Isothermal mechanical spectroscopy by means of a forced torsion pendulum (measuring internal friction/mechanical loss) was used to study the interplay of long-range atomic order and stress-induced diffusion (Zener relaxation) in Au57%Cu43%. Our results show that the relaxation strength of stress-induced diffusion exhibits the typical Curie-Weiss-type behavior in the disordered solid solution and then gradually goes to zero below the critical temperature marking the phase transition to the long-range-ordered AuCu II phase. The breakdown of the relaxation peak reflects the kinetics of the ordering process. The diffusion data were used to establish the transformation time vs temperature (TTT) diagram of the phase transformation.