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Eingeladener Vortrag
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Extensometer sind seit Jahrzehnten im Einsatz und bilden die Basis für eine präzise Dehnungsmessung in werkstoffmechanischen Versuchen. Jedoch ist die Temperaturstabilität von Extensometern häufig noch nicht ausreichend beachtet worden, um Fehler verursacht durch Temperaturschwankungen zu minimieren. Dieses wird derzeit nicht nur in Deutschland, sondern auch international zunehmend diskutiert. Datenblätter von Extensometern sind teils unvollständig besonders hinsichtlich von Temperatureinflüssen. Extensometer für den Einsatz bei höheren Temperaturen sollen z.B. laut Normen für dehnungsgeregelte Ermüdungsversuche mit Luft oder Wasser aktiv gekühlt werden. Jedoch schreiben die Normen keine Temperatur-Toleranzen für Extensometer vor. Dehnungsfehler verursacht durch Temperaturschwankungen am Extensometer bleiben meist verborgen, da die Temperatur des Extensometers standardmäßig gar nicht gemessen wird. Die Ursache von Einflüssen zur Temperaturstabilität von Extensometern wird erläutert. Ein einfaches Rechenbeispiel zeigt die Empfindlichkeit von Extensometern gegenüber Temperaturschwankungen und die Auswirkungen auf den Dehnungsfehler. Lösungsmöglichkeiten zur Optimierung werden aufgezeigt. Der Anwender von Extensometern soll sensibilisiert werden, um der Temperaturstabilität von Extensometern sowohl hinsichtlich Herstellerspezifikationen als auch bei der Anwendung von Extensometern im Prüflabor mehr Aufmerksamkeit zu schenken, um letztlich die Qualität von Versuchsergebnissen zu verbessern. Empfehlungen an Extensometerhersteller werden formuliert. Dieses Thema wird zukünftig auch in Prüfnormen mehr Aufmerksamkeit geschenkt werden. Es werden Maßnahmen zur Verbesserung der Temperaturstabilität von Extensometern Einzug in Prüfnomen finden, um letztlich verlässlichere Versuchsergebnisse zu erhalten. Es ist beabsichtigt, eine Verbesserung der Temperaturstabilität von Extensometern z.B. bei der nächsten Revision der Prüfnormen ISO 12106 und ISO 12111 für dehnungsgeregelte LCF- und TMF-Versuche zu berücksichtigen.
Extensometer werden seit langer Zeit erfolgreich für die Ermittlung von werkstoffmecha-nischen Kennwerten eingesetzt. Allerdings wurden in den letzten Jahren immer wieder durch Temperaturschwankungen bedingte Drift an Extensometern festgestellt, die be-sonders in Langzeitversuchen zu Problemen führen können. Es wurden Ursachen für die thermische Drift von Extensometern aufgezeigt. Anhand von einem Beispiel wurde gezeigt, wie sich Temperaturschwankungen an einem Extensometer auf die Ergebnisse von werkstoffmechanischen Ermüdungsversuchen auswirken können. An einem weiteren Beispiel wurde beschrieben, dass der durch Temperaturschwankungen verursachte Dehnungsfehler eines Extensometers stark von der Höhe der Temperaturschwankung und von der Dehnungsschwingbreite abhängt und, je nach Anwendungsfall größer als 1% werden kann. Es wurde verdeutlicht, dass bisherige nach Norm vorgeschriebene Maß-nahmen oft nicht ausreichen, um Einflüsse von Temperaturschwankungen am Extenso-meter zuverlässig zu unterbinden. Vorschläge wurden aufgezeigt, um die Temperatursta-bilität von Extensometern zu verbessern. Das gesamte Temperaturmanagement von Raumtemperatur, Kühlwassertemperatur, Temperaturmessung am Extensometer etc. sollte von Grund auf neu betrachtet werden, um Verbesserungen bei der Temperatur-konstanz von Extensometern zu erlangen. Ferner wurde vorgeschlagen, Extensometer in temperaturkompensierter Ausführung zu entwickeln und zu verkaufen. Dieses sollte Ret-rofits für existierende Extensometer miteinschließen. Konkrete Maßnahmen zur Imple-mentierung in Materialprüfungsnormen müssen weiter diskutiert werden.
