Nb-Al2O3 refractory composites have attracted significant attention due to their excellent performance at elevated temperatures. The incorporation of both metallic (Nb) and ceramic (Al2O3) phases enhances interest in these materials, i.e., the combination of the high toughness and plasticity of niobium with the refractoriness, high hardness, thermal stability, and oxidation resistance of alumina. In this study, the high-temperature mechanical behaviour of Nb-Al2O3 composites produced via field-assisted sintering (FAST) was investigated under compressive loading. Composites containing alumina particles of different sizes were subjected to compressive tests up to 1350 degrees C. The results were evaluated with respect to Nb content, alumina particle size, applied stress, and test temperature. Initial and post-mortem microstructural analyses were carried out to characterise the deformation behaviour. It was observed that an increase in the metal content led to an enhancement of plasticity, while changes in alumina particle size influenced the deformation behaviour.

In the present study, various thermo-mechanical tests under compressive load were performed on commercial MgO-C refractory brick grades over a wide temperature range up to 1600 degrees C in an argon atmosphere. The focus of the present study was the assessment of the influence of the incorporation of MgO-C recyclate on the thermomechanical behavior. Therefore, one MgO-C brick grade contained only fresh raw materials, while the other grade included MgO-C recyclate with a high mass fraction of 47.5 wt%. Subsequent investigations, including scanning electron microscopy and porosity analyses, provided insights into the mechanisms occurring within the material during the thermo-mechanical tests. The incorporation of MgO-C recyclate led to an improvement in the thermo-mechanical properties, as increased compressive strength values were observed within the tested temperature range, along with enhanced creep resistance and improved behavior in the refractoriness under load tests.

Die Kombination von quasi in situ Experimenten im Rasterelektronenmikroskop (REM) mit der Methode der digitalen Bildkorrelation (DIC) ist eine leistungsstarke Methode, um die Ausprägung von Dehnungslokalisationen zu untersuchen. Zusätzlich können Verfahren wie die Elektronenrückstreubeugung (EBSD) ergänzend zur Interpretation eingesetzt werden. Allerdings birgt die DIC an REM-Aufnahmen einige Herausforderungen. Die größte Herausforderung dabei ist die Erzeugung eines geeigneten Oberflächenmusters, das in engem Zusammenhang mit dem untersuchten Material und dem durchgeführten Experiment steht. Das Ziel der Kontrastierungsmethode ist es, ein zufällig verteiltes, kontrastreiches Speckle-Muster bis in den Submikrometerbereich zu erzielen, um sowohl eine hohe räumliche Auflösung als auch eine hohe Dehnungsauflösung bei den DIC-Berechnungen zu erreichen. Eine weitere Herausforderung stellt der Scan-Charakter der REM-Abbildungen dar, da komplexe Bildverzerrungen (räumlicher Verzerrung und Drift) treten können. Der Beitrag gibt zunächst einen Überblick über den Einfluss von Bildaufnahmeparametern und verschiedene Kontrastierungstechniken. Im Anschluss daran wird eine Fallstudie zur hochaufgelösten DIC an einem hochfesten, metastabilen, austenitischen rostfreien Stahl vorgestellt, der bei RT im REM einer zyklischen Belastung unterzogen wurde. Die hochaufgelösten zweidimensionalen DIC-Ergebnisse zeigen eindrucksvoll, dass Martensitkörner, die sich während der Ermüdung entweder im Zug- oder Druckhalbzyklus bilden, mit ihren orientierungsspezifischen Dehnungsfeldern im Bereich ≤ 1 µm identifiziert werden können.

To increase the safety of steels in high performance cases like crash energy absorption, even better properties of the materials are necessary. To advance this research, a TWIP and a TRIP steel were combined in a laminated composite via roll bonding at 450 °C with the goal of using accumulative roll bonding (ARB) in later research to further enhance the properties reaching an ultra-fine-grained material. Two different TWIP layer thicknesses (2 mm and 3 mm) were experimentally roll bonded with a 3 mm thick TRIP layer each using a 4-high rolling mill. A modular Python-based simulation incorporating coupled solving of ordinary differential equations of the temperatures and the horizontal stress changes of the layers were implemented to predict deformation and bonding behavior. Simulated results matched well with experimental data in terms of final geometry and temperature, while roll force deviations indicated the need for the refining of the used model. Furthermore, experimentally asymmetric layer relationships at the beginning and the addition of a thin (10 µm) Ni interlayer were found to enhance bond strength in high-strength steel laminates.

