Structural Evolution During Spark Plasma Sintering, Oxidation and Crystallization of Iron-Based Metallic Glass

Abstract

Metallic glasses are a novel class of amorphous materials that have diverse potential applications due to their superior mechanical, electrochemical and tribological properties as compared to their polycrystalline counterparts. However, the requirement of extremely high cooling rates in solidification processing severely limits the manufacturing of these materials to small dimensions such as powder and ribbons. In order to circumvent this problem, this research investigates three thermal responses, namely sintering, oxidation and crystallization in an iron-based metallic glass powder. The resulting original scientific contribution of this dissertation is the establishment of the mechanism and kinetics of sintering, crystallization and oxidation in this iron based metallic glass powder. During sintering at a constant rate of heating, the activation energy of viscous flow decreases linearly with the logarithm of the heating rate while the macroscopic applied pressure is amplified three orders of magnitude to the interparticle contact pressure that enhances densification by micro-viscous flow deformation of the particles. Thermal oxidation leads primarily to the formation of Fe2O3 oxide as a polycrystalline shell around the powder core that increases in thickness rapidly at the beginning and slows down towards the end during isothermal oxidation. Crystallization in this iron based metallic glass powder encompasses four exothermic transformations occurring with a decreasing nucleation rate. These conclusions provide a foundation towards the development of manufacturing bulk sized metallic glass components for structural applications.