FREE-ENERGY CHANGES ON FREEZING AND MELTING DUCTILE METALS

The variation in Landau free energy while melting platinum was investigated at a number of temperatures using computer simulation with a model potential. The technique used was to apply a biasing potential in a Monte Carlo simulation with umbrella sampling. From the Landau free energy curves one can determine the difference in free energies between the solid and liquid phases easily and accurately, the thermodynamic melting point (T(m)), and the limit of metastability of the crystalline phase. The latter occurs at approximately 1.2T(m). It was difficult to freeze the material, but, using a suitable order parameter, this was achieved. Unlike earlier results on a soft sphere system, there was no evidence for nucleation of a metastable body-centred-cubic phase. One possible reason is the existence of local icosahedral order in the liquid phase of the metal. The surface free energy of the solid-liquid surface was estimated from the free energy barrier to melting. Model rhodium behaved in a very similar way.