University of Siegen
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Publication Access status: Open Access , Optimierung der mechanischen Eigenschaften von Beta-Titanlegierungen durch die Verwendung von Wasserstoff als temporäres Legierungselement(2019)The use of hydrogen as a temporary alloying element as a part of the thermomechanical process is considered being a promising approach for improving mechanical properties especially of titanium cast alloys. The present study shows, that this so-called thermohydrogen processing (THP) improves monotonic and cyclic strength even of complex metastable beta titanium alloys by taking advantage of the reversibility of the titanium-hydrogen interaction and by exerting an selective influence on phase equilibria, kinetics of phase transformation and the distribution of alloying elements. Thermohydrogen processing is applied on two beta titanium alloys differing considerably with respect to beta stability. Thermohydrogen processing of highly beta stabilized titanium alloy Ti-3Al-8V-6Cr-4Mo-4Zr (Ti-38644) increases yield strength by more than 4 % and fatigue limit by 8 % as compared to a duplex aged (reference) microstructure. The 5-step Hi-Read-process (Hydride-induced Rearrangement of Dislocations) optimizes the precipitation behavior of the secondary alpha phase in a completely recrystallized beta microstructure, coming from hydride-induced multiplication of dislocations within beta solid solution. Designating the THP contributes to the mechanism of dislocation formation as initially described by Frank Read. The 4-step HiRe-Beta process (Hydride-induced Recrystallization of beta Phase) applied on high-strength beta titanium alloy Ti-10V-2Fe-3Al (Ti-1023) provides the best way to improve strength by maximizing the driving force for precipitation of the secondary alpha phase. Despite complete destabilization of globular primary alpha phase, hydride-induced recrystallization of the beta phase simultaneously preserves a fine beta microstructure. As compared to the conventionally heat-treated material the THP increases yield strength by more than 8 %. Fracture of samples occurred within the elastic region of the stress-strain curve is attributed to the existence of process-related crack formation. Both THP designed for beta titanium alloys fundamentally differ with respect to hydrogen uptake. The Hi-Read process utilizes time-independent Sieverts-type hydrogenation behavior at high temperature. In contrast, hydride formation already occurs during hydrogenation at low temperature in the HiRe-Beta process. Therefore, the volume fraction of precipitated hydrides is immediately affected by hydrogenation time (time-dependent hydrogenation behavior). Determination of THP-strategies for both metastable beta titanium alloys relies on studies on the relationship between microstructure and mechanical properties, exhibiting the detrimental effect of precipitate-free zones and grain boundary alpha phase, both decreasing yield strength and fatigue limit and additionally promoting the propagation of long cracks. Studying the kinetics and thermodynamics of hydrogen uptake and the effect of hydrogen on phase stability enables to establish specified hydrogen concentrations in metastable beta titanium alloys Ti-38644 and Ti-1023. Evaluation of hydrogen concentration profiles emerged from diffusion annealing of electrochemically hydrogenated small titanium bars enables the determination of hydrogen diffusion coefficients. Assuming an exclusively diffusion-controlled (ideal) hydrogen uptake, the hydrogen diffusion coefficients are used to calculate necessary hydrogenation times by means of numerical methods. Volumetric measurements reveal that even at temperatures above the stability limit of the titanium oxide an exclusively diffusion-controlled hydrogen uptake cannot be facilitated. The impact of the surface on the kinetics of hydrogen uptake is therefore considered by means of correction factors, which are used to estimate the times being necessary for complete hydrogen uptake and release. Specified hydrogen concentrations and homogeneous hydrogen distributions are established within the cross-sections of the samples in this way. Coating samples of both beta titanium alloys electrochemically with the hydrogen absorber palladium considerably accelerates the kinetics of hydrogen uptake, thereby facilitating feasible process times even at lower temperatures. Both beta titanium alloys studied exhibit a distinct reduction of the transition temperature (beta transus) with increasing hydrogen concentration. In case of Ti-38644 the modified beta transus decreases until the eutectoid transformation of the beta phase into alpha phase and hydride begins. For Ti-1023 the modified beta transus decreases continuously and remains constant over a wide range of hydrogen concentration after reaching the eutectoid temperature. Since hydride formation (eutectoid transformation) is considered to play the key role for microstructure optimization via thermohydrogen processing the location of the solubility limit of beta