Molecular-dynamics study of the decomposition of an unstable Ni-Ag alloy

In an attempt to understand how ballistically mixed alloys decompose under the influence of an ion-induced thermal spike, we perform model molecular-dynamics simulations in unstable random Ni_xAg_1-x alloys. We study the time dependence of decomposition as a function of the temperature, density and concentration in the alloy by monitoring the short-range order parameter and atom mobility. Mobility is diffusion-like; its temperature dependence is determined by activation energies of 2.6 (3.6) eV for Ag (Ni) atoms. The decomposition is strongly assisted by the reduced atom densities which are characteristic of thermal spikes. Decomposition works most efficiently in a mid-concentration range (x = 0.5), since here the heat of mixing liberated by the decomposition process heats the system and enhances diffusion.