Transient nucleation in zirconium-based metallic glasses

Abstract

The nucleation kinetics of zirconium-based metallic glasses has been studied by quantitative electron microscopy. The nucleation behaviour can be derived from crystal size distributions by computer simulations, i.e. by varying the steady state nucleation rate, time lag and number of nucleation sites as fitting parameters. A sufficient condition for such an analysis is a constant, isotropic growth rate as observed during polymorphic crystallization of CoZr 2 or FeZr 2; NiZr 2 exhibits a rather irregular shape. These glasses possess the same activation energy for growth, but growth rates increase significantly in the order Co → Fe → Ni. From crystal size distributions in partially crystallized Co 33Zr 67 or Fe 33Zr 67 glasses it is not possible to distinguish unambiguously between homogeneous or heterogeneous nucleation, but in any case rates are transient and the steady state nucleation rates increase in the sequence Ni → Co → Fe. In contrast to metal-metalloid glasses, the steady state nucleation rate can be described in a wide temperature range around the glass transition temperature by an Arrhenius equation with an activation energy of the same order of magnitude as for zirconium diffusion in CoZr glasses.