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Publication Access status: Open Access , Entwicklung und Erweiterung der thermodynamischen Datenbanken für die Systeme V-Si-B und V-Ti-B(2024)Refractory metals are increasingly becoming the focus of scientific research and development. They present a highly attractive alternative to the current state-of-the-art nickel-based superalloys. Their high melting temperatures and good formability at elevated temperatures align with the required properties for high-temperature applications. Despite their exceptional characteristics, refractory metal-based systems are inadequately explored, particularly regarding their chemical properties, such as corrosion resistance, the formation and composition of oxide layers, thermochemical stability, and similar aspects. The objective of this study is the development and expansion of a thermodynamic database, intended as a supportive tool for the research of new vanadium-based alloys. Using the FactSage™ software, literature-published datasets for the binary and ternary systems of the elements V, Si, B, Ti, and Cr were consolidated into a unified database. The dataset for the ternary system V-Ti-B was fundamentally newly generated. Microstructural analyses, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC), were conducted on eight V-Ti-B alloys to experimentally determine the phase compositions and fractions. The validation of the generated dataset was carried out by comparing the experimentally determined results with those calculated by the software. The prediction accuracy of the dataset exhibited a deviation range of 0% to 14% for the phase fractions. However, in the range of 34 at.% to 40 at.% titanium and 30 at.% boron, the deviation between experimental and calculated results was significantly higher, ranging from approximately 20% to 60%. The experimentally determined phase fractions indicate an inaccuracy in the calculated phase transition of the β-(V,Ti)bcc + TiB and the β-(V,Ti)bcc + TiB + V3B2 regions. The calculated phase transitions between the three-phase β-(V,Ti)bcc + TiB + V3B2 and the V3B2 + VB region, as well as the β-(V,Ti)bcc + V3B2 region, show good agreement with the experimental results. The developed V-Si-B-Ti-Cr database enables the calculation of phase diagrams, compositions, and fractions for all binary and ternary subsystems (excluding the V-B-Cr and B-Ti-Cr systems) of these five elements. Further integration of the datasets for the missing ternary systems would allow for the comprehensive calculation and investigation of the quaternary and quinary systems.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche Berichte1 5 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Integratives Design von neuartigen Mo-Si-Legierungen und Schutzschichten für Hochtemperaturanwendungen(2024)Mo Si based alloys with high Ti concentrations represent a new class of high temperature materials offering promising properties for structural applications such as gas turbines, exceeding the thermal capabilities of Ni based superalloys. The combination of these three elements provides high melting temperatures, substantial creep resistance, and exceptional phase stability. Recent alloys in this field have successfully addressed the primary challenge of inadequate oxidation resistance known as "pesting" in air environment. Furthermore, it is recognized that water vapor, constituting approximately 10 vol.% of relevant turbine environments, causes otherwise protective oxides such as SiO2 to react, forming volatile Si(OH)4. However, the exact mechanisms remain inadequately understood. This study evaluates the high-temperature oxidation behavior of both multi phase (MoSS, T1, T2, A15) (Mo9Si8B and Mo12,5Si8,5B27,5Ti2Fe (at.%)) and two phase (MoSS, T1/D88) (Mo20Si52,8Ti and Mo21Si34Ti0,5B) Mo Si (B) (Ti) (Fe) alloys under dry and water vapor containing (wet) atmosphere. To ensure reliable oxidation protection at lower temperatures in wet atmosphere, Si and Yb silicate Environmental Barrier Coatings (EBCs) are utilized. The goal is to acquire comprehensive knowledge about the oxidation behavior of Mo Si (B) (Ti) (Fe) alloys with and without coating systems in complex atmospheres. Oxidation tests are conducted at 1200 °C in both dry and wet atmospheres for up to 100 h. Techniques such as thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X ray diffraction (XRD), and transmission electron microscopy (TEM) including electron energy loss spectroscopy (EELS) are employed to assess the oxidation kinetics and to characterize the microstructural development of the oxide layers. The results from these investigations highlight that an increase in Ti content in general enhances the oxidation resistance in the Mo Si (B) (Ti) (Fe) system, manifesting itself in reduced oxide layer thicknesses and lower specific mass changes. This enhancement is closely linked to a finer and more homogeneous microstructure, which promotes the oxidation stability via shorter