Creep in compression experiments at 900 degrees C are conducted on coarse-grained steel-ceramic composites reinforced with different volume fractions of recycled magnesia particles. Strain accumulation within the austenitic steel matrix is characterized through detailed electron back-scattered diffraction (EBSD) analysis using the parameters kernel average misorientation (KAM) and grain reference orientation deviation (GROD). The creep resistance is significantly improved by increasing the volume fraction of ceramic reinforcements. Subgrain formation in the steel matrix is identified as the dominant creep mechanism. Both parameters KAM and GROD are suitable for evaluating lattice misorientation in the as-sintered and creep-deformed microstructures. Higher ceramic volume fractions increase the average lattice misorientation in the as-sintered states. Under the same applied creep stress, the unreinforced steel matrix and composite variants show comparable misorientation distributions. An increase in the applied stress significantly shifts the distributions to higher misorientation values. High-resolution electron channeling contrast imaging (ECCI) is used to confirm that KAM mapping reliably identifies subgrain structures within the steel grains. Average subgrain sizes range from 2 to 4 & micro;m and decrease moderately with increasing creep strain. No significant differences in subgrain size are observed between the unreinforced steel and the composite variants.

In the present study, the chemical processes occurring during high-temperature testing are investigated for two commercial MgO-C brick grades. One grade consists exclusively of virgin raw materials, while the other contains 47.5 wt.% MgO-C recyclate. High-temperature testing is conducted under argon atmosphere at ambient pressure using induction heating. To evaluate the effect of MgO-C recyclate incorporation on thermo-chemical processes, X-ray diffraction (XRD), electrothermal vaporization (ETV), differential thermal and thermogravimetric analysis (DTA/TG) coupled with mass spectrometry (MS) are applied. Scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) provide phase characterization after exposure at 1300 degrees C and above. The results show that mechanisms typically occurring during the service of refractory materials such as the carbothermal reduction of magnesia, the incorporation of impurity phases into the newly formed MgO surface layer, and the deposition of calcium-rich phases and whisker-like structures also occur when MgO-C is exposed to the applied gas atmosphere. This confirms the relevance of the observed thermo-chemical processes and surface phase formation for real contact with molten steel and slag. Under the present test conditions, however, no significant influence of MgO-C recyclate incorporation on the thermo-chemical behavior of the coked MgO-C materials is detected.

Iron-based shape memory alloys (SMAs) are promising alternatives to conventional SMA candidates such as Ni–Ti regarding cost-efficiency. The present study investigates cold-rolled Fe–Ni–Co–Al–Ti–B samples subjected to different recrystallization heat treatments. The impact on microstructure and thermomechanical behavior is discussed in detail. Two distinct recrystallization heat treatment (HT) procedures were conducted. After both HT processes, the samples were aged at 600 ℃ for 4 h to precipitate the γ'-phase, which is crucial for the thermomechanical behavior of the material. The cold-rolled material exhibits β-phase, which fully dissolves during a solution annealing above 1200 ℃. In addition, a Goss-type texture has formed with the main component in hkl < 100 > γ. During the HT process, both recrystallization and grain growth occurred. Thermomechanical experiments demonstrate reversible shape memory behavior and transformation strains up to 5

This study investigates the thermo-mechanical behaviour of steel-ceramic composites based on a matrix of austenitic stainless steel and coarse-grained magnesia particles. Composites reinforced with fresh-fused magnesia particles were compared to those reinforced with recycled magnesia sourced from spent MgO-C refractory lining bricks. The volume fraction of recycled magnesia varied from 20 vol% to 40 vol%. Quasi-static compression tests were performed at room temperature and within a temperature range of 800 degrees C-1100 degrees C. Results showed that incorporating coarse-grained ceramic reinforcements into the steel matrix increased the compressive strength at all temperatures. The composite reinforced with recycled magnesia demonstrated superior mechanical properties over the variant with fresh-fused magnesia due to a smaller and more uniform particle size distribution resulting from crushing steps in the recycling process. Although non-metallic inclusions from contaminated zones of the refractory bricks were present in the recycled powder, they did not affect the overall damage mechanisms.

