Sample-size effects in calorimetric studies of structural relaxation and crystallization of a gold-based metallic glass

Using flash differential scanning calorimetry (FDSC), samples of Au 49 Cu 26.9 Ag 5.5 Pd 2.3 Si 16.3 (at.%) can be repeatedly melted and quenched to form a glassy state without precursors of crystal nucleation. In this way, a detailed time-temperature-transformation diagram is determined for structural relaxation and crystallization of this metallic glass. This extends earlier studies to lower annealing temperatures, as much as 40 K below the conventional glass-transition temperature. The possible links between structural relaxation and crystallization are explored. The C-curve for crystallization is fitted using Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetics. Well below the nose of the C-curve, the fitted parameters have reasonable values, and the rates of crystal nucleation and growth are confirmed by relating to the grain size in cast samples. For higher temperatures, we map the transition into a stochastic regime in which the nucleation events within the small FDSC sample are too few for JMAK kinetics to be valid; in this regime Poisson statistics is successfully applied. Pre-anneals at low temperature can accelerate crystallization, and possible effects on C-curve shape are considered. We also examine how the small sample size essential for FDSC limits the capability of the technique for kinetic analysis.