Partial chemical reactions at phase interfaces

A schematic diagram illustrating the growth process of the layers of two chemical compounds ApBq and ArBs, with p, q, r and s being positive numbers, at the A B interface is shown in Fig. 2.1. Note that the lines showing the distribution of the concentration of components A and B in the phases involved in the interaction are parallel to the distance axis since (/) the formation of the layers of chemical compounds which have narrow, if any, ranges of homogeneity is considered and (ii) initial substances are assumed to be mutually insoluble. 

In such a case, immediate application of Fick s laws is clearly impossible. Therefore, it is not surprising that usually smooth concentration distributions characteristic of solid solutions are drawn, while the results obtained are then applied to chemical compounds. Many researchers, especially of physical orientation, even do not use the term chemical compound, preferring to analyse the formation of a new phase, thus actually making no difference between solid solutions and chemical compounds. 

Whether a particular phase is a chemical compound or a solid solution can hardly be subject to any doubt in obvious cases such as in the Ni-Bi binary system with the intermetallics NiBi (homogeneity range HR 0.3 at.%) and NiBi3 (stoichiometric phase) or in the Ti-Al binary system with the intermetallics Ti3Al (HR 12 at.% at 600°C), TiAl (HR 7 at.%), TiAl2 (HR 1 at.%) and TiAl3 (stoichiometric phase).142 145 193 

formation of a solid solution is often considered to be a prerequisite for the occurrence of a chemical compound layer, with the latter being a result of supersaturation of the former. In fact, however, these are two concurrent, competing processes, if both solid solutions and chemical compounds are present on the phase diagram of a binary system. In any 

Direct chemical reaction between elementary substances A and B clearly ceases after the formation of compound layers ApBq and ArBs, a few crystal-lattice units thick, which separate the reacting phases from each other. Subsequently, four partial chemical reactions take place at the layer interfaces. These are as follows  

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In any binary system, the sequence of formation of compound layers is governed by the rate of chemical transformations (partial chemical reactions) at phase interfaces. 

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