Activation energy for nucleation and crystal growth during the crystallization of palladium-based metallic glasses containing boron and beryllium

Abstract

Completely or almost completely amorphous ribbons have been obtained by melt spinning around the eutectic composition of Pd-B and Pd-Be. Activation energy for crystallization E c and the n exponent of the Avrami law which governs the kinetics of such a transformation, have been evaluated by DSC tests on Pd 76B 24 and Pd 72Be 28 glassy alloys. These measurements have been carried out both on as-quenched specimens and after different steps of preheating which bring about increasing amounts of crystalline fractions. E c and n are lowered by the presence of preformed nuclei and tend to reach constant values that would correspond to a condition of zero nucleation rate. Such values are considered to represent the activation energy for crystal growth E g and the geometrical contribution b contained in the n Avrami exponent, respectively. A relationship connecting the three activation energies E c, E g and E n (this latter for the nucleation process) is used to evaluate E n.