Thermal fluctuations do exist at extensometers but they remain quite often unknown because the temperature of the extensometer is not measured, recorded and assessed in strain controlled tests. The temperature fluctuation leads to thermal expansion of the gauge length of the extensometer resulting in force fluctuations as a materials response of the material tested in a strain controlled test. The thermal expansion leads to an error of the strain range applied.
A case study was conducted to calculate the strain range error when the strain range was varied as well as the thermal fluctuation of the extensometer. The most important outcome of the study is that the strain range error can exceed the allowed limits if the thermal fluctuation of the extensometer is too high. Sources of thermal fluctuation of the extensometer were analyzed and identified. Suggestions are given to reduce the thermal fluctuation of the extensometer.
Der Vortrag gibt einen Überblick über die wichtigsten Aspekte der anstehenden Revision der Norm ISO 12111 aus deutscher Sicht. Ferner wird ein neues Normungsprojekt für eine ISO-Norm für den kraft-geregelten TMF-Versuch unter deutscher Leitung vorgestellt. Es wird ein Ausblick über Rissfortschrittsmessungen bei TMF-Beanspruchung gegeben. Ferner wird über die Problematik der thermisch bedingten Drift von Extensometern gesprochen. Der Vortrag schließt mit Betrachtungen zur Messunsicherheit ab, die in Normen einfließen.
The authors, Li et al., of the paper entitled “Analysis on the Issues in ISO 6892-1 and TENSTAND WP4 Report Based on Data to Confirm Tests by 21 Laboratories” (J. Test. Eval.
DOI: 10.1520/JTE20150479 (online only)) have expressed views that the authors of this rebuttal believe to be based on fundamental misunderstandings and misinterpretations of the tensile testing standard ISO 6892-1:2009, ISO 6892-1:2016, and its former versions, thus leading to erroneous conclusions. This refutation is intended to clarify the understanding of ISO 6892-1 and to address the misunderstandings and the misinterpretations of the authors of the paper. The present standard ISO 6892-1:2016 has a long history dating back to the 1970s. At that time, the tensile testing procedure was standardized on the National and International scale in parallel. To understand the present standard, the knowledge of the history helps to understand the background of details of the testing procedure implemented today. The history of the tensile testing standard has been discussed extensively during the annual international standardization meeting of ISO committee TC 164 SC1 for the last few years, at which some of the authors of the Li et al. paper attended. The authors continue to disagree with facts that were agreed by the consortium of the European research project TENSTAND and by the present international experts involved in ISO TC 164 SC1. It appears that the principal objective of the authors regarding their present publication was to increase the testing speed during tensile testing. However, the international standardization community has previously declined similar proposals by some of the authors. Many Arguments presented by Li et al. were thus refuted. The conclusions of their paper are misleading and the international standardization community for tensile testing refused to revise the present standard, ISO 6892-1 (2016), according the authors’ proposals.
The presentation describes why the issue of thermal drift of extensometers has become important. It shows the possible sources of thermal drift of extensometers. It shows with a simple theoretical example calculation that the permitted strain error in standards can be exceeded if the temperature fluctuation at the extensometer is too high, but this effect remains mostly unknown because of missing data of the extensometer temperature throughout the test. This will be discussed in fatigue standard committees with the aim to improve the quality of fatigue testing results.
Strategic aspects for the development of measurement uncertainty annexes for ISO testing standards
(2017)
The presentation shows an overview over the current situation of the development of measurement uncertainty annexes for ISO mechanical testing standards. The comparability of measurement uncertainty statements is not ensured. The utilization of ready calculated measurement uncertainty values remains unclear. Partly the measurement uncertainty values clash with existing product standards where minimum values e.g. for the materials strength are determined. A route forward is shown to solve the current problems.
