Mathematische Modellierung der Ausscheidung von Sprödphasen in komplexen rutheniumhaltigen Nickelbasis-Superlegierungen

A new model has been developed in this work which is capable of simulating the precipitation kinetics of brittle phases, especially TCP-phases (topologically close packed phases) in ruthenium containing superalloys. The model simultaneously simulates the nucleation and the growth stage of precipitation for any number of precipitating phases. The CALPHAD method (Calculation of Phase Diagrams) is employed to calculate thermodynamic properties, such as the driving force or phase compositions in equilibrium. For calculation of diffusion coefficients, kinetic mobility databases which are also based on the CALPHAD-method are used. The model is fully capable of handling multicomponent effects, which are common in complex superalloys. Metastable phases can be treated and will automatically be dissolved if they get unstable. As the model is based on the general CALPHAD method, it can be applied to a broad range of precipitation processes in different alloys as long as the relevant thermodynamic and kinetic databases are available. The developed model proves that the TCP-phases precipitate in a sequence of phases. The first phase that is often formed is the metastable sigma-phase because it has the lowest interface energy due to low-energy planes at the interface between matrix and precipitate. After several hundred hours the stable µ- and P-phases start to precipitate by nucleating at the sigma-phase which is energetically favourable. During the growth of these stable phases the sigma-phase is continuously dissolved. It can be shown by thermodynamic CALPHAD calculations that the sigma-phase has a lower Gibbs free enthalpy than the µ- and P-phase. All required parameters of the model, such as interface energy and nucleate densities, have been estimated. The mechanisms of suppression of TCP-phase precipitation in the presence of ruthenium in superalloys were investigated with the newly developed model. It is shown by the simulations that ruthenium mostly affects the nucleation stage, while the growth stage of precipitation is nearly unchanged and the equilibrium phase fraction of the TCP-phases is hardly changed by ruthenium. Nucleation is partly slowed down by a reduction of the driving force through so called “reverse partitioning”, which means that rhenium is pushed into the y’-phase if adding ruthenium to the alloy. According to the results additionally the driving force is reduced by a lower y’-phase fraction, which can be caused by ruthenium in some alloys depending on other elements. Thermodynamic calculations prove that “reverse partitioning”, which is a disputed concept, exists in some alloys. However, this is not only depending on the ruthenium content but also on most other alloying elements. Beside the Ru content especially the Cr, Mo and Ti content is of importance for “reverse partitioning”. But the reduction of the nucleation rate by ruthenium cannot be explained only by these effects. As a conclusion of this work, it is postulated that a second maybe even more important mechanism is an increase of interface energy by ruthenium. The model explains the experimental results very well. Driving force of TCP-phase precipitation is controlling the size distribution of the precipitates and can be predicted qualitatively by CALPHAD calculations. High driving force leads to many small precipitates while a low driving force leads to few big precipitates. The calculations show that TCP-phase precipitation is preferred in the dendrite cores because the driving force for precipitation is much higher in these regions due to residual segregation of rhenium. For this work a validation of existing thermodynamic and kinetic databases was needed. All results demonstrate that the database TTNi7 is well suited for the application with superalloys containing up to 3 weight percent ruthenium. A new mobility database was developed in this work, which is capable of simulating diffusion in ruthenium-containing superalloys. In conclusion it can be said that simple thermodynamic calculations with the CALPHAD method are a much more generally applicable tool for the comparison of the instability of alloys in respect to TCP phase formation than the conventional PHACOMP (Nv-method) or New-PHACOMP (Mv-method) method because it is not limited to sigma-phase precipitation and includes rhenium and is capable of considering both the matrix concentration and microsegregation. The simulations of microsegregation as well as solidification modelling with one-dimensional models using the software DICTRA confirmed that the diffusion perpendicular to the axis of the dendrite is larger than in the direction of the dendrite axis as the diffusion distances are much smaller in this direction. The effect is