solid solution for hydrogen is determined roughly within the phase diagrams of both beta titanium alloys. Knowing the modified beta transus and the hydrogen solubility limit of beta solid solution is essential since it enables specification of thermohydrogen process routes within the phase diagrams of beta titanium alloys Ti-38644 und Ti-1023. Thermohydrogen processing of metastable beta titanium alloys Ti-38644 und Ti-1023 led to microstructures which cannot be facilitated through conventional thermomechanical processes. Additionally, the Hi-Read and HiRe-Beta thermohydrogen process provide the opportunity for adjusting the microstructures of both beta titanium alloys gradually according to prevailing load situations. Therefore, both THP show all features of an innovative heat treatment.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:244 11 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Hochtemperaturoxidationsstudien zum Wasserdampfeinfluss auf thermisch wachsende Chromoxidschichten(2019)Most high temperature alloys rely on the formation of a protective chromia scale during service. In this thesis the kinetic data of oxidation is strongly connected to a microstructural analysis and to stress measurements of the Cr2O3 layer formed on Cr, Ni-25Cr, Fe-10Cr and Fe-18Cr. The alloys were exposed to different dry and wet atmospheres with different oxygen and water vapour contents. Samples with different thicknesses were oxidized for 24 hours in the temperature range from 700°C - 1000°C. Thermogravimetric data were obtained using a microbalance system. The microstructures of oxidized samples were analysed using various experimental techniques such as scanning electron microscopy, X-ray diffraction and transmission electron backscatter diffraction in conjunction with transmission electron microscopy analysis. Significant differences in the oxidation rates, observed during oxidation experiments, can be explained by different atmospheric conditions and compressive stresses within the scales. Obviously, the defect structure of chromia changes depending on the factors mentioned above. The differences of chromia growth in dry and water vapour containing atmospheres are a result of interactions between chromia and various gas species, such as hydrogen and hydroxide and also protons. It can be concluded that some oxidation phenomena cannot be explained by classical oxidation theory. However, it is reasonable to assume that water vapour influences the point defect structure in the chromia scale notably. As an important consequence of the precisely adjusted gas compositions, the effects of the oxygen partial pressure and the water vapour addition on the oxidation behaviour and the corresponding defect structure of chromia have been studied thoroughly and systematically.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:234 14 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Simulation sowie experimentelle Untersuchung von Materialschädigungen bei Umformprozessen(2019)Threads that are typically produced on flat thread rolling machines often have seams, cracks, closes wrinkles and hollow spaces leading to customer complaints, productivity losses and scrap costs. So far, the fundamental understanding of the thread rolling process is missing, so that in this work systematic experimental and simulation-based investigations were carried out. For the description of the material flow behavior of the steel 23MnB4, a material model of tensile and compression tests was developed. In a next step, a suitable friction model was created, which was calibrated by a slide upsetting test. The focus of damage modeling was on a macro-mechanical model proposed by Cockcroft and Latham and a micro-mechanical model was based on Lemaitre. Employing the findings of material, friction and damage modeling, the simulations could be set up with the software SIMUFACT.FORMING. For the first time, it was possible to simulate such a complex thread rolling process in a 3D setting. The application of this simulation model to similar tool geometries was successfully tested on two examples that are relevant in practice. Finally, the fatigue behavior of screws with strong and less pronounced material damage was investigated. The work is completed with TEM investigations of all individual steps in a life cycle of a screw, starting from the cut-off, over the press and rolling blank up to screws subject to fatigue tests.