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A study of the order-disorder L12 to A1 transformation near a congruent point of the Co-Pt phase diagram is presented. This work was performed by combining in situ transmission electron microscopy observations, resistivity measurements carried out in conditions near thermodynamic equilibrium and high-resolution imaging on quenched samples. Resistivity measurements display three characteristic temperatures: 994 K, 1020 K and 1032 K. The evolution of the microstructure with temperature at different heating rates was followed in dark field images, until the order-disorder temperature was reached. It was found that at temperatures above 994 K ( approximately=40 K below Tc), the antiphase boundaries (APB) undergo a structural change; the phenomenon becomes more pronounced as the temperature increases. High-resolution images of modified APB show thin layers of disordered phase in the core of these APB. This can be described as a wetting of the antiphase boundaries by the disordered phase. Above 1020 K, up to 1032 K, a second mechanism is superimposed; the disordering occurs through a nucleation, growth and coarsening of disordered regions inside the ordered domains. This nucleation and growth process starts at the beginning of the two-phase region (1020 K). These results are in excellent agreement with those from cluster variation method calculations performed by Kikuchi and Cahn (1979) and Finel and co-workers (1990).

Expressions are derived for the kinematical amplitudes of a perfect crystal, modulated by planar translation interfaces. The model is generalised so as to account for longitudinal and transversal relaxation effects at the interfaces and for non‐equidistant interface distributions. The cases of short range order (nearest neighbour correlation) and long range order (uniform sequences) are treated in detail. The results can be applied to antiphase boundaries in long period superstructure alloys as well as to discommensuration walls in deformation modulated structures.