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

Differential scanning calorimetry (DSC) and in situ X-ray diffraction exper iments on stoichiometric AuCu are reported. Enthalpy peaks of successive tr ansitions AuCu I (L1(0)) --> AuCu II and AuCu II --> disorder (Al) are obse rved, and their integration leads to Delta H values of 2.5 and 9.4 J/g, res pectively. These successive transitions are well seen in in situ diffractio n. 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 l ow as 350 degrees C. Some kinetic studies were performed. The long-range or der parameter evolution against time is estimated for the ordering at 300 d egrees C into L1(0) from quenched disordered AuCu. The AuCu I (L1(0)) --> A uCu II was also studied from a kinetic point of view, using the intensities of characteristic lines. Various results are compared with previously repo rted observations and discussed. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd All rights reserved.