The order-disorder transition at the (001) surface of a 3 at. %-Au-rich ${\mathrm{Cu}}_3$Au crystal has been studied using surface x-ray scattering and Auger-electron spectroscopy. ${\mathrm{Cu}}_3$Au is a prototypical system for ordering alloys which undergo a first-order bulk phase transition. In contrast to previous studies on ${\mathrm{Cu}}_3$Au(001), we find that the order-to-disorder transformation at this Au-rich surface occurs at a temperature about 20 K above the bulk transition temperature (${\mathit{T}}_{\mathrm{bulk}}$=644.5±2 K). Changes in the near-surface composition during these transitions are less than 0.5%. Type-I antiphase domain boundaries, which leave nearest-neighbor configurations unchanged, form preferentially parallel to the surface. A phase is revealed with a large repeat distance normal to the surface and possibly involving type-II antiphase domain boundaries. The time dependence of the growth of the ordered domains during annealing times between ${10}^3$ to ${10}^5$ s was investigated through the x-ray measurement of the superstructure beam profile following a quench from the disordered state to a range of final temperatures in both the surface and bulk phases. In both cases the kinetics of the domain growth was substantially slower than the previously reported ${\mathit{t}}^{1/2}$ growth for the bulk. The nature of surface ordering is discussed in terms of surface segregation and formation of two types of antiphase domain boundaries.

• Received 21 July 1995

DOI:https://doi.org/10.1103/PhysRevB.52.12601