considerably larger than it has been proposed earlier and limits application of one-dimensional models for this purpose.In dieser Arbeit wurde ein neues Modell zur Simulation der Ausscheidungskinetik von Sprödphasen, insbesondere von TCP-Phasen in rutheniumhaltigen Superlegierungen, entwickelt. Das Modell simuliert die Keimbildung und das Wachstum für beliebig viele Phasen simultan unter Anwendung der CALPHAD-Methode (Calculation of Phase Diagrams). Diese Methode wird zur Berechnung der thermodynamischen Größen wie z.B. der Triebkraft oder der Phasenzusammensetzung im Gleichgewicht und zur Berechnung der Diffusionskoeffizienten bzw. Mobilitäten eingesetzt. Dabei werden erstmals vollständig die Effekte berücksichtigt, die durch die Vielzahl der Legierungselemente in Superlegierungen bedingt sind. Metastabile Phasen werden in dem Modell automatisch aufgelöst, sofern sie thermodynamisch instabil werden. Da das Modell auf der CALPHAD-Methode beruht, ist es allgemein ohne großen Aufwand auch auf andere Ausscheidungsvorgänge in beliebigen Legierungssystemen anwendbar, sofern die entsprechenden thermodynamischen und Mobilitätsdatenbanken vorliegen. Die Modellierung bestätigt die experimentellen Erkenntnisse, welche Ausscheidungssequenzen bei der TCP-Phasenbildung vermuten lassen. Aufgrund der Kristallstrukturen scheidet sich zunächst meist die metastabile sigma-Phase aus, weil sie die geringste Grenzflächenenergie aufweist. Nach einigen hundert Stunden kommt es dann zum Wachstum der thermodynamisch meist stabileren µ- oder P-Phase, welche aus energetischen Gründen vor allem an der sigma-Phase ihre Keime bildet. Die metastabile sigma-Phase wird anschließend wieder aufgelöst. Thermodynamische Berechnungen mit der CALPHAD-Methode zeigen, dass die sigma-Phase eine niedrigere Gibbs’sche Freie Enthalpie als die µ- oder die P-Phase aufweist. Alle erforderlichen Modellparameter wie Grenzflächenenergie und Keimdichten wurden bestimmt. Anhand des neu entwickelten Modells konnte untersucht werden, welche Mechanismen die experimentell beobachtete Hemmung des Wachstums der TCP-Phasen bei der Zugabe von Ruthenium erklären können. Es stellt sich heraus, dass durch Ruthenium vor allem die Keimbildung gehemmt wird, während der Einfluss auf das Wachstum der Ausscheidungen gering ist. Der Volumenanteil der TCP-Phasen im thermodynamischen Gleichgewicht wird durch Ruthenium kaum verändert. Ruthenium bewirkt insbesondere eine Verringerung der Triebkraft durch Beinflussung der y’-Phase. Dies geschieht einerseits durch eine Verschiebung von Rhenium in die y’-Phase („reverse partitioning“) und andererseits durch eine Reduzierung des y’-Volumenanteils. Thermodynamische Rechnungen konnten das „reverse partitioning“ in bestimmten Legierungen belegen. Dies hängt aber nicht nur vom Rutheniumgehalt, sondern auch von der Konzentration der meisten anderen Legierungselemente ab, insbesondere von Cr, Mo und Ti. Weiterhin muss aufgrund der Ergebnisse eine Erhöhung der Grenzflächenenergie zwischen Matrix und TCP-Phase in rutheniumhaltigen Legierungen postuliert werden. Dieser Effekt hat offenbar einen großen Einfluss auf die Hemmung der Keimbildung durch Ruthenium. Mit Hilfe des Modells können die experimentellen Ergebnisse gut verstanden werden. Das Modell verdeutlicht, dass die thermodynamische Triebkraft den größten Einfluss auf die Größenverteilung der Ausscheidungen hat und mit CALPHAD-Berechnungen vorhergesagt werden kann. Bei hoher Triebkraft bilden sich viele kleine Ausscheidungen, während bei niedriger Triebkraft wenige große Ausscheidungen entstehen. Weitere Simulationen zeigen eine bevorzugte TCP-Phasenbildung in den Dendritenkernen, da dort die thermodynamische Triebkraft zur TCP-Phasenausscheidung aufgrund der Restsegregation von Rhenium deutlich erhöht ist. Eine Untersuchung der verfügbaren Datenbanken belegt die Eignung der thermodynamischen Datenbank TTNi7 auch für rutheniumhaltige Legierungen bis mindestens drei Gewichtsprozent Ruthenium. Außerdem wurde eine neue Mobilitätsdatenbank entwickelt, welche auch für rutheniumhaltige Superlegierungen anwendbar ist. Die CALPHAD-Methode erweist sich als wesentlich allgemeineres und leistungsfähigeres Werkzeug als die herkömmliche PHACOMP-Methode (Nv-Methode) bzw. New-PHACOMP-Methode (Md-Methode) für die einfache und schnelle Abschätzung der Instabilität einer Legierung bezüglich der TCP-Phasenbildung. Sie ist im Gegensatz zu den genannten Methoden für alle TCP-Phasentypen und auch für rheniumhaltige Legierungen anwendbar und kann direkt die Konzentrationen in der Matrixphase und die Mikrosegregation berücksichtigen. Sowohl die Simulation der Mikrosegregation als auch der Wärmebehandlung mit eindimensionalen Modellen ergibt, dass Diffusionseffekte quer zur Längsachse des Dendriten aufgrund der kurzen Diffusionswege dominieren und bei der Modellierung berücksichtigt werden müssen