diffusion paths. However, a higher Ti content in coated alloys leads to thicker oxide layers and higher specific mass changes, particularly in the Mo20Si52,8Ti alloy as compared to the Mo12,5Si8,5B27,5Ti2Fe alloy. The use of a wet atmosphere results in increased specific mass changes and thicker oxide layers across all alloys, primarily due to the formation of volatile oxides such as MoO3, which accelerate the oxidation rate. Coated alloys demonstrate exceptional oxidation resistance with minimal specific mass changes and oxide layer thicknesses, especially evident for the Mo12,5Si8,5B27,5Ti2Fe alloy. Additionally, the investigation of the oxide layer on the Mo20Si52,8Ti alloy confirms the significance of a nearly pore-free SiO2 layer for effective oxidation resistance. The supplementary application of Yb silicate layers effectively minimizes the impact of water vapor and shows only a slightly worsened oxidation behavior under wet conditions, underscoring the protective nature of the coatings. These results emphasize the necessity of an effective coating system to ensure satisfactory oxidation resistance in wet atmospheres.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:391 5 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Charakterisierung des Ermüdungsverhaltens von metallischen Strukturwerkstoffen unter Einbeziehung der energiedispersiven Laue-Beugung(2024)The microstructural processes that occur within cyclically stressed metallic components, which result in damage and sudden failure, remain poorly understood. In order to gain further insights, it is planned to use a special detector that is capable of measuring the energy of hard X-ray radiation with a high spatial resolution. The objective of this research is to determine the mechanisms of damage and lattice strains in metallic structural materials subjected to cyclic stress using a novel measurement technique that integrates an energy-resolving detector with high-energy white X-rays. In order to achieve this objective, a series of dislocation arrangements were set in a range of polycrystalline materials, each exhibiting distinct dislocation glide behaviours. This was achieved by varying two key parameters: the amplitude of plastic strain and the number of cycles. Subsequently, the diffraction behaviour was correlated with the aforementioned dislocation arrangements. The diffraction patterns that are formed are dependent on the state of the internal stresses with respect to the geometry and the energy that is measured. The radially extended diffraction reflections are referred to as streaks. In the example material pertaining to wavy glide behaviour, nickel, a correlation between the diffraction patterns and the dislocation arrangements is evident. In particular, individual streaks are identified as long as cell structures are observed which consist largely of random dislocation walls. These streaks can be evaluated with regard to the lattice strains. As the number of geometrically necessary dislocation walls increases in response to elevated cyclic loading, the evaluation process becomes more challenging due to the lateral displacement and the resulting overlap of the streaks. In contrast, in the example material for planar glide behaviour, brass, the radiation is diffusely scattered by the high density of stacking faults and statistically stored dislocations, resulting in the absence of streaks that can be evaluated with regard to the lattice strains. In a nickel-chromium alloy exhibiting mixed glide behaviour due to the presence of precipitates, the formation of streaks can be observed and correlated to a lattice strain. This indicates that there are few to no geometrically necessary dislocations present on the dislocation walls, which are formed in addition to the parallel dislocation segments. Moreover, the repetitive sliding processes of the dislocations to overcome the chromium carbides result in a reduction of the material’s strength and an overall decrease in the strains within the lattice. The two-phase example material, duplex steel, exhibits individual clear streaks that can be attributed to the ferritic or austenitic phase and indexed accordingly. The alteration in the diffraction pattern observed in ferrite can be attributed to the clustering of dislocations, which correspond to random dislocation walls. In contrast, the change in the diffraction pattern in austenite is caused by an increase in statistically stored dislocations, stacking faults and twin grain boundaries. Due to the spatial restriction of dislocation arrangements to the vicinity of phase boundaries, lattice strains exhibit significant fluctuations.