NONEQUILIBRIUM CRYSTALLIZATION, CRITICAL COOLING RATES AND TRANSFORMATION DIAGRAMS

Two different approaches are currently used to study non-equilibrium crystallization. One is based on crystallization kinetic theories and the other is based on nucleation and crystal growth theories. The second, exact, treatment of the kinetics of crystallization under a non-isothermal regime is presented, based on models of nucleation and growth, to establish the temperature versus heating rate-transformation (T-HR-T) and temperature versus cooling rate-transformation (T-CR-T) diagrams. The usefulness of the method is illustrated by analysis of the crystallization process in three model materials: an oxide glass (SiO2), a chalcogenide glass (Se61.5Ge15.4Sb23.1), and a metallic glass (Nd2Fe14B). The calculations also yield the justification and limitations both for the additivity assumption, often used to describe the relationship between isothermal and continuous heating/cooling crystallization studies, and for the empirical method of constructing the low temperature part of the time-temperature-transformation (T-T-T) and T-HR-T diagrams, assuming an Arrhenius form of the crystallization rate constant. By using an exact method to construct the T-CR-T diagram, the crystallization temperatures predicted by the model are established. The critical cooling rate calculated is between two and four times lower than that obtained by use of the additivity assumption and between six and eight times lower than that resulting from the T-T-T diagram. By extension of the model, the T-HR-T diagram is obtained and a critical heating rate is defined. In all cases, the exact heating rate is about two orders of magnitude higher than that obtained by use of the additivity assumption.