Resistive Oxygen Gas Sensors for Harsh Environments

Resistive oxygen sensors are an inexpensive alternative to the classical potentiometric zirconia oxygen sensor, especially for use in harsh environments and at temperatures of several hundred °C or even higher. This device-oriented paper gives a historical overview on the development of these sensor materials. It focuses especially on approaches to obtain a temperature independent behavior. It is shown that although in the past 40 years there have always been several research groups working concurrently with resistive oxygen sensors, novel ideas continue to emerge today with respect to improvements of the sensor response time, the temperature dependence, the long-term stability or the manufacture of the devices themselves using novel techniques for the sensitive films. Materials that are the focus of this review are metal oxides; especially titania, titanates, and ceria-based formulations

Platform for a Hydrocarbon Exhaust Gas Sensor Utilizing a Pumping Cell and a Conductometric Sensor

Very often, high-temperature operated gas sensors are cross-sensitive to oxygen and/or they cannot be operated in oxygen-deficient (rich) atmospheres. For instance, some metal oxides like Ga2O3 or doped SrTiO3 are excellent materials for conductometric hydrocarbon detection in the rough atmosphere of automotive exhausts, but have to be operated preferably at a constant oxygen concentration. We propose a modular sensor platform that combines a conductometric two-sensor-setup with an electrochemical pumping cell made of YSZ to establish a constant oxygen concentration in the ambient of the conductometric sensor film. In this paper, the platform is introduced, the two-sensor-setup is integrated into this new design, and sensing performance is characterized. Such a platform can be used for other sensor principles as well

MultOpt++: a fast regression-based model for the development of compositions with high robustness against scatter of element concentrations

Abstract Alloys-by-design is a term used to describe new alloy development techniques based on numerical simulation. These approaches are extensively used for nickel-base superalloys to increase the chance of success in alloy development. During alloy production of numerically optimized compositions, unavoidable scattering of the element concentrations occurs. In the present paper, we investigate the effect of this scatter on the alloy properties. In particular, we describe routes to identify alloy compositions by numerical simulations that are more robust than other compositions. In our previously developed alloy development program package MultOpt, we introduced a sensitivity parameter that represents the influence of alloying variations on the final alloy properties in the post-optimization process, because the established sensitivity calculations require high computational effort. In this work, we derive a regression-based model for calculating the sensitivity that only requires one-time calculation of the regression coefficients. The model can be applied to any function with nearly linear behavior within the uncertainty range. The model is then successfully applied to the computational alloys-by-design work flow to facilitate alloy selection using the sensitivity of a composition owing to the inaccuracies in the manufacturing process as an additional minimization goal

Dual Mode NOx Sensor: Measuring Both the Accumulated Amount and Instantaneous Level at Low Concentrations