Bei den Normen ISO 15363 und EN 10275 handelt es sich um den hydraulischen Ringaufweitungsversuch, der in der Rohrprüfung in der Industrie genutzt wird. Auf Vorschlag von Salzgitter Mannesmann Forschung wurde im Arbeitsausschuss NA 062-01-53AA – Rohrprüfung - der Vorschlag eingebracht, beide Normen zu harmonisieren. Beide Normen sind fast identisch. Lediglich die verwendeten Symbole sind unterschiedlich. Daher soll in der ISO 15363 ein weiterer Anhang eingeführt werden, der eine Tabelle enthält, in der die Symbole der ISO 15363 und der EN 10275 gegenübergestellt werden. Die in der EN 10275 verwendeten Symbole entsprechen denen, die in Produktnormen für Stahlrohre verwendet werden. Der Normenharmonisierungsvorschlag, der unter dem Wiener Abkommen umgesetzt werden soll, wurde 2016 auf die Sitzung ISO TC 164 SC2 erfolgreich von deutscher Seite eingebracht.
Extensometer für Hochtemperatur-Ermüdungsversuche sind vielfach aus Aluminium gebaut, an dem Glas- oder Keramikstangen befestigt sind, die in den heißen Bereich des Versuches hineinragen. In einer physikalischen Betrachtung wird die Wärmeausdehnung von Aluminium berechnet und der relative Fehler aus der thermischen Drift ermittelt. Der Fehler ist stark von der thermischen Fluktuation abhängig. Ferner steigt der Fehler besonders bei kleinen Dehnungen stark an. Es wird deutlich, dass bei kleinen Dehnungen die erlaubte Temperaturschwankung im Bereich von 0,5 – 1K betragen darf, um den nach Norm (z.B. ISO 12106) erlaubten Fehler von maximal 1 % nicht zu überschreiten.
In dehnungsgeregelten Ermüdungsversuchen äußert sich die Temperaturschwankung am Extensometer in Kraftschwankungen an der geprüften Probe. Daher bewirkt eine Konstanthaltung der Temperatur eine Verbesserung der Versuchsergebnisse. In der Praxis sind jedoch Temperaturmessungen am Extensometer nicht üblich. Von daher kann der thermisch bedingte Fehler aufgrund von Temperaturschwankungen am Extensometer gar nicht ausgewertet werden. Für eine Verbesserung der Qualität von Versuchsergebnissen wird generell eine Messung der Temperatur am Extensometer vorgeschlagen.
Es ist eine seit ca. 10 Jahren bestehende Politik des ISO Gremiums TC 164, für Normen von mechanischen Prüfverfahren einen Anhang zu entwickeln, der Angaben zur Berechnung der Messunsicherheit enthält. Bis auf wenige Ausnahmen enthalten heute ISO-Normen überwiegend informative Anhänge, die nur allgemeine Informationen zur Berechnung der Messunsicherheit enthalten. In dem Vortrag sollen die Nachteile einer solchen Vorgehensweise dargestellt werden. Es wird ein Vorschlag zur Verbesserung unterbreitet. Anhand von zwei einfachen Beispielen wird veranschaulicht, dass solche informativen Anhänge Messunsicherheiten erzeugen, die bei Erstellung durch unterschiedliche Prüflaboratorien nicht miteinander vergleichbar sind. Das ist nicht befriedigend. Von daher wird vorgeschlagen, einen normativen Anhang zu erstellen, der die Auftraggeber und Antragnehmer verpflichtet, eine Methode zur Berechnung der Messunsicherheit vertraglich auszuhandeln. Ferner wird in dem normativen Anhang darauf hingewiesen, dass beim Vergleich von Messunsicherheiten die Berechnungsgrundlage identisch sein muss. Ferner wird vorgeschlagen, einen informativen Anhang mit einem Beispiel zur Berechnung der Messunsicherheit einzuführen, um Auftraggebern und Auftragnehmern eine entsprechende Möglichkeit zur Berechnung an die Hand zu geben. Diese Methode ist ein pragmatischer Einstieg in die Entwicklung von Anhängen für ISO Normen, in denen die Messunsicherheit berechnet wird. Der Vorschlag enthält Mindestanforderungen. Eine Weiterentwicklung wird in der Zukunft angestrebt.
