JOURNAL DE PHYSIQUE IV

Abstract. After having recalled the crystal structures of the three equilibrium phases of stoichiometric Au,,Cu, , alloy, new results are presented in the frame of a general discussion on the transition series (with increasing temperature): AuCu I (L l o)-t AuCu I1-t disordered AuCu (AI). Both transitions have been followed by in-situ temperature X-ray, confirming earlier electron diffraction studies, and their transition enthalpies have been measured by differential scanning calorimetry. These results are commented, and a possible explanation for the large two-phase hysteresis (35°C) is proposed 1. THE THREE EQUILIBRIUM PHASES OF A u ~. ~ C U ~. ~ The equiatomic AuCu alloy can display three phases: the tetragonal (c/a=0.92) ordered L10 structure (AuCu I) at low temperature, the orthorhombic incommensurate structure AuCu I1 (first investigated through X-ray diffraction by Johansson and Linde [I]) from less (see below) than 385°C up to about 420°C, and the disordered fcc A1 structure at higher temperature. The crystal structures of the L10 and AI phases are well known: A l '-10 Figure 1. The cells of the AI (disordered) and L l o (ordered) structure