Mechanism of low-temperature ordering in equiatomic CuAu alloy : I. X-ray diffraction studies

The c/a ratio of lattice parameters of CuAu was measured as a function of the ordering annealing time at 250°C. It is shown that the c/a ratio determined using only fundamental and only superstructure reflections differ considerably from each other, because the centroid position of the 220 fundamental reflection is strongly affected by the presence of the 220 reflection of the disordered matrix. A three-stage mechanism of ordering is proposed. It is shown that partial disordering is one of the stages of the process upon approaching the equilibrium stage.