Interface reaction and a special form of grain boundary diffusion in the Cr-W system

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.