Ingenieurwissenschaften und zugeordnete Tätigkeiten
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- Coarsening (6)
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- Alloy 2618A (3)
- Degradation (3)
- LCF (3)
- Transmission electron microscopy (3)
- Creep (2)
- Dark-field transmission electron microscopy (DFTEM) (2)
- EN AW-2618A (2)
Organisationseinheit der BAM
The increasing importance of resource availability and closed-loop material cycles are driving materials research to reduce alloying content in conventional materials or even substitute them with more sustainable alternatives. Intermetallic iron aluminide alloys (FeAl) present a potential alternative. Many alloy concepts for improved high-temperature properties or ductility have already been successfully implemented in casting technologies on a laboratory scale. However, successful testing of FeAl alloys on an industrial scale was still pending at the beginning of the project.
Therefore, the aim of the project was to develop simulation based casting concepts for industrial casting processes using the base alloy Fe-26Al-4Mo-0.5Ti-1B and to narrow down process limits by means of hot cracking tests. Findings were transferred into practice-oriented guidelines for casting of iron aluminides, which is accessible to future applicants in SMEs. The focus was placed on centrifugal casting combined with investment casting or die casting. In addition to numerous design and casting process parameters, heat treatments and alloying additions (Al, Mo, B) were varied to determine the influence of alloying elements on castability, microstructure and mechanical properties. Data from microstructure analyses (microscopic imaging, determination of grain sizes as well as phase compositions and volume fractions, fractography), mechanical tests (hardness measurements, compression tests, ambient and high-temperature tensile tests, creep tests) as well as measurements of thermophysical properties could be generated on the base alloy. Correlations of materials data with process variables allowed conclusions to be drawn on strengthening mechanisms and ductility of the alloy and how they can be controlled in terms of processing and component design. Successful casting of highly complex components with thin wall thicknesses and optimised alloy compositions points out prospects for new fields of application.
In nickelbasierten Superlegierungen, die für Gasturbinenschaufeln verwendet werden, sind bei hohen Betriebstemperaturen und -belastungen Kriechschäden, Ermüdung, Korrosion und Oxi-dation zu beobachten. Die betroffenen Turbinenschaufeln müssen ersetzt oder repariert werden, um Effizienzverluste und ein eventuelles Bauteilversagen zu vermeiden. In letzter Zeit wurden additive Fertigungs- und Lötverfahren für die Reparatur und das Fügen von Nickelbasiswerk-stoffen entwickelt, um die Einschränkungen der traditionellen Reparaturverfahren zu überwinden. Für eine auslegungsrelevante Bewertung des Risswachstumsverhaltens in den Fügezonen, die durch diese neuen Technologien erzeugt werden, sind Rissfortschrittsuntersuchungen erfor-derlich. Entsprechende Untersuchungen an gefügten Proben werden in diesem Beitrag vorge-stellt. Die Rissfortschrittsversuche werden an gefügten SEN-Proben (Single Edge Notch) bei einem Spannungsverhältnis von 0,1 und einer Temperatur von 950 °C durchgeführt. Das Riss-wachstum wird mit der DCPD-Methode (Direct Current Potential Drop) überwacht, und die ge-messenen Potenzialsignale werden anhand der optisch vermessenen Risslänge und einer Finite-Elemente-Analyse kalibriert. Der Spannungsintensitätsfaktor (SIF) für die gefügte SEN-Geometrie wird mit Hilfe von Finite-Elemente-Analysen berechnet. Auf diese Weise können Rissfortschrittskurven aus den experimentellen Daten abgeleitet werden.
The use of miniaturized specimen geometries in mechanical testing allows extracting the sample material directly from critical components of power plants like e.g. headers and pipes. In this way, both the impacts of the manufacture of the component (including all shaping and tempering influences) and of the complex aging/loading/oxidation conditions during Service can be analyzed and representative mechanical properties are obtained. In the present study, example results of a comparative creep and creep rupture study on P91 tempered martensite ferritic Steel, involving Standard and miniature specimens that were all taken from one batch of material, are presented. Cylindrical creep specimens with a minimum gauge length diameter of 3 mm were used, representing the smallest recommended test geometry of ISO 204. The test results of miniature specimens exhibit all characteristic creep features of tempered martensite ferritic steels, and analysis of the stress and temperature dependence of creep results in values that correspond well to literature data for P91. However, direct comparison with large scale specimen data reveals small but systematic variations in minimum creep rates, elongations at fracture and times to rupture. In our contribution, these differences are discussed in the light of literature findings on specimen size influences in other heat resistant alloys. Size effects need to be considered for a correct interpretation of results from miniature specimen creep tests.
Es konnte anhand einer numerischen Voruntersuchung gezeigt werden, dass anhand der kombinierten Auswertung der im Versuch verwendeten Sensorik eine Einteilung der unter Ermüdung in Bohrlochproben auftretenden Rissformen in verschiedene Hauptkategorien (Eckriss, Oberflächenriss, Durchgangsriss) möglich ist.