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:371 15 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Einfluss von Eigenspannungen auf die Ermüdungsschädigungsentwicklung des martensitischen Federstahls 54SiCr6 im HCF-Bereich(2024)This work contributes to the elucidation of the influence of residual stresses on the fatigue damage evolution of the martensitic spring steel 54SiCr6 in the high cycle fatigue range. The results of the fatigue tests performed show that the fatigue damage evolution starts with the formation of parallel slip bands, whose orientation follow the slip planes with the highest Schmid factors. As fatigue progresses, crack networks form on the sample surface and their crack density rises with increasing number of cycles and the level of the applied stress amplitude. In this process, higher crack densities can be observed under cyclic torsional loading compared to uniaxial cyclic loading. Initiation of fatal cracks occurs exclusively intergranularly along prior austenite grain boundaries with subsequent intergranular and/or transgranular short crack propagation. The short crack propagation represents the dominating phase of the fatigue life and shows an oscillating crack propagation rate, which is due to a barrier effect of prior austenite grain boundaries and grain boundary triple points. In the case of cyclic torsional loading, the comparatively higher crack density leads to a frequently occurring crack coalescence, which is also reflected in the oscillation of the crack propagation rate. Generally, residual stresses lead to a higher fatigue life and crack initiation at the sample surface is inhibited, resulting in lower crack densities. Furthermore, under uniaxial cyclic loading, the initiation of the fatal crack shifts to the sample interior, where the crack propagation rate is reduced due to vacuum conditions. The cyclic torsion tests on shot peened samples provide evidence that the transition from shear stress controlled short crack propagation to normal stress controlled long crack propagation is impeded due to compressive residual stresses, which probably leads to the higher fatigue life in the case of shot peened samples.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:363 9 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Wasserstoff als temporäres Legierungselement zur Erzeugung spezifischer Gefügegradienten in der (α+β)–Ti-Legierung Ti–6Al–4V(2023)Technical components are subject to increasing demands with regard to durability and reliability. To meet these expectations, the development of thermochemical processes for an unerring and reproducible microstructure adjustment is necessary. Titanium alloys allow temporary alloying with hydrogen as part of the heat treatment, thus called thermo-hydrogen treatment (THT). The present work intends to produce a local microstructural adjustment of Ti–6Al–4V by means of THT by changing the local distribution of strengthening precipitates and the grain size as a function of the distance to the surface (microstructural gradient), which should improve the fatigue properties of the material. For the THT parameter values selection, the hydrogen diffusion coefficient and the hydrogen solubility are determined as material parameters. In addition, the THT parameters are evaluated by determining fatigue crack propagation resistance and fracture toughness as a function of solution heat treatment and evaluating different microstructural gradients after hydrogen loading and degassing by means of numerically simulated hydrogen concentration profiles, hardness curves, metallographic investigations and phase analysis using X-ray diffractometry and TEM. The results prove that a desired microstructural gradient can be achieved by hydrogen charging at 500°C and degassing at 750°C followed by an aging at 550°C. Modelling of kinetics and thermodynamics of the Ti–H interaction reproduces the experimentally generated microstructural gradients in a good approximation and provides the basis for further optimization of the process. The evaluation of the resulting mechanical properties is carried out by tests on a miniature testing machine, which allows in-situ observation of fatigue crack initiation and propagation on a laser microscope, alternating deformation tests to determine the fatigue life as a function of the stress amplitude as well as tensile tests. The investigation shows that THT-induced microstructural grading extends the fatigue life of conventionally and additively manufactured specimens by a factor of 3 – 6 compared to a homogeneous reference microstructure at HCF, and also offers the prospect of enhanced fatigue strength on a near-component demonstrator by means of hardness gradient existing before THT, which THT increases by 4 – 5 %.