The accumulating-type (or integrating-type) NOx sensor principle offers two operation modes to measure low levels of NOx: The direct signal gives the total amount dosed over a time interval and its derivative the instantaneous concentration. With a linear sensor response, no baseline drift, and both response times and recovery times in the range of the gas exchange time of the test bench (5 to 7 s), the integrating sensor is well suited to reliably detect low levels of NOx. Experimental results are presented demonstrating the sensor’s integrating properties for the total amount detection and its sensitivity to both NO and to NO2. We also show the correlation between the derivative of the sensor signal and the known gas concentration. The long-term detection of NOx in the sub-ppm range (e.g., for air quality measurements) is discussed. Additionally, a self-adaption of the measurement range taking advantage of the temperature dependency of the sensitivity is addressed

Penggambaran superhero pada tokoh Deadpool dalam film Deadpool

Deadpool merupakan film superhero yang diproduksi oleh Marvel Studio pada tahun 2016. Film ini menceritakan sosok superhero yang memiliki sisi lain dari superhero pada umumnya. Penelitian ini bertujuan untuk melihat bagaimana tanda-tanda penggambaran superhero pada tokoh Deadpool dalam film Deadpool. Superhero merupakan sosok yang memiliki kekuatan diluar nalar manusia dan mereka selalu menutupi identitasnya. Metode analisis semiotika Charles Sanders Pierce digunakan untuk meneliti tanda penggambaran pada tokoh Deadpool dalam film Deadpool. Melalui tanda-tanda yang muncul dalam film Deadpool, peneliti menemukan bahwa menjadi superhero tidak harus menjadi sosok yang sempurna baik fisik maupun sifatnya. Selain itu film ini mempertegas bahwa menjadi superhero hanya membutuhkan kemauan yang kuat, jiwa kepahlawanan yang besar dan usaha yang maksimal

Metal-Organic Frameworks for Sensing Applications in the Gas Phase

Several metal-organic framework (MOF) materials were under investigated to test their applicability as sensor materials for impedimetric gas sensors. The materials were tested in a temperature range of 120 °C - 240 °C with varying concentrations of O2, CO2, C3H8, NO, H2, ethanol and methanol in the gas atmosphere and under different test gas humidity conditions. Different sensor configurations were studied in a frequency range of 1 Hz -1 MHz and time-continuous measurements were performed at 1 Hz. The materials did not show any impedance response to O2, CO2, C3H8, NO, or H2 in the gas atmospheres, although for some materials a significant impedance decrease was induced by a change of the ethanol or methanol concentration in the gas phase. Moreover, pronounced promising and reversible changes in the electric properties of a special MOF material were monitored under varying humidity, with a linear response curve at 120 °C. Further investigations were carried out with differently doped MOF materials of this class, to evaluate the influence of special dopants on the sensor effect

Planar Zeolite Film-Based Potentiometric Gas Sensors Manufactured by a Combined Thick-Film and Electroplating Technique

Zeolites are promising materials in the field of gas sensors. In this technology-oriented paper, a planar setup for potentiometric hydrocarbon and hydrogen gas sensors using zeolites as ionic sodium conductors is presented, in which the Pt-loaded Na-ZSM-5 zeolite is applied using a thick-film technique between two interdigitated gold electrodes and one of them is selectively covered for the first time by an electroplated chromium oxide film. The influence of the sensor temperature, the type of hydrocarbons, the zeolite film thickness, and the chromium oxide film thickness is investigated. The influence of the zeolite on the sensor response is briefly discussed in the light of studies dealing with zeolites as selectivity-enhancing cover layers

Determination of the NOx Loading of an Automotive Lean NOx Trap by Directly Monitoring the Electrical Properties of the Catalyst Material Itself

Recently, it has been shown that the degree of loading of several types of automotive exhaust aftertreatment devices can be directly monitored in situ and in a contactless way by a microwave-based method. The goal of this study was to clarify whether this method can also be applied to NOx storage and reduction catalysts (lean NOx traps) in order to obtain further knowledge about the reactions occurring in the catalyst and to compare the results with those obtained by wirebound NOx loading sensors. It is shown that both methods are able to detect the different catalyst loading states. However, the sensitivity of the microwave-based method turned out to be small compared to that previously observed for other exhaust aftertreatment devices. This may limit the practical applicability of the microwave-based NOx loading detection in lean NOx traps