Kinetic competition during glass formation

For vitrification of an alloy melt during cooling there is a kinetic competition with the nucleation and growth of metastable and stable crystalline phases. Many of the measures of glass forming ability (GFA) attempt to capture some of the features of the kinetic competition, but the GFA metrics are static measures and the kinetic processes are dynamic in nature. In fact, the critical cooling rate for glass formation should be viewed in terms of a critical cooling rate range to acknowledge the stochastic nature of crystal nucleation behavior. Direct measurements of the critical cooling rate range confirm this behavior and also provide useful input for kinetics analysis. Usually kinetics analyses are based upon crystallization behavior that is measured either isothermally or upon heating to temperatures near the crystallization onset, Tx and the results are extrapolated to much higher temperatures. This practice is based upon a number of assumptions about transport behavior in the undercooled liquid. With rapid up-quenching of amorphous samples, the high temperature crystallization behavior can be measured and used to refine the kinetics analysis and provide useful insight on the kinetic competition and glass forming ability.