Crystallization in glass-forming liquids: Effects of fragility and glass transition temperature

• The relevance of fragility concepts for understanding crystallization is analyzed.
• Classical fragility does not describe appropriately crystallization tendencies.
• A new definition of the fragility index is introduced.
• In terms of this new definition, fragility becomes one of the main factors affecting crystallization.
• The origin of correlations between glass transition temperature and crystallization is specified.

The relevance of the concepts of fragility, m, of the liquid and of the reduced glass transition temperature, Tg/Tm (Tg: glass-transition temperature, Tm: melting or liquidus temperature) for the understanding of crystal nucleation and growth in glass-forming liquids is explored. Based on the analysis of crystallization processes in glass-forming melts, it is shown that classical fragility can be relevant for the understanding of the crystallization behavior only if several severe conditions are fulfilled that are rarely met. By this reason, a new definition of liquid fragility is introduced. This new definition does not involve the commonly utilized temperature ratio Tg/T (T: actual temperature). Instead, we employ the reduced variable Tm/T and do not restrict the computation of m to the glass transition temperature, Tg, as in the standard approach. Moreover, we specify the fragility index by computing it for the temperature appropriate for the particular kinetic process being analyzed. With this modified definition of fragility applied to both the diffusion coefficient controlling crystallization and viscosity, fragility becomes one of the main factors determining the temperatures and magnitudes of the maxima of nucleation, growth, and overall crystallization rates. In addition, the origin of the previously reported correlations between reduced glass transition temperatures and intensity of crystallization processes is specified.