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:342 6 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Einfluss von Defekten auf die Streuung der Lebensdauer in einer Lost Foam gegossenen Al-Si-Legierung unter thermomechanischer Ermüdung(2022)In Al-Si cylinder heads, start-stop cycles lead to thermomechanical fatigue (TMF), which is accompanied by the initiation and growth of cracks in the critical web regions. A major issue is the high scatter of the resulting lifetime. The cylinder heads investigated in the framework of this study were manufactured by the lost foam casting process. Due to the low cooling rates caused by the process, the material contains a high number of large and sharp-edged pores. Under cyclic loading, these defects have a significant influence on crack initiation and growth and thus on the lifetime. During the work, uniaxial TMF tests were performed on specimens extracted from the thermomechanically critically loaded region of the cylinder heads. For the determination of the scatter of the initial crack length a method based on the detection of pores using microcomputer tomography (μCT) was developed. In combination with scanning electron microscopy of the fracture surfaces, the pores in the primary crack origin and the relevant criteria for crack initiation were identified. A program developed in this work enables the computational localization of the primary crack origin from the μCT data. The program contains the identified crack initiation criteria. The scatter of the elastic-plastic crack propagation rate was determined by uniaxial TMF crack propagation tests. On the basis of the experimentally obtained results, a new method was developed for the assessment of the lifetime scatter of thermomechanically loaded cylinder heads based on the scatter of defects within the material. For this purpose, a lifetime model based on fracture mechanics was used. This model estimates the lifetime scatter from the scatter of the initial crack length and the crack propagation rate. The deviation between the calculation and test results is quite small for both the lifetime and lifetime scatter. The combination of the μCT for the detection of pores within the material and the lifetime assessment method developed enables a non-destructive quality assurance. In the series process, negative effects of the material quality on the lifetime can be identified at an early stage.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:32368 153 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Einfluss der Zusammensetzung auf das Hochtemperaturoxidationsverhalten im refraktären Hochentropielegierungssystem Ta-Mo-Cr-Ti-Al(2023)Refractory High Entropy Alloys (RHEAs) or Refractory Compositionally Complex Alloys (RCCAs), by contrast with conventional alloys, are composed of a large number of base elements in equimolar or near- equimolar ratios. The alloy class of RHEAs/RCCAs is regarded to be promising candidates for high-temperature applications due to their high melting temperature and outstanding high-temperature strength. However, a serious disadvantage of most RHEAs/RCCAs is the insufficient oxidation resistance attributed to the high refractory metal content. In previous studies on equimolar TaMoCrTiAl, it was revealed that an almost unexplored, continuous oxide layer of rutile-type Cr-Ta oxides provides reliable protection against oxidation. The work presented aims at a sound understanding of the oxidation mechanism of TaMoCrTiAl alloys as well as a mechanistic comprehension of the effects of the elements on the oxidation behavior. Another emphasis lies in the characterization of the micro-structure in the Ta-Mo-Cr-Ti-Al system. To achieve these objectives, several series of alloys derived from the reference alloy TaMoCrTiAl, within which the concentrations of a single base element were varied, were investigated before and after the oxidation experiments carried out at 1200°C in air. X-ray diffraction and cross-scale electron micro-scopy studies in combination with various analytical methods including electron energy loss spectroscopy were applied for characterization. Microstructural investigations in conjunction with thermodynamic calculations reveal a decrease of Ta or Cr to suppresses the formation of the intermetallic Laves phase Cr2Ta (C14-type), whereas a reduction of the Ti or Mo concentration results in higher volume fractions of Laves phase. By reducing the Al content, the disordered A2 phase (W-type) is stabilized. It is demonstrated that the equimolar TaMoCrTiAl alloy possesses the highest oxidation resistance, which is related to a complex interaction of all base elements. The oxidation resistance is attributed to a non-stoichiometric rutile (Cr,Ta,Ti)O2 oxide layer formed at the oxide/substrate interface, which grows inward as a result of oxygen inward diffusion. The experimental results lead to the conclusion that the Ti4+ cations reduce the oxygen vacancy concentration. Reduction of the Ta concentration causes the formation of thicker and multi-phase oxide layers, which do not prevent evaporation. Quantitative assessments indicate significantly increasing Mo volatilization with lower Ta concentrations. A positive effect of Al exists above the content of 15 at.% that is explained by the nucleation effect, the getter effect of Al and an accelerated Cr diffusion due to Al2O3 particle formation. Decreasing the Cr concentration below 15 at.