Given the growing emphasis on refractory recycling, the performance characteristics of commercial MgO-C bricks incorporating MgO-C recyclates are of considerable economic and environmental relevance. In the present study, four-point bending tests are conducted in an argon atmosphere on three commercial MgO-C brick grades over a temperature range from room temperature to 1300 degrees C. One MgO-C brick grade is produced exclusively from virgin raw materials, whereas the other two grades incorporate MgO-C recyclate at a high mass fraction of 47.5 wt%. Due to the different sources of the recyclates, their carbon contents vary, resulting in different total carbon contents in the two MgO-C brick grades containing recyclate. Post-mortem SEM investigations show predominantly transgranular fracture through the MgO-C recyclates. The evaluation of the four-point bending tests reveals no detrimental effect of the MgO-C recyclates on bending strength. Instead, a decisive influence of the total carbon content is observed, with lower total carbon contents correlating with higher bending strength values. Furthermore, the heterogeneous structure of the MgO-C recyclates contributes to enhanced stress dissipation caused by anelastic deformation processes in the respective MgO-C brick grades, despite the accompanying decrease in virgin flake graphite content.

In light of the increasing focus on the recycling of refractories, the properties of commercial MgO-C refractory bricks containing MgO-C recyclate are of significant economic and ecological importance. In the present study, MgO-C recyclate and two commercially available MgO-C bricks - one grade consisting exclusively of fresh raw materials while the other containing 47.5 wt% MgO-C recyclate - were comprehensively characterized. This involved the use of X-ray computed tomography, scanning electron microscopy, X-ray fluorescence analysis, Xray diffraction, as well as density and porosity measurements. The MgO-C recyclate exhibited a heterogeneous composition consisting of MgO aggregates and contained an increased content of impurities compared to fresh MgO raw materials. The incorporation of MgO-C recyclate as a raw material for commercial MgO-C bricks resulted in a decrease of the average MgO aggregate size, a higher porosity with a decrease in the median pore size and a reduced CaO/SiO2 ratio of the corresponding MgO-C brick grade. Furthermore, the MgO-C grade with 47.5 wt% MgO-C recyclate exhibited a higher cold crushing strength, but a reduced oxidation resistance.

ABSTRACTThree variants of 42CrMo4 steel were investigated regarding the fatigue life at two test frequencies of 90 Hz and 20 kHz and temperatures up to 500°C. The materials varied in different levels of sulfur and oxygen content and steel cleanliness in terms of nonmetallic inclusions. The steels had a tempered martensitic microstructure with a hardness of 300 HV. A frequency dependence was observed only for the medium‐sulfur steel, whereas for low‐ and high‐sulfur levels, both test frequencies yielded in a common S‐N curve. The fracture surface analysis of the fatigue specimens was compared with the inclusion rating done on unloaded samples. A correlation of both methods was observed by introducing a size factor. Based on the distribution functions and by using Murakami's approach of a projected area of an inclusion, a method was developed to predict the S‐N curve. This offers the opportunity for an inclusion‐based lifetime prediction.

Elastocaloric cooling (eCC) is the most promising alternative to conventional refrigeration, leveraging the reversible martensitic transformations in nickel titanium (NiTi) shape memory alloys (SMAs) to achieve substantial adiabatic temperature changes. This study investigates the feasibility of commercially available NiTi sheets for such applications. Samples processed via electrical discharge machining (EDM), laser machining, and diamond smoothing were characterized using scanning electron microscopy (SEM), optical microscope (OM), differential scanning calorimetry (DSC), IR thermography (IR-TG), digital image correlation (DIC), and acoustic emission (AE). The sheets exhibited superelastic behavior with a critical transformation stress of 400 MPa and an adiabatic temperature change of ± 20 K. However, rapid functional degradation occurred in the initial loading cycles, marked by a 50 MPa drop in transformation stress, 0.5

