses Modell geht davon aus, daβ die Leerstellenkonzentration im Chrom viel gröβer ist als im Wolfram und daβ die Chromatome eine viel gröβere Beweglichkeit besitzen. Aus diesen Betrachtungen würde man eine “Inkubationszeit” erwarten, während der die Bewegung der Kirkendall-Markierung verzögert wird. Ein analoges atomistisches Modell wurde zur Beschreibung des Mechanismus benützt, der zur speziellen Form der Diffusion von Chrom in einer Wolframkorngrenze führt. Es wird vorgeschlagen, daβ in dieser als Phase mit sehr hoher Leerstellenkonzentration behandelten Korngrenze eine Reaktion stattfindet, die zur Ablagerung einer wolfram-reichen W-Cr-Legierung führt; diese Legeirung ist epitaktisch zum angrenzenden Chrom-Kristallit oder zu einem der angrenzenden Wolfram-Kristallite. Als Folge wird die ursprüngliche Korngrenze in zwei Korngrenzen aufgespalten oder sie wird sich von ihrer ursprünglichen Lage fortbewegen.
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This work has been carried out in the Laboratory for Physical Chemistry of the Solid State at the Technological University of Delft, Netherlands, as part of a thesis for the degree of Doctor of Technical Sciences.