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

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 \ub0C, 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 \ub0C 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

Alloying of C40-structured Mo(Si,Al)2 with Nb, Ta and V

Alloying of Mo(Si,Al)2, prepared by sintering, with Nb, Ta or V has been studied. All alloyed materials retained the C40-structured matrix phase of the un-alloyed reference material. The concentration of alloying elements in the C40 phase reached 0.4–1.6 at.% (approximately 20–50 % of the intended levels), and the remaining alloying content was dissolved in the minority D8m- and D88-structured (Mo,X)5(Si,Al)3 phases. V-alloying also promoted the formation of C54-structured (Mo,V)(Si,Al)2

High temperature compression of Mo(Si,Al)2-Al2O3 composites

The aim of this study was to investigate the effect on high temperature of mechanical properties of adding Al2O3 particles to polycrystalline Mo(Si,Al)2. Mo(Si,Al)2-Al2O3 composites, containing 0–25 wt% Al2O3 particles have been compression tested at 1300 \ub0C, and the microstructure after deformation was studied using electron backscatter diffraction. It was shown that even small amounts (5 wt%) of Al2O3 particles resulted in a grain-refined material through inhibition of grain growth during sintering, which lead to lower flow stresses compared to the coarse-grained Al2O3-free material. The inverse grain size effect and post-test microstructure investigations suggest that creep-like deformation mechanisms dominate in fine grained Mo(Si,Al)2-Al2O3 composites at 1300 \ub0C. In the materials containing 5–15 wt% Al2O3, the maximum stress decreased with increasing Al2O3 content. In materials with higher Al2O3 additions, the maximum stress increased with the Al2O3 addition, but did not reach the strength levels in the Al2O3-free reference material. It is suggested that the deformation behaviour is affected by electroplasticity effects as resistive heating was used. Electroplasticity contributes to the decrease in maximum stress observed in the lower Al2O3 containing materials, while this is outweighed by particle strengthening at higher Al2O3 contents

High temperature deformation of polycrystalline C40 Mo(Si,Al)2

Polycrystalline Mo(Si,Al)2 with C40 crystal structure was deformed in compression with a strain rate of 10−4 s−1 at 1300 \ub0C. The specimens were deformed to a strain of 10%–15% and showed maximum stresses around 150 MPa prior to pronounced softening. No crack formation or significant increase in porosity could be observed. Post-test microstructure analysis revealed that the material was inhomogeneously deformed on both inter- and intragranular levels. Dynamic recrystallization occurred alongside low angle grain boundary formation in highly deformed grains. Furthermore, complex intragranular deformation fields suggest that slip systems other than 21̄1̄0[0001] may be active during deformation

Exploring the Effect of Silicon on the High Temperature Corrosion of Lean FeCrAl Alloys in Humid Air

A new approach to reduce the chromium and aluminium concentrations in FeCrAl alloys without significantly impairing corrosion resistance is to alloy with 1-2 wt.% silicon. This paper investigates the "silicon effect" on oxidation by comparing the oxidation behavior and scale microstructure of two FeCrAl alloys, one alloyed with silicon and the other not, in dry and wet air at 600 degrees C and 800 degrees C. Both alloys formed thin protective oxide scales and the Cr-evaporation rates were small. In wet air at 800 degrees C the Si-alloyed FeCrAl formed an oxide scale containing mullite and tridymite together with alpha- and gamma-alumina. It is suggested that the reported improvement of the corrosion resistance of Al- and Cr-lean FeCrAl\u27s by silicon alloying is caused by the appearance of Si-rich phases in the scale

Alloying of C40-structured Mo(Si,Al)2_2 with Nb, Ta and V

Alloying of Mo(Si,Al)$_2$, prepared by sintering, with Nb, Ta or V has been studied. All alloyed materials retained the C40-structured matrix phase of the un-alloyed reference material. The concentration of alloying elements in the C40 phase reached 0.4–1.6 at.% (approximately 20–50 % of the intended levels), and the remaining alloying content was dissolved in the minority D8$_{m-}$ and D8$_{8-}$structured (Mo,X)$_5$(Si,Al)$_3$ phases. V-alloying also promoted the formation of C54-structured (Mo,V)(Si,Al)$_2$

Influence of Yttrium Doping on the Oxidation of Mo(Si,Al)2 in Air at 1500\ua0\ub0C

Mo(Si,Al)2 with different yttrium (Y) additions (up to 2\ua0at.%) was synthesised by dry powder mixing followed by compaction and sintering. In as-sintered materials, Y was present as yttrium aluminium garnet. The materials were exposed in air at 1500\ua0\ub0C for up to 250\ua0h to study the effect of Y on oxidation behaviour. The oxides formed were investigated using scanning electron microscopy (SEM)-based techniques and X-ray diffraction. While the Y-free Mo(Si,Al)2 formed a scale consisting of Al2O3 and a small amount of mullite, the Y-containing samples formed oxides containing both yttrium silicate and larger fractions of mullite, in addition to Al2O3. Oxidation rate, scale spallation, as well as the evaporation of Mo, all increased with Y addition