An X-ray scattering and simulation study of the ordering kinetics in CuAu

– A detailed numerical and experimental study of the ordering of the low-temperature tetragonal phase of CuAu is presented. The numerical simulations are based on a coarsegrained free energy derived from electronic structure calculations of CuAu, while the experimental results are obtained from in situ X-ray scattering. Both theoretical and experimental work indicate a subtle kinetic competition between the ordered tetragonal phase and the metastable modulated phase. What determines the microstructure of a typical alloy? While there is no simple answer to this question it is understood [1, 2] that a variety of morphological instabilities can be encountered in non-equilibrium processing which create spatial inhomogeneities that are magnified during coarsening. In this paper experimental and theoretical techniques are combined to investigate the intricate morphologies that emerge during the sublattice ordering of the low-temperature tetragonal phase of CuAu. The results of this study indicate a subtle competition between a metastable modulated phase and the stable low-temperature ordered phase. At shallow quench temperatures or early times the modulated state is favored while at lower temperature or later times the ordered phase dominates. The results are supported by in situ X-ray scattering experiments and numerical simulations. The phases of CuAu that are relevant to this study are a high-temperature disordered face-centered cubic phase, an intermediate-temperature modulated orthorhombic phase and a low-temperature ordered tetragonal phase. The modulated phase consists of a periodic array of antiphase domain walls arranged perpendicular to the tetragonal direction. The wavelength of the modulated phase is 10 underlying fcc unit cells. The transitions from the ordered phase