Es wird der DIN-Arbeitsausschuss Ermüdungsprüfung und seine Aufgaben vorgestellt. Ferner wird über die Thematik der thermischen Drift von Extensometern und die daraus resultierenden Fehler berichtet. Darüber hinaus werden aktuelle Aspekte der dynamischen Temperaturmessung im thermo-mechanischen Ermüdungsversuch vorgestellt.
Investigations on the tensile testing procedure conducted within the European project TENSTAND
(2015)
The current presentation shows the intention of the former European TENSTAND project on computer controlled tensile testing of metals. The history of the tensile testing standards is explained. Some test results of the TENSTAND project are shown. The presentation is focused on criticism of Chinese delegates in ISO TC 164 SC1 WG4 who stated that the TENSTAND project which was finished about 10 years ago came to wrong conclusions and were misleading. This presentation points out misunderstandings of the Chinese delegation in the test program and the results of the TENSTAND project and the former development of the tensile testing standard ISO 6892-1. Some tests of the TENSTAND project were newly evaluated using the original test data. The evaluation showed again that the conclusions of the TENSTAND project are correct.
Components in the Aerospace, Power and Automotive engineering sectors are frequently subjected to cyclic stresses induced by thermal fluctuations and mechanical loads. For the design of such components, reliable material property data are required which need to be acquired using well accepted and reproducible test procedures for thermo-mechanical fatigue (TMF) loading. Available materials TMF property data are limited so that there is a need for further TMF data generated by TMF testing. The TMF behaviour of materials is often desired to be simulated in models which describe the cyclic stress-strain behaviour, the fatigue life and the cyclic crack growth behaviour. There is a continuous need for the development and amendment of such models. Models can be validated by using materials in industrial applications which are subjected to TMF loading.
DIN 50100 definiert die Bedingungen für den Schwingfestigkeitsversuch, der zur Ermittlung von Kennwerten dient, die das Verhalten metallischer Werkstoffe und Bauteile bei zyklischer Beanspruchung mit einer konstanten Lastamplitude beschreiben (Wöhlerversuch). Die Norm umfasst die Versuchsdurchführung, die Versuchsauswertung sowie die Dokumentation von lastgeregelten Wöhlerversuchen im Zeit- und Langzeitfestigkeitsbereich. Mit DIN 50100 soll erreicht werden, dass die Wöhlerlinien verschiedener Prüfstellen miteinander vergleichbar und die wesentlichen Randbedingungen der Versuchsdurchführung dokumentiert werden. Da die Ergebnisse von Schwingfestigkeitsversuchen streuen, ist die Treffsicherheit für die Schätzung der Kennwerte von der Anzahl der Versuche einer Versuchsreihe abhängig. In Abhängigkeit von einer geforderten Treffsicherheit wird durch die DIN 50100 der erforderliche Stichprobenumfang vorgegeben.
In dem Vortrag werden aktuelle Entwicklungen zum thermo-mechanischen Ermüdungsversuch aufgezeigt. In der Vergangenheit wurden ausschließlich dehnungsgeregelte TMF-Versuche durchgeführt. Zukünftig werden auch spannungsgeregelte TMF-Versuche eine Rolle spielen, besonders bei kleinen Belastungen und hohen Schwingspielzahlen. Daneben werden Rissfortschrittsmessungen bei TMF-Beanspruchung eine zunehmende Bedeutung zugemessen. Die Versuchstechnik hierzu ist noch nicht sehr verbreitet verfügbar und ist sehr anspruchsvoll. Es wird das Europäische Forschungsprojekt TEACH vorgestellt, welches beantragt, aber noch nicht bewilligt ist. Ferner wird der 3rd TMF-Workshop 2016 angekündigt, der bei der BAM in Berlin stattfinden wird.
The continuing increase of steam parameters of fossil fuelled high efficiency power plants and new combustion concepts for the capture and storage of carbon dioxide lead to harsher service conditions for the components and structural materials of such facilities. The present work introduces a test concept that allows testing of candidate materials under simultaneous mechanical and corrosive loading. The material's reaction can be directly investigated under simulated temperature, load and corrosion conditions of modern installations. First results obtained for different heat resistant steels suggest a strong influence of the environmental medium on the fatigue and creep behaviour. Such findings complement the data that is available from the classical qualification process of the materials and may support the material selection for new power plant installations.
