The diffusion couple technique in phase diagram determination

A schematic illustration of the method, and of the correlation between binary phase diagram and the one-phase layers formed in a diffusion couple, is shown in Fig. 2.42 adapted from Rhines (1956). The one-phase layers are separated by parallel straight interfaces, with fixed composition gaps, in a sequence dictated by the phase diagram. The absence, in a binary diffusion couple, of two-phase layers follows directly from the phase rule. In a ternary system, on the other hand (preparing for instance a diffusion couple between a block of a binary alloy and a piece of a third 

With this technique, under an especially equipped electron microscope, high-energy electrons are focused on a fine probe and directed at the point of interest in the specimen. The electrons interacting with the sample atoms cause the emission of the characteristic X-rays, which are detected and identified for qualitative analysis and used, generally through suitable standardization, to perform also a quantitative analysis. 

As concluding remarks about these techniques, their increasing interest and the advantages of their combination with other techniques, we may mention, as an example, that within the European research project COST 535, concerning the Thermodynamics of Alloyed Aluminides , a meeting (Diisseldorf, December 2004) was dedicated to The Diffusion Couple Technique , presenting the principles of the method and the results obtained in the examination of several alloy systems. Zhao (2004) has developed an efficient variant of the diffusion couple technique (the diffusion multiple approach ). 

A general treatment of a diffusion-controlled growth of a stoichiometric intermetallic in reaction between two two-phase alloys has been introduced by Paul et al. (2006). A reaction couple in which a layer of Co2Si is formed during inter-diffusion from its adjacent saturated phases was used as a model system. In the discussion it has been emphasized that the diffusion couple is undoubtedly one of the most efficient and versatile techniques in solid-state science it is therefore desirable to have alternative theories that enable us to deduce the highest possible amount of information from the data that are relatively easily attainable in this type of experiments. 

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