Numerous A3B binary alloys are ordered at low temperature on a given lattice (BCC or FCC) and become disordered on this lattice at high temperature. This transition can be of first or second order depending on whether the long range order parameter goes to zero discontinuously or not. In the first case, there is a two-phase field except at the congruent point. In any case, the transition is characterized by a loss of symmetry and the simplest situations occur when the ordered structure is

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... in this case, there is only a loss of translational symmetry which implies the formation, during the ordering process, of translational variants (or domains) bounded by domain walls, called antiphase boundaries (APB). In this paper, we present experimental studies of the thermodynamic behaviour of these boundaries when approaching the order-disorder transition from below. In the case of a first order transition studied in the Cu-17%Pd alloy, which orders on the FCC lattice according to the L12 suucture, we show that the APB's are wetted by the disorder and that their width logarithmically diverges at the transition in agreement with statistical physics calculations. A second order transition has been studied in the Fe-27%A1 alloy between the ordered structures DO3 and B2 built on the BCC lattice. A different behaviour is observed and the observations are consistent with the theoretically expected power law. FIRST ORDER TRANSITION The simplest situation is the transition L12disordered FCC. The Ll2 structure is obtained by placing B atoms at the vertices of the cube and the A atoms at the face centers. The translation vectors lost during the transition being of type <1/2,1/2,0>, there are three kinds of APB's characterized by one of the displacement vectors: 1/2[110], 1/2[101] and 1/2[01 I]. As a consequence APB's obey precise extinction contrast rules: considering a given dark field image formed with a superstructure reflection, two kinds of APB's are visible and the third one is out of contrast [I] . An example is given in figure 1. Figure 1: Complementary dark field images (DF) of perfect APB in Cu-17%Pd: (1 10) in a), (210) in b), (120) in c). It has been shown qualitatively that in various L12 compounds [1,2 31, APB's have a behaviour typical of 2 interfacial wetting: when increasing the temperature up to a few 10-Tc below the transition temperature, a boundary extends over a few atomic planes which become partially disordered. As shown in figure 2a , the APB is progressively split into two new interfaces of width 5. At the transition, the width of the domain walls diverges i.e. Article published online by EDP Sciences and available at http://dx.