Modeling the lifetime reduction due to the superposition of TMF and HCF loadings in cast iron alloys
(2016)
The superposition of small amplitude, high frequency loading cycles (HCF) to slow, large amplitude loading cycles (TMF) can significantly reduce the fatigue life. In this work, the combined TMF+HCF loading has been experimentally investigated for a cast iron alloy. In particular, the influence of the HCF frequency of the HCF amplitude and of the location of the superposed HCF cycles has been assessed. It was observed that the HCF frequency has a limited impact on the TMF fatigue life. On the other side, the HCF-strain amplitude has a highly non-linear influence on the TMF fatigue life. A simple estimate for the fatigue life reduction due to the superposed HCF cycles has been derived from fracture mechanics considerations. It is assumed that the number of propagation cycles up to failure can be neglected after a threshold for the HCF loading has been reached. The model contains only two adjustable parameters and can be combined with any TMF life prediction model. The model predictions are compared with the test results for a large range of TMF+HCF loading conditions.
Charakterisierung von Hochtemperaturwerkstoffen durch Zug- und Ermüdungsversuche an Kleinproben
(2019)
Die Verwendung von miniaturisierten Probengeometrien in der mechanischen Prüfung ermöglicht die Entnahme des Probenmaterials direkt aus kritischen Bereichen wie Fügeverbindungen und ermöglicht die Prüfung von kleinen Bauteilen wie additiv gefertigten Strukturen. In der vorliegenden Arbeit werden exemplarisch die Ergebnisse von vergleichenden Zug- und niederzyklischen Schwing-versuchen (LCF) an dem austenitischen Stahl AISI 316L und der Nickelbasislegierung IN718 vorgestellt. Die Prüfergebnisse der Kleinproben aus AISI 316L weisen die charakteristischen Eigenschaften des Werkstoffs auf, und die Analyse der Zug- und Ermüdungsdaten führt zu Werten, die den Literaturdaten weitgehend entsprechen. Der direkte Vergleich mit Standardprobendaten zeigt jedoch systematische Abweichungen bei Zugfestigkeit, Dehngrenze und Gleichmaßdehnung, die in diesem Beitrag diskutiert werden.
The studied aluminium alloy is EN AW-2618A (2618A). It is very widely used for exhaust gas turbo-charger compressor wheels. Due to long operating times, high cycle fatigue (HCF) and material aging under the influence of temperatures up to 230 °C is particularly relevant for the wheels. The wheels are typically milled from round wrought blanks. From such round blanks, different testpieces are extracted and a comprehensive series of HCF tests is conducted at room temperature. The tests investigate the materials fatigue performance in the T61 state for two load-ratios, namely R = -1 and R = 0.1. Additionally, two overaged material states are tested, accounting for the aging process the material undergoes during long operating times at high temperatures. The experimental results are evaluated and compared to each other.
Furthermore, the design process of notched specimens is presented. With the notched specimens, it is aimed to quantify the notch sensitivity of the material. Relating thereto, two potential model parameters for the fatigue lifetime model are introduced. Finally, the extended research data management in this project is highlighted and its advantages for sustainable use in material science and engineering applications are shown.
The plastic deformation results in irreversible microstructure changes in the steel, which can be considered as the initial stage of the fracture process. However, detecting, monitoring and evaluating, damage states and small defects non-destructively in advance still proves challenging. It was reported in literature the phenomenon of the spontaneous emergence of weak magnetic fields in structural steels and pipelines, which originates due to heterogeneous mechanical and / or thermal stresses. This observation is not associated with induced phase transformations by deformation and appears to be a promising tool for the prior characterization of damage in ferromagnetic steels. To provide a better understanding of the physical bases of the process, the magnetic microstructure of such materials and the change of magnetic domains after undergoing plastic deformation were studied. For this purpose, a colloidal solution with paramagnetic particles in the nanometer scale (ferrofluid) was used, through the Bitter technique, in order to, not only observe a change in size of the magnetic domains of the material, but also changes in their morphology. Ferritic steels with different carbon contents (0.08%; 0.22% and 0.45%) were studied in this work.
The plastic deformation results in irreversible microstructure changes in the steel, which can be considered as the initial stage of the fracture process. However, detecting, monitoring and evaluating, damage states and small defects non-destructively in advance still proves challenging. Dubov reported the phenomenon of the spontaneous emergence of weak magnetic fields in ferritic structural steel and pipelines, which originate due to heterogeneous mechanical and / or thermal stresses. This observation is not associated with induced phase transformations by deformation and appears to be a promising tool for the prior characterization of damage in ferromagnetic steels. To provide a better understanding of the physical bases of the process, the magnetic microstructure of such materials and a change of magnetic domains after undergoing plastic deformation were studied. A colloidal solution with paramagnetic particles in the nanometer range (ferrofluid), which allowed, through the Bitter technique, not only to observe a change in size of the magnetic domains of the material, but also changes in their morphology. Ferritic steels with their concentrations of carbon in its composition (0.12%; 0.17% and 0.45%) were studied in this work.
The current competitive situation on electricity markets forces power plants into cyclic operation regimes with frequent load shifts and starts/shutdowns. In the present work, the cyclic mechanical behavior of ferritic-martensitic 9-12 % Cr steels under isothermal and thermomechanical loading was investigated for the example of grade P92 material. A continuous softening was observed under all loading conditions. The introduction of hold periods to the applied cycles reduced material lifetime, with most prominent effects at technologically relevant small strain levels. The microstructural characterization reveals a coarsening of the original “martensitic” lath-type microstructure to a structure with polygonal subgrains and reduced dislocation density. The microstructural data forms the input for a physically-based modelling approach.