Factors influencing heterogeneous nucleation in an NiSiB glass

Abstract

A crystallization study of isothermally annealed Ni-7at.%Si-17at.%B ribbon using quantitative metallography is presented. Particular attention was devoted to the investigation of the possible factors influencing nucleation and growth rates. In spite of the care taken during sample preparation, crystal nucleation at low temperature was dominated by impurity effects, independent of the quenching rate used to prepare the ribbon. At higher temperatures, nucleation and growth were continuous and constant, irrespective of the quenching rate. The high sensitivity to contaminants (earlier observed by rapid crystallization on the ribbon surfaces) was demonstrated by deliberate internal oxidation. Once more, the main effect of these impurities was observed at low temperature, adding to the pre-existing impurity effect, resulting in a significant increase in nucleation rate. In order to quantify the influence of impurities on crystallization, fine dispersoid particles were voluntarily added to the alloy before spinning. These particles present the advantage of being directly observable, and nucleation behaviour can be related to known properties such as stability, interfacial energy, structure or shape. From the high melting temperature carbides, nitrides and oxides chosen, several illustrations of the crystallization behaviour are shown and discussed. The aim is to develop an understanding of the influence of such dispersoid or impurity particles on overall crystallization kinetics. This information may be useful in enhancing or inhibiting heterogeneously nucleated crystallization and may lead to well-controlled transformations and structures.