Free energy of melts and intermetallic compounds of binary alloys determined by a molecular dynamics approach

We present an atomistic approach aimed at determining the free energy g(liq) of binary alloy liquids, a quantity which governs the thermodynamics of phase transformations and whose evaluation has long been a challenge to modeling methods. Our approach, illustrated here for a metallic system model NiZr, combines two methods: the quasiharmonic approximation, applied for some existing (real or hypothetical) intermetallic compounds, and the liquid-solid coexistence conditions. The underlying equations for g(liq) are solved by means of a subregular-solution approximation. We demonstrate the high reliability of our calculated free energies in determining the phase diagram of a binary system and describing quantitatively the growth kinetics. The latter issue is illustrated by linking molecular dynamics simulations to phase-field modeling with regard to directional solidification and melting in a two-phase system [Ni(x)Zr(1-x)](liq)-Zr(cryst) out of chemical equilibrium.