Rafting during high temperature deformation in a single crystal superalloy: experiments and modeling
(2012)
Experimental characterization and mechanical modeling of creep induced rafting in superalloys
(2012)
A constitutive model has been developed for the high temperature mechanical behavior of single crystal superalloys, including rafting and its consequences. The flow stress depends on the γ channel width via the Orowan stress. An evolution equation for channel widening during high temperature straining has been derived and calibrated with measurements. Therein, rafting is assumed to be driven by the relaxation of internal stresses. The model is able to represent the mechanical softening at high stresses consecutive to rafting. The model has been applied to simulate rafting during uniaxial creep in several crystal orientations, in notched specimens as well as in cyclically loaded specimens.
The uniaxial, torsional and axial-torsional thermomechanical fatigue (TMF) behavior of the near-γ TiAl-alloy TNB-V5 was investigated. TMF tests were performed at 400-800 °C with mechanical strain amplitudes ranging from 0.15% to 0.7%. The tests were conducted thermomechanically in-phase (IP) and out-of-phase (OP).
For the same lifetimes, uniaxial IP tests required the highest strain amplitudes, while OP test conditions were most damaging and needed the lowest strain amplitudes. The Mises equivalent mechanical strain amplitudes of pure torsional tests were found in between uniaxial in-phase and out-of-phase tests for the same lifetimes. The non-proportional multiaxial out-of-phase test showed a lower lifetime at the same equivalent mechanical strain amplitude compared to the other types of tests.
The microstructure has been characterized applying electron microscopy and microstructural parameters such as fraction of twinned grains, grain size, lamellar distance and dislocation density have been quantified.
Mechanical properties of superheater materials after ageing and corrosion in CO2 rich flue gases
(2010)
Creep damage of single-crystal nickel base superalloys: mechanisms and effect on low cycle fatigue
(2010)
The main mechanisms of creep damage of single-crystal nickel-base superalloys are the loss of the interface coherency, coarsening of the γ / γ'-microstructure, precipitation of topologically closed packed phases and growth of porosity. This degradation deteriorates the mechanical properties such as yield stress, creep lifetime and especially low-cycle fatigue life, which can be reduced nearly by a factor of 10. The degradation kinetics during creep was characterised quantitatively on the superalloy CMSX-4. A new non-destructive testing technique was applied to cover a wide test parameter range with few specimens in a relatively short time: repeated load annealing of wedge shaped specimens.
A new experimental technique (repeated load annealing of flat wedge shaped specimens) was proposed for characterization of microstructural degradation under creep conditions. This technique was applied to investigate the microstructural degradation of the nickel-base superalloy CMSX-4 in a wide range of temperatures and stress levels. The results obtained allowed to describe analytically the kinetics of rafting, which is important to predict the reduction of fatigue lifetime and yield stress.
A major international inter-comparison exercise on strain-controlled thermo-mechanical fatigue (TMF) has been undertaken to validate a new European Code of Practice for TMF Testing and to provide underpinning information for an ISO Standard. This paper focuses on (a) distribution of samples of Nimonic 90, (b) the establishment of a protocol for testing and reporting results, and (c) the analysis of the results. Participants in the inter-comparison exercise comprised eight inner-circle partners who primarily used test pieces which were all manufactured at the same workshop, albeit of three different test piece geometries, and ten outer-circle participants who manufactured their own test pieces, of their own in-house geometry. Each participant undertook three repeat in-phase (IP) tests and three repeat out-of-phase (OP) tests. The tests were conducted at temperatures cycling between 400 °C and 850 °C, with a strain range selected to give a failure life of approximately 1000 cycles, resulting in a stress range of up to ~1000 MPa. The testing conditions were chosen following a preliminary evaluation of critical testing parameters. Results from solid circular and solid flat test piece geometries, together with hollow tubular test pieces have been compared. The influence of temperature measurement using different types of temperature sensors has also been investigated. In-house repeatability has been assessed, together with inter-laboratory reproducibility. The results have been correlated with modulus and thermal expansion data for individual tests. Initially, the largest contribution to scatter in the results was attributed to human errors in reporting the results, compounded by computer assisted cut-and-paste errors. Once these obvious discrepancies had been corrected, it was possible to use the data sets to point to some recommendations regarding testing procedures that can be incorporated into the Code of Testing Practice [Hähner P et al. Code of practice for thermo-mechanical fatigue testing TMF-Standard-Work Package 6 Report, September 2005] and provide technical underpinning for the ISO Standard. The results and the procedures used for analysis are presented.
