Relationships Between Phase Diagrams, The To, and Tn Temperatures, Cooling Rates and Glass Forming Ability

The notion that it should be possible to assess the glass forming ability (GFA) of a given alloy, or alloy system, is a well established concept. If crystallization in the liquid on cooling can be prevented relatively easily, making it possible for a liquid alloy to be cooled through the glass transition temperature (Tg), while employing only a relatively slow cooling rate, the GFA is said to be large. A numerical measure of GFA is frequently expressed in terms of the reduced glass temperature ratio Trg=Tg/Tl, where Tl is the equilibrium liquidus temperature [1]. While the role of Tg in this ratio is clear, the role of Tl is less obvious. Tl can be derived from the features of the phase diagram, but attempts have also been made to calculate Tl from the heats of fusion and the melting points of the constituents involved in a given alloy [2]. At the same time, however, it must be recognized that in a crystallization process under rapid cooling conditions both the temperature of solidification and the composition and structure of the crystalline phase, or phases, which compete with glass formation, are likely to be highly nonequilibrium. Even if a suitable adjustment of the Tl value is made to allow for these features, the Tl fails to take into account the kinetic aspects involved in the glass forming process. Yet these aspects undoubtedly also influence the GFA.