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:2215 18 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , VHCF7 Seventh International Conference on Very High Cycle Fatigue : July 3-5, 2017, Dresden, Germany(2017)The proceedings contain the contributions wich are delivered as presentations during the Seventh International Conference on Very High Cycle Fatigue taking place from 3rd through 5th of July 2017 in Dresden, GermanySource Type:Conference ProceedingsSeries:Siegener werkstoffkundliche BerichteVolume number:142 9 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Oxidationsverhalten refraktärmetallbasierter Konstruktionswerkstoffe für Hochtemperaturanwendungen(2017)New high temperature materials which would lead to a higher thermal efficiency of gas turbines are urgently required in aircraft industry. Refractrory metal-based materials are very perspective candidates as they exhibit high melting points and attractive mechanical properties. However, they often possess poor oxidation resistance that prohibits the practical use of these materials. This work summarizes results of the research activities on high temperature corrosion behavior of refractory-metal based alloys conducted at the Institute of Materials Science at University of Siegen. The research was focused on following alloy systems: (i) Mo-Si-B-Ti alloys, (ii) Co-Re alloys and (iii) refractory high entropy alloys. Due to the macro- and microalloying of the reference alloys, significant improvements of the oxidation resistance were achieved for all alloy systems studied. Results presented in this work clearly show that the main drawback of the refractory metal-based materials, namely the poor oxidation resistance, can be overcome and these alloys can be inherently protective in a wide temperature range.Source Type:HabilitationSeries:Siegener werkstoffkundliche BerichteVolume number:204 12 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Werkstoffmodellierung und Kennwertermittlung für die Simulation spanabhebender Fertigungsprozesse(2018)Finite element simulation of machining processes enable cutting tool and process design for specific requirements of cutting applications. Applying simulation techniques leads to a reduction of real cutting experiments, development times and costs. The physical input data of a simulation is of special importance, since it significantly affects the simulation quality. Especially the material law and the material data for the approximation of the flow stress are important factors. In the research of this thesis a new method for the determination of material data, based on linear-orthogonal cutting tests, is developed. Applying optical high-speed measurements in the chip formation zone the shear strain rate in the material is determined by Digital Image Correlation (DIC). Together with the cutting forces this data is used to determine material data sets via an inverse approach. Split-Hopkinson as well as compression tests provide additional material data sets, the data is also used to evaluate the material behavior at increased strain rates and temperatures. Based on these results, a modified material law is presented, considering specific characteristics in the thermal softening curve of the steel material investigated. The material data sets and the material laws are applied in simulations and the results are compared with measurements from real cutting tests.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:217 16 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Charakterisierung der Schädigungsmechanismen einer equinen oberflächlichen Beugesehne zur gezielten Behandlung von läsionierten Arealen unter Anwendung eines morphofunktionell und biomechanisch bewerteten Fasergerüstes(2018)A damage to the superficial digital flexor tendon (SDFT) on the anterior extremities of the horse often leads to an out-of-service position, which in serious cases results in an euthanasia of the animal. The aim of this work is to evaluate the suitability of an implanted decellularized replacement tissue for the treatment of a topical lesion section in addition to the investigation of the damage mechanisms of an initially intact equine SDFT. After fatigue loading, the onset of damage is indicated by inhomogeneities of the matrix,in the form of interfibre space widening and discontinuities within the collagen structure, up to complete rupture in the middle metacarpal region. A larger crimp angle in the peripheral area leads to a larger toe strain, which in turn results in an early loading of the central fibres. In addition to the purely structural factors, biochemical deformation effects were registered. A higher proportion of fibrils, which is decisive for the strength of the tendon, measured in the entire tissue construct, ensures a higher modulus of elasticity in the central area, which additionally leads to a higher load in this zone with a identical strain of the peripheral fibres. In order to avoid an inferior scar tissue in the lesioned tendon section,the decellularized donor tissue is suitable as an implanted substitute structure up to a defined load. On the basis of existing in vivo studies during the convalescence, only a low-loading step movement of the horse, combined with a suitable ground, can be recommended as to prevent a renewed rupture in the treatment period. Treatment of the ruptured tendon section with a suitable suture technique will continue to be necessary for the fixation of the collagen structure. Rather, the decellularized implant as a filling material could positively influence the healing process. A sufficient strength of the implant structure above the permissible load-bearing capacity of the suture connection can be demonstrated by means of this work‘s findings.