The thermodynamics and mechanisms of ordering systems

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Abstract
In certain alloy systems a random solid solution known as the disordered state, transforms into a regular array called the ordered state, below a critical temperature Tc. Over the past decade or so, a considerable controversy has arisen over the transient path(s) by which a disordered alloy is replaced by an ordered structure at stoichiometric compositions, and also concerning the nature of the phases under equilibrium conditions when the alloy departs from the ideal composition. The present review is an attempt to unify the different approaches with reference to both theoretical grounds and experimental findings. Following an introduction to the general characteristics of superlattices and their peculiarities, the article lays the thermodynamic basis and some experimental techniques relevant to the field of ordering reactions and compares and contrasts sometimes quite different interpretations of experimental findings, both at and away from stoichiometry. One method of overcoming some of the confusion regarding mechanisms would be to adopt the Gibbsian classification of a Type I or Type II phase change, while equilibrium thermodynamic notations should only be employed at Tc or for that region of the phase diagram away from stoichiometry.

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