New enthalpies determination and in situ X-ray diffraction observations of order disorder transitions in Au0.5Cu0.5

Differential scanning calorimetry (DSC) and in situ X-ray diffraction experiments on stoichiometric AuCu are reported. Enthalpy peaks of successive transitions AuCu I (L1(0)) --> AuCu II and AuCu II --> disorder (Al) are observed, and their integration leads to Delta H values of 2.5 and 9.4 J/g, respectively. These successive transitions are well seen in in situ diffraction. Each transformation is spread over a temperature range, and it is shown that the transition AuCu I (L1(0)) --> AuCu II starts at a temperature as low as 350 degrees C. Some kinetic studies were performed. The long-range order parameter evolution against time is estimated for the ordering at 300 degrees C into L1(0) from quenched disordered AuCu. The AuCu I (L1(0)) --> AuCu II was also studied from a kinetic point of view, using the intensities of characteristic lines. Various results are compared with previously reported observations and discussed. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd All rights reserved.