% leads to the formation of non-protective multi-phase buckling oxide layers consisting of (Al,Cr,Ta,Ti)O2 and Ta2O5. The removal of the element Mo causes an enhanced internal corrosion attack due to additional phase boundaries. In addition, it is concluded that the oxidation of Ti nitrides leads to the formation of pores and consequently to a degradation of the oxidation resistance.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:333 8 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , On the development and investigation of the high-temperature oxidation behavior of Co-Re-base alloys(2021)As an alternative potential materials class of so-called “Beyond Ni-base Superalloys”, Co-Re-base alloys were firstly proposed by Rösler from the Technical University of Braunschweig in 2007 for applications at temperatures beyond 1200°C. Various studies have been consequently carried out in the framework of the German Research Foundation (DFG) research group “FOR 727” aiming to find additional alloying elements to improve both the mechanical properties and oxidation performance of the Co-Re-base alloys. Based on Gorr’s previous work within the framework, the study presented aims to investigate and develop further alloying concepts (Si(-B), Cr, Al, Y, and Ni) to improve the oxidation resistance of the alloy system. The focus is given to the formation of thermally grown protective SiO2, Al2O3, and/or Cr2O3 scales on the Co-Re-base alloys. Model alloys Co-17Re-9Si-8B, Co-17Re-23Cr-xSi (x = 0, 1, 2, 3, 4), Co-17Re-23Cr-xAl (x = 5, 10), Co-17Re-xCr-2Si (x = 23, 25, 27), Co-17Re-25Cr-2Si-xY (x = 0.05, 0.1), and Co-17Re-23Cr-15Ni were produced through arc melting followed by proper heat treatment (numbers are given in at.%). The oxidation tests of these alloys were performed in air over the temperature range from 800˚C to 1300˚C with the aid of thermogravimetry. The microstructure of the alloys and the oxidation products was characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), electron backscatter diffraction (EBSD) technique, and transmission electron microscopy (TEM). Thermodynamic studies using commercial software FactSage were employed helping to establish the oxidation mechanism. Results show that the formation of a thermally grown SiO2 layer on the Co-Re-base alloys through macro-alloying with Si is problematic with respect to high-temperature applications of such an alloy system, since the Si addition to the Co-Re alloys strongly reduces their melting temperature. The preferential formation of Co2SiO4 in the model alloy Co-17Re-9Si-8B restricts the formation of a continuous SiO2 layer. The addition of both 5% and 10% Al significantly improves the oxidation resistance of the reference alloy Co-17Re-23Cr. While the main contribution of 5% Al addition is that it acts as an oxygen “getter” promoting the formation of a protective Cr2O3 layer, the 10% Al addition is almost sufficient for the formation of a continuous Al2O3 layer up to 1100˚C. The severe internal nitridation of Al at 1200˚C and above leads to intensive oxidation attacks due to the lack of a protective Al2O3 layer. The investigation of Co-17Re-23Cr-(1-4)Si and Co-17Re-(23-27)Cr-2Si alloy series indicates that both the Si and Cr significantly promote the formation of brittle and poorly oxidation-resistant primary σ phase in the alloy. Despite the positive effect on isothermal oxidation resistance, the increase of Si content successively intensifies the spallation of the Cr2O3 layer during cooling. The minor addition of Y up to 0.1% tremendously improves the adhesion of the Cr2O3 scale forming on the alloy Co-17Re-25Cr-2Si, especially under the thermocyclic condition. A 15% Ni addition to alloy Co-17Re-23Cr is not only beneficial for the mechanical properties but also improves the oxidation resistance significantly. The refinement of the σ phase through Ni addition allows short-peening treatment to further facilitate the formation of the Cr2O3 scale on the alloy Co-17Re-23Cr-15Ni.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:304 17 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Entwicklung und Charakterisierung neuartiger refraktärer Hochentropielegierungen für Hochtemperaturanwendungen(2021)The aim of this work was, among others, to extend the previously available thermodynamic database for the multicomponent Mo-Cr-Ti-Al system by Ta and Nb and to validate it with various experimental methods of microstructure analysis. Furthermore, the oxidation behavior of various ternary, quaternary, quinary refractory multicomponent alloys in a temperature range of 900–1500 °C in air was investigated in detail and oxidation mechanisms were derived from the studies. Published data on binary and ternary subsystems were essentially used to extend the thermodynamic database to include Nb and Ta. Phase formation and phase stability under equilibrium conditions were studied experimentally on quinary alloys. For this purpose, samples were aged under inert gas (Ar) for 80 h at 700 °C, 300 h at 800 °C, 100 h at 1000 °C, and 24 h at 1300 °C, and the forming phases and microstructure were studied and compared with the thermodynamic