Thermo-mechanical fatigue (TMF) testing plays an increasingly important role in the design, the reliability assessment and the lifecycle management of safety critical components used, for instance, for power generation, in the process industry and in aeronautical and automotive applications, with a view to increasing the fuel efficiency, safety and service intervals, while reducing production (and material) costs. In a European Commission funded research project (acronym: TMF-Standard) of the 5th Framework Programme, 20 European laboratories have undertaken a joint research effort to establish a validated code-of-practice (CoP) for strain-controlled TMF testing. Starting from a survey of the testing protocols and procedures previously used by the partners, a comprehensive pre-normative research activity into various issues has been completed, addressing the dynamic temperature control, the effects of deviations in nominal temperatures and phase angles, the influences of temperature gradients, as well as the practicalities of test interruption and restart procedures. Meaningful allowable tolerances for the various test parameters were identified and practical recommendations as to the test techniques were formulated. From this a preliminary CoP was compiled and used to guide an extensive round robin exercise among the project partners. From the statistical analysis of that exercise, a validated CoP was derived dealing with strain-controlled constant amplitude TMF of nominally homogeneous metallic materials subjected to spatially uniform temperature fields and uniaxial mechanical loading. It is intended to give advice and guidance on the appropriate test setup, testing procedures and the analysis of results, in particular for newcomers in the field of strain-controlled TMF. This paper highlights some of the results of the TMF-Standard project. Moreover, commonalities and differences of the present CoP with respect to the standard documents for strain-controlled TMF, which have been developed at ISO and ASTM levels, are presented in this paper.
SC16 at 950 °C under LCF loading on smooth cylindrical specimen: influence of secondary orientation
(2008)
SC16 at 950°C under LCF loading on smooth cylindrical specimen: influence of secondary orientation
(2008)
Time dependence of y/y' Lattice Mismatch in Creep-deformed Single Crystal Superalloy SC16 at 1173 K
(2007)
Specimens of single crystal superalloy SC16 were creep deformed at 1223 K along [0 0 1] up to ±0.5% creep strain using stresses of -150 MPa and +150 MPa, respectively. Line widths and peak positions of superlattice reflections were measured by means of X-ray diffraction parallel and perpendicular to the load axis in the temperature range between 293 K and 1173 K. The line widths were found to decrease with the increase of temperature for both directions on the two specimens after tensile and compressive creep deformation. After both kinds of creep deformation the crystal lattice showed tetragonal distortion which decreased with increasing temperature. The tetragonality after tensile creep deformation was larger than unity while it was smaller than unity after compressive creep deformation. The peak positions and widths restored after cooling back to room temperature. The experimental results can qualitatively be explained by the creation of dislocations during deformation and their anisotropic arrangement at the γ/γ' interfaces.
Für viskoplastische Stoffgesetze wird das prinzipielle Verfahren der Identifikation von Materialmodell-Parametern für das Spannungs-Verformungs- und Versagens-Verhalten metallischer Werkstoffe am Beispiel von Hochtemperatur-Ermüdung aufgezeigt. Dies betrifft zum Einen die Verwendung geeigneter und auch möglichst weniger Experimente zur Hervorbringung der zu beschreibenden Phänomene des Werkstoffverhaltens und deren gezielte Auswertung sowie zum Anderen die physikalisch kontrollierte Führung einer numerischen Optimierung zur Ermittlung vertrauenswürdiger Materialparameterwerte. Diese Vorgehensweise wurde erfolgreich auf Warmarbeitsstähle sowie poly- und einkristalline Superlegierungen angewendet.