This study investigates the microstructural and mechanical properties of lattice structures made of two high-alloy austenitic CrMnNi steels with TRIP (TRansformation-Induced Plasticity) and TWIP (TWinning-Induced-Plasticity) effect. For this purpose, lattice structures consisting of a unit cell of type f2ccz were produced by additive manufacturing using the electron beam powder bed fusion (PBF-EB/M) technique. The influence of the alloy composition of the steel during the PBF-EB/M process on the microstructure in the thin struts of the lattice structures are discussed by the results of electron backscattered diffraction (EBSD) investigations. It is shown that the texture of lattice structures made of steel with tendency to a pronounced columnar grain growth and a strong preferential orientation of the grains after PBF-EB/M in bulk material is interrupted in the thin lattice struts. The mechanical characterisation of the lattice structures is carried out under Out-of-plane (OOP) and In-plane (IP) quasi-static compression tests. The occurrence of the TRIP and TWIP effect in the lattice structures is described in dependence of the chemical composition of the steels and the impact of these effects on the mechanical properties of the lattice structures will be discussed. Additionally, the local strain distribution in the struts is analysed using Digital Image Correlation (DIC) and Kernel average misorientation (KAM). The results revealed that lattice structures with a pronounced TRIP effect achieve higher strength and energy absorption capacity under Out-ofplane compressive load than lattices with TWIP effect. In contrast, the TRIP/TWIP effect is of minor importance for the mechanical behaviour of the lattice structures tested under In-plane loading conditions.

The present study explores the properties of both fine- and coarse-grained refractory composites made from niobium and alumina, designed for high-performance applications at extreme temperatures, in abrasive environments, and under mechanical stresses. The effects of various production methods, including casting and extrusion, and different alumina particle sizes (fine or coarse) on the mechanical properties under compressive loads are investigated. Compression tests, as well as stress-relaxation and creep tests under compressive loads, are conducted at room temperature and high temperatures up to 1500 degrees C. The results are analysed concerning initial open porosities and particle sizes. The findings reveal that both particle size and manufacturing method significantly affect the mechanical properties of Nb-Al2O3 refractory composites. Additionally, the choice of production technique substantially influences the microstructure of the composites, leading to noticeable differences in mechanical performance. This research highlights the essential role of optimizing particle size and production methodologies to enhance the operational properties of Nb-Al2O3 refractory composites.

In the present study a metastable austenitic stainless steel X2CrMnNi16-7–4.5 was investigated. The alloy composition was adjusted by mixing steel powder X2CrMnNi16-7–9 and steel powder X2CrMnNi16-7–3, whereby the first steel exhibits a primary-austenitic and the latter one a primary-ferritic solidification of the melt, in order to achieve a fine-grained, predominantly austenitic microstructure. After mixing of the powder blend the material was subsequently processed by in situ alloying during powder bed fusion electron beam melting (PBF-EB/M), using two different build parameter sets. The study demonstrates how powder blending and in situ alloying can be used to tailor microstructural features like grain size, texture and phase composition in PBF-EB/M processing by changing the chemical composition of an alloy. The microstructure and phase composition of manufactured specimens were examined by different techniques, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), electron backscatter diffraction (EBSD) and measurements of ferromagnetic phase content. The steel was predominantly austenitic and exhibited a fine-grained microstructure for one of the build parameter sets, with a slight < 011 > texture in build direction (BD) after the PBF-EB/M process. Mechanical properties of alloy X2CrMnNi16-7–4.5 were characterized by tensile as well as low cycle fatigue (LCF) tests. In tensile tests the material possesses excellent mechanical properties due to the occurrence of the TRIP (TRansformation-Induced Plasticity) effect under loading, whereby the orientation of the loading axis (LA) relative to the build direction plays a detrimental role. Fatigue tests revealed that surface polishing did not show any improvement in fatigue lifetime compared to the as-built specimens with a natural surface, which was attributed to the presence of numerous inclusions and lack of fusion (LOF) defects.

