A thermodynamic approach to homogeneous nucleation via fluctuations of concentration in binary liquid alloys

A thermodynamic approach to homogeneous crystalline nucleation in binary li quid alloys is presented, which is based on a two-step mechanism for critic al embryo formation. It postulates that suitable concentration fluctuations in the undercooled liquid state can provide favoured sites for nucleation. Thus, a distinction is operated between local chemical order fluctuations and structure fluctuations, as opposed to the classical nucleation theory a pproach. To quantify the importance of concentration fluctuations, the prop osed mechanism is applied to binary liquids exhibiting a tendency towards p hase separation. The energy required to form a relevant fluctuation in the liquid, prior to nucleation, is calculated as a function of the undercoolin g amount. Furthermore, the ratio between the population of relevant fluctua tions and that of critical nuclei is assessed statisticallyt the probabilit y for a fluctuation to become a critical embryo increases with the amount o f undercooling. Finally, the two-step mechanism for nucleation is supported from a kinetic point of view.