Tensile testing according to EN 10002-1 is one of the basic mechanical tests to characterise the mechanical properties of metallic materials. This testing procedure has been regularly under development for many decades in order to modify and amend it and to bring it up to an up to date standard. Today tensile testing for quality control in metals manufacturing industry is routinely performed automatically with computer controlled testing machines. Due to economical needs of industry proposals were made by a European Standard Committee to amend the standard EN 10002-1. A European research project with the acronym TENSTAND was started to validate the proposed modifications of the tensile testing standard. The work package 4 of the project was to validate the machine control characteristics. A comparison test program was started with ten partners, 4 test piece geometries and 3 test parameters for 6 materials, partly with upper and lower yield strength and partly with 0.2 % proof strength to compare experimental results according to the proposal to modify the standard EN 10002-1. Initially testing in the strain control mode was introduced as well as switching of the control mode to crosshead control 1 and switching of the testing speed at appropriate points during the test. The comparison test was evaluated statistically and scientifically. The following conclusions were derived from the comparison test and summarised as recommendations to the standard committees. The scatter of the material properties were not significantly reduced by introducing the amended testing procedure. Few of the reasons for the continuously observed scattercan be identified as follows: The material properties are observed to be widely dependent on the testing speed. As a consequence the range for the allowed testing speed must be reduced. The closed loop control was not optimised sufficiently in some tests, the use of complicated testing machine software led to misinterpretation of software commands, individual testing equipment and implementation of the tensile testing procedure led also to scatter of the material properties which lay in a range of few percent. This is blurred by the inhomogeneity of the material.
High-resolution diffraction using synchrotron x-ray radiation was applied to study γ´ precipitates with an L12 superlattice crystal structure in the single crystal superalloy SC16 after creep deformation at 1223 K with a creep strain of ±0.5% for tensile and compressive loads, respectively. The measurements of full width at half maximum (FWHM) of 001 and 100 γ´ superlattice reflections were performed at various temperatures from ambient temperature to 1173 K in vacuum. The experimental results revealed that the FWHM of both reflections decrease with increasing temperature. It is well-known that changes in particle size and lattice distortion in materials could lead to a variation of FWHM. The observed behaviour is discussed in the light of both the above-mentioned aspects. The decrease in the peak width is mainly attributed to the temperature dependence of the internal strain state.
A material family to replace the current superalloys in aeronautical gas turbine engines is
considered to be that of gamma Titanium Aluminide (γ-TiAl) alloys. Structural components in
aeronautical gas turbine engines typically experience large variations in temperatures and multiaxial
states of stress under non-isothermal conditions. The uniaxial, torsional and bi-axial thermomechanical
fatigue (TMF) behaviour of this γ-TiAl alloy have been examined at 400 800°C with
strain amplitudes from 0.15% to 0.7%. The tests were conducted at both in-phase (IP) and out-ofphase
(OP). The effects of TMF on the microstructure were also investigated. For the same
equivalent mechanical strain amplitude uniaxial IP tests showed significantly longer lifetimes than
pure torsional TMF tests. The non-proportional multiaxial OP test showed the lowest lifetimes at
the same equivalent mechanical strain amplitude compared to the other types of tests.
Microstructural and Mechanical Performance Assessment of Diffusion Bonded Bimetallic Model Discs
(2003)
The stability of microstructure and the microhardness of a NiCoCrAlY alloy was studied after thermal treatment at high temperatures and subsequent quenching into ice water. The alloy revealed mainly two ordered phases, a gamma' phase with L12 lattice structure and a beta phase with B2 structure. The gamma' phase is shown to become unstable in the temperature range between 1073 K and 1373 K where it undergoes an order-disorder phase transformation. The low transformation temperature compared to pure Ni3Al is ascribed to deviations from the Ni3Al stoichiometry and to the additional alloying elements Co and Cr. The gamma'-gamma order-disorder transformation is shown to be attended by a decrease in microhardness of about 30 %.