Open Access

The first of its kind, the AlMo0.5NbTa0.5TiZr alloy coined the term "refractory superalloy" (RSA) for it exhibits Ni-Base-like dual-phase microstructure with a high fraction (≈ 40 %) of disordered body-centered cubic (A2) cuboidal particles that are coherently embedded in a continuous or-dered (B2) matrix. Here, the tensile creep behavior and the micromechanisms occurring at the minimum creep rates reached in this RSA are presented for specimens tested in vacuum at 900 – 1100 °C and 30 – 160 MPa. Microstructural changes were assessed on the head and gage of crept samples using scanning and transmission electron microscopy to respectively study the effects of temperature and external load. In both stressed and stress-free regions, a significant volume of B2 phase transforms into a mixture of hexagonal Al4-xZr5-based and new A2 phase with needle, domain and grain boundary substructures. There, the A2 solid solution becomes the continuous phase, which is surrounded hard by Al4-xZr5 platelets. This "topological inversion" probably promotes plasticity and thus degrades creep resistance. This phase transformation is more pronounced at the lower temperatures regardless of the applied stress (σ), which likely ex-plains for the very low apparent activation energy for creep of this RSA (≈ 111 kJ/mol). Further deformation micromechanisms are discussed in light of the aforementioned microstructural in-stability. At stress levels < 90 MPa, the Norton, n, reaches ≈ 3 at all temperatures, which is asso-ciated with solute-drag creep, while behavior dislocation creep (n ≈ 5) is inferred at higher stresses.

In order to transition to a more sustainable and environmentally friendly future, a change in power generation is necessary. Reducing CO₂ output is a crucial factor to combat climate change. A major contributor to CO₂ emissions is the generation of electricity, mainly from coal and lignite combustion, emitting billions of tons of CO₂ every year. Additionally, coal and lignite are fossil fuels and therefore are not replenishable in human time frames. One solution to this problem is firing existing power plants with biomass as a substitute. This has the advantage of both being a renewable energy source and significantly reducing CO₂ emissions. However, the use of biomass can cause issues within the reactor by corroding the super heater tubes. This phenomenon occurs due to the chemical differences between biomass and coal, as well as the composition of super heater tubes, which were engineered to withstand coal combustion, not biomass combustion. During the firing of biomass, salt deposits form on the surface of the steel, quickly corroding the underlying metal. This hinders the efficiency and lifetime of super heater tubes, requiring replacement and causing excessive costs. In this work, model alloys, designed after commercially utilized steels, are investigated in varying environmental conditions. One ferritic alloy (Fe-13Cr, similar to X20 steel) and two ferritic-austenitic alloys(Fe-18Cr-12Ni (similar to TP347H), Fe-25Cr-20Ni (similar to HR3C)) are exposed to experimental temperatures of 560 °C. The corrosion attack is investigated using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) to determine the structure and elemental distribution of both alloy and the forming scale. Phase composition of the scale is determined using X-ray diffraction (XRD), and thermodynamic boundary conditions are calculated using FactSage. A common phenomenon observed in biomass combustion is the formation of KCl deposits. Therefore the effect of KCl deposits on steel corrosion is investigated in chapter 2. For these experiments steels are tested in a SO₂ bearing atmosphere with and without KCl deposits. Steels without KCl show good corrosion resistance in the form of a thin Cr₂O₃ layer. Samples with KCl deposits show a multilayered scale comprised of metal sulfides and oxides. The alloy composition also shows a large impact, with higher alloyed steels showing stronger Cr depletion and Ni bearing samples exhibiting large internal corrosion for samples with KCl. The third chapter studies the initial growth (5 h) of the scale in samples with and without KCl coating using synchrotron radiation. The initial formation of the scale is important, since some phases only exist for a short time before reacting further. This allows for a better understanding of the corrosion mechanism and how KCl impacts the scale growth. The impact of alloy composition and different microstructure of the alloys was studied. Samples without KCl coating showed a protective scale of Cr₂O₃ throughout the experimental length. All samples coated with KCl showed a weight gain about one order of magnitude higher than their non-coated counterpart. This increase in weight gain is linked to the higher proportion of formed Fe-oxides and higher Fe-Cr ratios in mixed (Fe,Cr)₂O₃. The coated samples also show a large Cr depletion towards the surface for highly alloyed samples. This prevents the formation of new Cr₂O₃ after possible spallation events. The results show that KCl is detrimental for all sampled steels and that highly alloyed Ni bearing steels aren't beneficial in this kind of environment, with Fe-13Cr performing the best. The fourth chapter studies the impact of humidity on the corrosion behavior in biomass fired power plants. To simulate the impact, Fe-18Cr-12Ni, with KCl deposits, is exposed at 560 °C in either laboratory air or SO₂. In each atmosphere one experiment is performed under dry conditions while a second one is conducted under humid conditions. This variation in humidity represents both the possible range of humidity from different biomass sources as well as the possibility of influencing the humidity artificially by drying the biomass. In dry lab air the sample showed about double the weight gain and porosity when compared to humid air. In SO₂ bearing atmosphere the difference is even more pronounced. The dry sample showed a scale width about one order of magnitude larger than the humid counterpart. Additionally, the dry sample showed internal corrosion, which wasn't observed in the humid sample. The better performance of Fe-18Cr-12Ni is linked to the chlorine compounds found in this environment. H₂O allows for the formation of HCl, instead of Cl₂, which has a lower reaction rate with steels. When SO₂ is present, the H₂O facilitates the formation of K₂SO₄, which together with Cr₂O₃ form a protective layer preventing further scale growth. These results show that artificial drying is counterproductive for biomass combustion and significantly accelerates the corrosion attack.