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:192 16 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , On the origin of crack initiation in duplex stainless steel during cyclic loading in the VHCF regime(2017)Formation of micro-cracks of a duplex stainless steel (DSS) in the very high cycle fatigue (VHCF) regime was studied and discussed. Several high-resolution techniques were applied to fatigued specimens, identifying the mechanical properties of the DSS and relating them to the mechanisms of the micro-crack initiation. Results acquired from optical microscopy, nanoindentation tests and synchrotron radiation investigations revealed that the austenite phase of the DSS in VHCF exhibits cyclic softening-hardening-softening behaviour. The first cyclic softening takes place during the initial millions of load cycles, where a continuous growth of slip markings and a decrease in micro-hardness were observed. Afterwards, austenite hardens in the tested austenite grains which show a stop in the growth of slip markings and an increase in the micro-hardness. Synchrotron radiation experiments performed at this stage revealed that the internal residual stresses between the two phases increase compared to the initial state. After this period, slip markings in austenite continue to grow and the micro-hardness decreases, indicating the reoccurrence of cyclic softening. The synchrotron radiation experiments executed at this stage showed a decrease in internal residual stresses. The cyclic hardening and softening behaviour of the austenite can be attributed to dislocation motion and rearrangement, and was supported by transmission electron microscopy (TEM) micrographs. Phenomenological observations applied to the specimens showed two types of micro-cracks: intergranular micro-cracks form continuously from an early to a relatively late stage, and transgranular micro-cracks that initiate suddenly. The formation of these cracks is interpreted by the resultant shear stress τ at the phase boundary, which consists of 1) shear stress τ 1 due to the accumulation of intrusions and extrusions (slip of dislocations), 2) shear stress τ 2 due to dislocation pile-ups and 3) shear stress τ 3 due to external load stress. The three components evolve during cyclic hardening and cyclic softening processes, yielding different resultant shear stresses and resulting in the formation of dis-tinct plastic deformation in the ferrite grains. The shear stress τ 1 increases dramatically within the initial 10 5 cycles, which can be expected from the observation of extrusion-intrusion growth. The shear stress τ 3 in ferrite decreases when austenite cyclically hardens, and increases when austenite cyclically softens. These were observed in the results obtained for stress and strain partitioning experiments applying in-situ X-ray diffraction (XRD) and in-situ digital image correlation (DIC) techniques. Importantly, the shear stress τ 2 which is largely dependent on phase boundary orientations determines the morphology of the micro-cracks. On the occasion of an austenitic grain forming slip markings, the formation of micro-cracks in the adjacent ferritic grain will depend on the following conditions. 1) If the angle between the phase boundary and the ferritic surface is less than 90°, plastic deformation easily occurs and intergranular micro-cracks gradually form at the phase boundary. 2) If the angle between the phase boundary and the ferritic surface is greater than or equal to 90°, slip markings are frequently arrested by the phase boundary and no plastic deformation takes place. When the resultant shear stress is large enough to overcome the barrier effect, transgranular micro-cracks are mostly present. These hypotheses were confirmed by scanning electron microscopy (SEM) images acquired with focused ion beam (FIB)-cutting.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:171 23 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Ermüdungsmechanismen eines Duplexstahls bei niedrigen Spannungsamplituden : experimentelle Charakterisierung und modellmäßige Beschreibung des Einflusses der Mikrostruktur auf die Lebensdauer(2017)This work contributes to the elucidation of those fatigue mechanisms in two phase metallic materials which are relevant for low stress amplitudes up to very high numbers of load cycles. The investigations were carried out on an austenitic-ferritic duplex stainless steel and revealed that plastic deformation occurs in form of dislocation glide along slip bands mainly in the austenitic phase leading to slip traces at the surface. Frequently, stress concentrations are in-duced by dislocations piling up in such slip bands against phase boundaries, possibly causing the formation of fatigue crack nuclei at intersection points between austenite slip traces and phase boundaries. Resulting short fatigue cracks often permanently stop in the middle of the first grain without interacting with microstructural barriers, such as a grain or phase boundary. The reason for this crack stop lies in the inhomogeneous stress