In situx-ray-scattering studies of polymorphic crystallization of metal-boron glasses

Time-resolved x-ray scattering has been used to study the isothermal crystallization kinetics of the binary metal-boron glasses ${\mathrm{Co}}_2$B, ${\mathrm{Fe}}_{76}$ ${\mathrm{B}}_{24}$, and ${\mathrm{Co}}_3$B, for crystallization times as short as ≊1 s. For alloys which crystallize into a single phase, a simple model of the transformed volume fraction, based on steady-state homogeneous nucleation of crystallites which then grow at a constant rate, explains the results. There is no evidence for a transient in the nucleation. A one-parameter fit to the slowest diffraction data allows volume fractions as small as ${10}^{\mathrm{-}4}$ to be measured. The observed crystallization kinetics agree well with calorimetric and resistivity measurements. For each alloy, the kinetics are well described by a single activation energy, even at the highest transformation rates. For alloys which crystallize into a single phase, the transformation curves can be scaled onto each other by renormalizing the transformation time. Comparisons are drawn between the behavior of these metal-metalloid glasses and previously studied metal-metal systems.