Fast and slow crystal growth kinetics in glass-forming melts

Published values of crystal growth rates are compared for supercooled glass-forming liquids undergoing congruent freezing at a planar crystal-liquid interface. For the purposes of comparison pure metals are considered to be glass-forming systems, using data from molecular-dynamics simulations. For each system, the growth rate has a maximum value U-max at a temperature T-max that lies between the glass-transition temperature T-g and the melting temperature T-m. A classification is suggested, based on the lability (specifically, the propensity for fast crystallization), of the liquid. High-lability systems show "fast" growth characterized by a high U-max, a low T-max/T-m, and a very broad peak in U vs. T/T-m. In contrast, systems showing "slow" growth have a low Umax, a high T-max/T-m, and a sharp peak in U vs. T/T-m. Despite the difference of more than 11 orders of magnitude in U-max seen in pure metals and in silica, the range of glass-forming systems surveyed fit into a common pattern in which the lability increases with lower reduced glass-transition temperature (T-g/T-m) and higher fragility of the liquid. A single parameter, a linear combination of T-g/T-m and fragility, can show a good correlation with U-max. For all the systems, growth at Umax is coupled to the atomic/molecular mobility in the liquid. It is found that, across the diversity of glass-forming systems, T-max/T-g = 1.48 +/- 0.15. (C) 2014 AIP Publishing LLC.