AbstractThis chapter presents results on the analysis of nonmetallic as well as intermetallic inclusions within a metal matrix. In both, steel and aluminum matrix these impurities cause detrimental effects during production as well as in service, e.g. under mechanical load. In steel, nonmetallic inclusions originate from the steelmaking process and range in the magnitude of ppm. In recycled aluminum alloys, iron-rich intermetallic phases exhibit a volume fraction in the range of percent caused by insufficient scrap separation. Both types of detrimental inclusions/precipitates were investigated within different materials such as case hardening steel, quenched and tempered steel as well as Al-Si cast alloy. In order to reduce the amount of impurities, the effects of appropriate crucible materials, reactive and active melt filtration and chemical composition of the used materials were studied. Therefore, extensive metallographic investigations on sections were conducted with optical microscopy, manual and automated scanning electron microscopy, focused ion beam preparation and transmission electron microscopy aiming to determine the compositions of inclusions and intermetallic phases. Focusing on the morphology of inclusions and intermetallic phases, experiments with electrolytic and chemical extraction as well as X-ray micro tomography were performed. The gained knowledge can be utilized to improve filtration and reduce volume fraction and size of nonmetallic inclusions and intermetallic phases. This enables the design of long-lasting and safe materials.

AbstractThis chapter is focused on the fatigue life and damage mechanisms of steel 42CrMo4 in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes at temperatures up to 773 K. For this purpose, resonance fatigue testing was used at different test frequencies (90 Hz and 20 kHz). The influences of the manufacturing process (wrought or cast condition), as well as the core hardness (various heat treatment conditions), were investigated. Fractographic examinations of the fracture surfaces allowed the analysis of crack-initiating defects. Together with light microscopic observations of the defect distribution, the fatigue mechanisms of the steel 42CrMo4 were investigated at different temperatures (RT, 473 K and 773 K). A short crack model according to Chapetti applied to the present results was used to describe the change in the fatigue damage mechanisms operating at RT/473 K and 773 K, respectively. It is shown that high-temperature fatigue at 773 K was dominated by crack growth, whereas fatigue at RT and 473 K was dominated by crack initiation. These investigations complete the work presented in Chap. 24, in which the influence of nonmetallic inclusions on the ultrasonic fatigue behaviour of steel 42CrMo4 is being analysed at room temperature. The present results provide important insights into the crack-initiating defects and their distributions as they are relevant in typical industrial applications.

Ultrasonic fatigue testing (USFT) is a time-efficient method for evaluation of the fatigue limit of metallic alloys in a high and very high cycle range. Propagation of a final crack at the very end of the fatigue life can be clearly detected by USFT parameters and monitoring techniques. In contrast, initiation of fatigue damage during USFT remains unclear. Despite its excellent sensitivity, the implementation of acoustic emission (AE) for damage monitoring is hindered by severe noisiness of acoustic signals from USFT. In addition to resonance-related quasi-stationary noises, USFT in a pulse-pause mode is accompanied by non-stationary ones, which makes recognition of material-related AE signals even more difficult. A special AE processing algorithm was developed to overcome this issue and get useful insights from USFT monitoring. Consistent cropping, Fourier transformation, adaptive filtration and thresholding allowed to calculate noise-free AE activity during USFT of 42CrMo4 steel Remarkably, most of this activity was located in the very beginning of fatigue loading. In comparison to non-failed runout samples, AE activity of samples with fatigue cracks was significantly higher, indicating its relation to fatigue damage. The proposed AE processing principle can be helpful for monitoring of USFT samples and other parts, operating in resonance conditions.

X-ray computed tomography (XCT) is extremely useful for the non-destructive analysis of additively manufactured (AM) components. AM components often show manufacturing defects such as lack-of-fusion (LoF), which are detrimental to the fatigue life of components. To better understand how cracks initiate and propagate from internal defects, we fabricated Ti-6Al-4V samples with an internal cavity using electron beam powder bed fusion. The samples were tested in high-cycle and very high-cycle fatigue regimes. XCT was used to locate crack initiation sites and to determine characteristic properties of cracks and defects with the aid of deep learning segmentation. LoF defects exposed to the outer surface of the samples after machining were found to be as detrimental to fatigue life as the internal artificial defects. This work can benefit industries that utilize the AM of high-strength, lightweight alloys, in the design and manufacturing of components to improve part reliability and fatigue life.