equilibrium calculations. Based on the thermodynamic calculations, it could be shown that the formation of a single-phase bcc microstructure can be expected in the Ta/Nb Mo Ti system in a very wide concentration range. However, the addition of Cr and Al to alloys of the Ta/Nb Mo Ti system favors the formation of (i) Laves phases (Cr2Nb and Cr2Ta, respectively), (ii) an A15 phase (AlMo3 type), and (iii) an ordered B2 phase (CsCl type). In addition to the extension of the thermodynamic database and the extensive microstructural investigations, the high-temperature corrosion behavior of the complex multicomponent alloys from the Ta-Nb-Mo-Cr-Ti-Al system was investigated in this work, essentially at 900–1100 °C and in some cases even up to 1500 °C in air. On the one hand, the macroalloying influence of various base elements was analyzed using several model alloys. On the other hand, a microalloying effect due to the addition of small amounts of Si or Y was investigated using the two high-entropy alloys NbMoCrTiAl and TaMoCrTiAl as examples. For the Ta and Cr containing alloys (TaMoCrAl, TaMoCrTiAl), the formation of a complex protective oxide layer of CrTaO4 was observed. The formation of CrTaO4 in these alloys occurred after a short incubation period and appears to reduce metal outward diffusion. Therefore, oxygen inward diffusion was found to be rate-determining in these alloys and the growth of the CrTaO4 layer occurred at the metal-oxide interface following a parabolic rate law. For the Nb-containing alloys (NbMoCrAl, NbMoCrTiAl), the formation of Nb2O5 near the metal-oxide interface caused the formation of a poorly adherent and non-protective oxide layer due to the high anisotropic thermal expansion. Microalloying NbMoCrTiAl with 1 at. % Si improved the oxidation resistance at 1000 °C and 1100 °C in air, at least temporarily; however, no significant effect was observed at 900 °C. Unlike NbMoCrTiAl, the Si-containing alloy formed predominantly compact and protective Cr2O3- and Al2O3-rich oxide layers, which appear to be largely responsible for the higher oxidation resistance of this alloy. The addition of 1 at. % Si to TaMoCrTiAl had a slightly detrimental effect on the oxidation resistance of the alloy in air at 900–1100 °C. The Si addition to TaMoCrTiAl resulted in a significant increase in the volume content and grain size of the Cr2Ta Laves phase, increasing internal corrosion attack within and along the Laves phase grains, and significantly greater precipitation depths of Cr-/Ti-nitrides and Al2O3. Finally, the effect of Y additions of 0.5 at. % and 1 at. % on the high temperature oxidation resistance of the equimolar alloy NbMoCrTiAl at 1000 °C in air was investigated. It was found that the additions of Y lead to the precipitation of Al2Y phase along the grain boundaries, which during the stationary phase of oxidation, causes local stresses leading to cracking and subsequent increase in oxidation rates.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:29484 591 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Skalenübergreifende Charakterisierung kurzer Ermüdungsrisse in artverschiedenen Edelstählen unter dem Einfluss von Wasserstoff(2021)The present work deals with the characterisation of the propagation behaviour of short fatigue cracks in three stainless steels under cyclic loading and under the influence of hydrogen. For this purpose in-situ tests were carried out on a servo-hydraulic testing machine as well as on a miniature testing machine. This characterisation is important because the lifetime of components can be strongly dominated by short crack propagation. Two meta-stable austenitic stainless steels (X2CrNi19-9 and X2CrNi19-12), which differ in their nickel content, and one completely martensitic steel (X3CrNiMo13-4) were investigated. For this purpose, with hydrogen precharged samples were compared with corresponding uncharged samples as reference. Both the X2CrNi19-9 and the X2CrNi19-12 tend to deformation-induced martensite formation during fatigue, whereas the X2CrNi19-9 shows, due to its lower nickel content, a higher tendency to deformation-induced martensite formation. While the crack propagation rate in X2CrNi19-12 samples is hardly influenced by the hydrogen present, X2CrNi19-9 is significantly more susceptible to hydrogen. Therefor, the crack propagation speed in these samples is significantly increased by the present of hydrogen. A pronounced response to the presence of hydrogen is the change in crack initiation sites and slip band morphology in both austenitic stainless steels. While cracks initiate in reference samples at slip bands and grain boundaries, cracks in precharged samples start at large micro-structural defects such as grain boundary triple points. The martensitic X3CrNiMo13-4 samples show the greatest possible influence of hydrogen on the fatigue properties. Precharged samples show a significantly higher crack growth rate.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:282 7
