Phase Transitions and Thermal Expansion Behavior in AuCu Alloy

Thermal expansion behavior in a stoichiometric AuCu alloy was investigated from a point of view of a phase transition. Thermal expansion measurement was carried out from room temperature to 873 K at various heating and cooling rate. The crystal structures and the phase transitions were analyzed by X-ray diffractmetry and differential scanning calorimetry, respectively. The structure of the casted alloy was a disordered face-centered cubic and changed to a face-centered tetragonal structure or mixed structure of fcc and AuCu I after thermal expansion measurement. In the thermal expansion curves, break points were corresponded to order-disorder transition. During the heating process, three break points corresponding to three different ordering stages were observed. The activation energy for each stage was obtained using Kissinger method. It was considered that first and second stages are ascribed to the migration of excess vacancies induced by quenching, and third stage is predominated by self-diffusion.