The conversion of heat- into electrical energy requires the combustion of matter, which is harmful to components of combustion power plants, automotive, air- and space industry because of the corrosive impact of combustion products i. e. alkali chlorides NOx, CO2, O2, H2O, sulphurous gases (H2S, SO2, S2, SO3) and ash. Essential structural alloys are austenitic and ferritic steels that can be protected from high-temperature corrosion by a slowly growing, dense but fine-grained chromium oxide (Cr2O3) layer. The Cr content above a critical value or the addition of alloying additives i. e. Al or reactive elements such as Ce, Zr, La, Hf and Y, in principle promote protection by Cr2O3. However, diffusion and transport mechanisms at various interfaces of the alloy’s microstructure and the formed scale (alloy/oxide; oxide/oxide; oxide/gas) are proceeding during the dynamic corrosion process. In austenitic stainless steels, the diffusivity of the protective layer forming elements is much slower than in ferritic stainless steels. The microstructural impact in atmospheres where oxygen is the only oxidising agent is well known. In contrast, the understanding of the microstructure impact (considering: grain boundaries, carbides, grain size and a duplex phase) of ferritic model alloys with high Cr content (> 13% Cr weight fraction) on corrosion in media containing SO2 and H2O is missing to improve the materials corrosion resistance. In the present work, Fe−16Cr−0.2C model alloys were used as a model system to investigate the microstructural impact on the high-temperature corrosion behaviour in environments where SO2 and H2O were the harmful species. The alloys grain size, grain boundary misorientation angle and corresponding grain boundary line length, the carbides diameter and area fraction were extracted employing Electron Backscatter Diffraction (EBSD) to attribute the microstructural effect after exposure at 650 ◦C. The focus of this work was the derivation of the microstructure-dependent corrosion mechanisms after short- and long-term exposure (30 min up to 1000 h) considering thermodynamic calculation principles as well as qualitative phase analysis with electron and X-ray diffraction methods, mass spectrometry and electron microscopy. By the comparison of samples with different microstructural states, it was shown that phase boundaries carbide/alloy and High-Angle Grain Boundary (HAGB) are the preferred diffusion paths for initial oxide and sulphide growth at the material interface and that the material’s damage can be reduced by a targeted heat-treatment. The correlation between phase growth sequence and microstructural state was observed in real-time via in situ X-ray diffraction in the early stage of oxidation (5 sec. up to 4 h) at 800 ◦C in dry air. All samples show the initial growth of Cr2O3, and the time to breakaway oxidation was significantly reduced in the fully recrystallised state. The dissolution of sub-micrometre sized carbides of low area fraction is proposed to explain a contributing effect on Cr2O3 formation. The hypothesis that high fractions of short circuit diffusion paths contribute to protective oxide layer formation cannot be confirmed for 800 ◦C in dry air because of fast proceeding recrystallisation. The improved understanding of the causes of internal sulphidation and higher oxidation rates for microstructure manipulated Fe − 16Cr − 0.2C model alloys in hot SO2 and H2O environments, the following implications for appreciating ferritic microstructures can be deduced to decrease the corrosive attack. A 16 wt.% Cr content in ferritic model alloys is high enough for Cr2O3 formation. Sulphidation, as well as high oxidation rates, can be reduced by a targeted heat-treatment to reduce the HAGB line length. Furthermore, the carbides diameter must be in the sub-micrometer scale and their area fraction low to effect the chromia maintainance at prolonged exposure times by decarburisation.

Concentrated solar power (CSP) systems commonly use solar salt as a high temperature heat transfer and storage medium. This study examines the effect of oxygen (O2) and nitrogen oxide (NO) gas concentrations on solar salt thermal stability and 310N stainless steel corrosion at 600°C up to 1224 h. The impact of the gas atmosphere (5–80 vol% O2 , 400–600 ppm NO) on salt chemistry, including nitrate, nitrite, oxide, and chromate ions, was analyzed, and corrosion behavior was evaluated through weight change, corrosion rate, and microstructural analysis. The results show that introducing NO gas concentrations ≥ 400 ppm with at least 5 vol% O2 stabilizes salt decomposition by controlling nitrite and oxide ion formation and promotes the development of a protective corrosion layer on the steel surface. Once this protective layer is formed, variations in oxygen concentration (5–80 vol%) in the presence of ≥ 400 ppm NO gas have a minimal long‐term effect on the corrosion behavior of stainless steel in solar salt. These findings underscore the importance of maintaining minimum O2 and NO levels to optimize salt chemistry that effectively mitigates steel corrosion in solar salt systems.

Titanium aluminide components fabricated by wire-laser directed energy deposition (DED-LB/M) with a high deposition rate of ~ 205 cm3 * h-1 are investigated. To minimize oxidation during processing, deposition is conducted under vacuum with argon (Ar) shielding. The effect of substrate preheating on reducing thermal gradients and promoting room-temperature (RT) ductility was explored. Microstructures are characterized using optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and X-ray diffraction (XRD). In the as-deposited condition, the microstructure is heterogeneous, comprising large spheroidal regions of gamma-TiAl alongside smaller equiaxed grains containing an alpha2/gamma lamellar structure. Heat treatment at 1375°C for 2h produces a fully alpha2/gamma lamellar microstructure within very uniform, equiaxed grains (∼0.5–2 mm). The interlamellar spacing is 100–400 nm. In the heat-treated condition, a homogeneous hardness distribution of 300–350 HV1 is obtained across the entire deposited block.