Compositional Exploration and Microstructural- Environmental Evaluation of Alloys for Glass Vitrification Applications
This thesis investigates the multi-environment performance of alternative alloys for high-temperature glass vitrification, with the aim of improving the durability of Co-101 alloy spinners used in glass mineral wool production. These components operate under aggressive thermal and chemical conditions that accelerate oxidation and corrosion, limiting service life. To address this challenge, two alloying strategies were explored for Co-101: (i) variations in C (0.25 and 0.5 wt%) and Si (1 and 4 wt%) content, and (ii) the addition of Ta (up to 3.1 at.%). The microstructural evolution and oxidation behaviour of these modified alloys were assessed at 800°C, 1000°C, and 1200°C, followed by evaluation of their corrosion resistance under partial immersion in a glass melt at 1100°C. Among the C and Si variations tested, Si had the most pronounced influence on microstructure and performance. Increasing Si promoted the formation of M12C over M23C6 carbides and introduced the χ phase in the low-C, high-Si variant. These changes enhanced oxidation resistance at 800°C and 1200°C through oxidation-induced Laves phase formation and the development of continuous silica scales, albeit at the expense of room-temperature ductility. Ta additions promoted MC carbide formation and, at the highest concentration, replaced the Cr-rich M23C6 network with σ and Laves phases. Moderate Ta contents improved oxidation resistance at 800°C and 1000°C by disrupting continuity of the Cr-rich carbide network; however, at 1200°C, resistance declined due to CrTaO4 formation, which disrupted the silica subscale. In static partial-immersion in modified soda-lime glass at 1100°C, high-Si alloys exhibited the greatest corrosion resistance across air-exposed, transition, and melt-immersed regions. Two low-Si alloys suffered catastrophic oxidation in the air-exposed region, attributed to reduced Cr mobility, exacerbated by volatile-species interactions and galvanic effects. Ta improved corrosion resistance only in the melt-immersed region, where CrTaO4 appeared more stable under low oxygen fugacity. To overcome the limitations of the Co-based system in accommodating higher Si contents, a CALPHAD-guided Fe-Cr-Si alloy series was developed. These alloys were designed to exploit the wide α-Fe + Cr3Si phase field and accommodate high Cr (15-21 at.%) and Si (24 at.%) contents. After solution heat treatment at 1200°C, all alloys formed single-phase solid solutions. Ageing at 700°C and 1000°C promoted Cr3Si precipitation, particularly in Fe-21Cr-24Si, where it remained stable at both temperatures. A hierarchical microstructure, comprising D03- and B2-type ordering within the α-Fe matrix, developed during cooling and contributed to high hardness (≥575 HV1). Oxidation testing at 1000°C confirmed improved resistance with increasing Cr content, with the Fe-21Cr-24Si alloy exhibiting a parabolic rate constant ~33% lower than that of the best-performing Co-101 derivative. This work establishes composition-microstructure-performance relationships and identifies Fe-Cr-Si as a promising foundation for future alloy design in aggressive glass melt-contact environments.
Co-based Superalloy, Corrosion, Materials Science, Metallurgy, Oxidation
Networked Digital Library of Theses and Dissertations