Crystallization processes in NiTiB glassy alloys of near-ternary-eutectic composition

Abstract

The crystallization kinetics and mechanisms of three NiTiB glasses have been examined with a view to elucidating the roles of chemical composition and quenched structure on subsequent behaviour. Alloys of composition near a ternary-eutectic point have been chosen because they represent a real and complex situation where several crystalline phases may form simultaneously. Crystallization processes are analysed in terms of nucleation and growth stages. Different nucleation mechanisms have been observed for the different alloys: these seem to be best explained in terms of the detailed, short range ordered structure of the quenched glass. Analysis of crystal glass interface energies, based on structural examination using High Resolution Electron microscopy, indicates that it is not this energy term which controls the nucleation of crystals on annealing the glass. Crystal growth may involve a eutectic mechanism, or a single-phase mechanism controlled by interface or matrix-diffusion kinetics. Crystallization is fastest when eutectic nucleation and growth occurs: formation of the eutectic colony requires a specific sequence of phase nucleation, namely the initial formation of the phase of complex structure followed by the phase of simpler structure.