Microstructure and high temperature properties of Mo(Si,Al)2 - The effect of particle strengthening and alloying

Doctoral thesis, 2024

High temperature heating processes within the steel industries result in significant emissions of CO2, primarily due to the combustion of fossil fuels. Electrification of these processes, such as through the implementation of resistive heating elements, holds great promise for reducing emissions. However, a bottleneck in the transition to a more environmentally friendly industry is related to the materials used for these heating elements.

Mo(Si,Al)2 is a ceramic material commonly used for heating elements in various high temperature furnaces and is being considered for large-scale industrial-scale applications. While its oxidation properties have been extensively studied, its mechanical properties, which are crucial when increasing the size of the heating elements, have received limited attention. In this thesis, the high temperature deformation behaviour of Mo(Si,Al)2-based materials, and potential routes for their improvement, have been investigated.

This work has shown that diffusion-driven grain boundary sliding is the main deformation mechanism in polycrystalline Mo(Si,Al)2, particularly in fine-grained materials. In coarse-grained materials, the slip of dislocations also contributes to deformation. Moreover, coarse-grained Mo(Si,Al)2 relaxes through the formation of low-angle grain boundaries and dynamic recrystallization. The addition of Al2O3 particles, to achieve particle strengthening, results in a competition between a negative effect from grain refinement at low fractions (up to 15 wt.%), and a positive effect from inhibition of grain boundary sliding at higher fractions.

Also alloying with W, Nb, Ta, and V has been studied, among which W was the most promising alternative. The solid solubility of W in Mo(Si,Al)2 was high, and it also led to a slight improvement in high temperature strength. The solubility of the alloying elements Nb, Ta, and V was found to be low in Mo(Si,Al)2. Instead, these elements were enriched in secondary phases. Additionally, Y alloying has been explored to investigate its effect on oxidation behaviour. However, the oxide adhesion was adversely affected.