High temperature properties of polycrystalline Mo(Si,Al)2: compression and oxidation

Licentiate thesis, 2021

Electrification of industrial heating processes holds great promise for reducing CO2 emissions. Large furnaces operating at elevated temperatures and in demanding atmospheres are complicated but indeed important to electrify. A material often used as heating elements in small-scale furnaces operating in harsh environments is Mo(Si,Al)2, hence, the material is a high-potential candidate for the electrification of more complicated industrial heating processes. Such applications require Mo(Si,Al)2 heating elements with increased dimensions, which poses new challenges with respect to the high temperature performance of the material. With increasing element size, the mechanical properties are expected to become more important. In this thesis, the mechanical response and the resulting microstructure of polycrystalline Mo(Si,Al)2, tested in compression at 1300 °C, was investigated. The main findings were: (1) the material could sustain large plastic strains without cracking, (2) the deformation was inhomogeneous on both intra- and intergranular scale, and low angle grain boundaries were formed in severely deformed grains, (3) the material softens due to dynamic recrystallisation. In spite of the excellent oxidation and corrosion resistance of Mo(Si,Al)2, there are indications that oxide spallation could present a potential issue for larger heating element dimensions. In this thesis, the effect of reactive element addition, which is known to effectively reduce spallation in e.g. FeCrAl alloys, was investigated. Mo(Si,Al)2 was alloyed with yttrium and exposed at 1500 °C for up to 250 h. The study showed that (1) oxide adhesion was not improved, as the oxide scale spallation increased with increasing yttrium content, (2) also the oxidation rate increased with yttrium addition, (3) yttrium silicate and mullite were formed from a melt within the otherwise pure alumina scale.