distribution in the grains due to anisotropic elastic properties and residual stresses. Obviously, the shear stress at the crack tip decreases stronger along the anticipated crack path than the stress field in front of the crack increases with crack extension. Under these conditions, the irreversible fraction of cyclic dislocation motion at the crack tip vanishes and, hence, the driving force for the short fatigue crack propagation no longer exists. In addition, grain or phase boundaries (i) retard short fatigue cracks, (ii) block them temporarily or even (iii) stop them permanently. The obstacle strength against crack growth depends on the crystallographic orientation difference between the grain before and behind the grain or phase boundary. The experimentally revealed mechanisms of fatigue crack nucleation and microstructure-governed short fatigue crack growth were simulated taking into account crystal plasticity, anisotropic elasticity as well as residual stresses due to the heat treatment and manufacturing process of the material. It was clearly shown by means of the developed simulation model that in addition to the anisotropic elasticity of the grains and the residual stresses, the three-dimensional geometry of the microstructure has a significant impact on the fatigue damage development.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:161 8 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Entwicklung eines Lebensdauervorhersagekonzepts im VHCF-Bereich auf Basis kovariater mikrostruktureller Merkmalsgrößen(2016)The prediction of fatigue life of metallic materials in the very high cycle fatigue (VHCF) regime is still a challenge for material science. Recent studies show that a true durability limit does not exist for many metallic materials and fatigue failures can occur even after 107 loading cycles. Moreover, the huge scatter of experimental results at low stress amplitudes relates to a strong influence of microstructural heterogeneities such as varying grain boundary character, distribution of grain size, inclusion size etc., which together with the applied stress amplitude determine fatigue life. The influence of the microstructure has a probabilistic character and increases the scatter band width in the area of VHCF up to three decades for the SN-curve, thus impeding the application of a reliable fatigue life prediction concept by means of traditional statistical approaches and requiring a detailed investigation of the relevant fatigue damage mechanisms. In the present work the dependence between the size and space distribution of defects relating to the material quality and the size and location of failure initiation defects in fatigue specimens correlating with their fatigue lives in the VHCF-range was investigated. For this purpose fatigue tests with clarification of typical crack initiation sites and damage mechanisms were carried out. The investigations were made for three reference materials that belong to different material types introduced by H. Mughrabi (2006). A nickel-based superalloy Nimonic 80A (material type I), a metastable austenitic stainless steel 1.4301 with a high deformation-induced martensite volume fraction as well as a welding joint of aluminium sheets from EN AW-6082 T651 (material type II) were investigated. The effect of typical damage-relevant defects for the investigated materials was modeled by corresponding failure-relevant parameters. The stress concentration at crack initiation twin boundaries as well as regular grain boundaries in Nimonic 80A was quantified using a misorientation factor by Blochwitz et al. (1997) and a developed crack initiation parameter. The effect of size and location of extrinsic defects in the type-II-materials was estimated by means of a stress intensity factor with consideration of the local stress at defects. The investigation of distribution of failure-relevant parameters in the single specimens showed that crack initiation predominately takes place at defects with the maximum values of the defined parameter. Using the fatigue test results the observed dependence between the failure-relevant parameters and corresponding numbers of cycle until failure or crack initiation was modeled. The analysis and statistical modeling of the defined damage-relevant defects was carried out on the basis of metallographic investigation of the reference materials in as-received condition. Using the extreme value statistics the size and (if necessary) space distributions of the larger values of defined damage-relevant defects was modeled in metallographic samples. These models were used in order to evaluate the value of failure-relevant parameters in fatigue specimens. The fatigue life of different specimens was predicted on the basis of evaluated failure-relevant parameters and compared with own as well as adopted fatigue test results. The agreement of experimental and modeling results as well as application of the used method on other alloys were discussed in the conclusions.Source Type:Doctoral ThesisSeries:Siegener werkstoffkundliche BerichteVolume number:157 9