Indentation creep testing of superalloys

Great progress has been made over the last years in high temperature nanoindentation testing and quite reliable test systems are available to operate at temperatures up to 800°C. With such systems the high temperature strength is measured via the hardness of materials. However, for high temperature materials especially the creep strength is of interest and therefore also many attempts have been undergone to probe also the creep properties with high temperature nanoindentation. In most cases pointed indenters as Berkovich or conical indenters have been used for this. A major challenge, however, then is, how the nanoindentation data are converted into uniaxial creep properties, i.e. those which are needed for constructional purposes. Although, it seems that the stress exponent can be derived quite successfully with such indenters, an evaluation of a full creep curve for materials with significant primary creep does not seem possible, since the strain a pointed indenter is inducing is fixed by the indenter shape and stays more or less constant during the whole test [1]. Please click Additional Files below to see the full abstract

γ/γ\u27 Co-base superalloys – new high temperature materials beyond Ni-base Superalloys?

In 2006 a new L12 phase, Co3(Al,W), was discovered in the Co-Al-W system which has led to the development of novel Co-base superalloys with g/g¢ microstructures similar to those of the well-established Ni-base superalloys. First investigations on simple ternary alloys could show that these Co-Al-W based alloys exhibit higher solidus temperatures and show less segregations after casting compared to typical Ni-base superalloys. This leads to the question whether this g/g¢ Co-base superalloys can be regarded as new class of high temperature materials that can compete with or even supersede established Ni-base superalloys. In the first part of the talk it will be shown how alloy properties change, when the base element Ni is gradually substituted by Co in a series of Ni-Co-Al-W-Cr alloys with otherwise constant element contents of Al, W and Cr. All alloys form g/g¢ microstructure after a standard aging treatment with a similar g¢ volume content. Liquidus and solidus temperatures are hardly influenced by the Ni/Co content, but the g¢ solvus temperature is strongly decreasing with increasing Co content. This indicates that the potential application temperature of g/g¢ Co-base superalloys will not be beyond the maximum application temperature of advanced single crystal Ni-base superalloys. However, this also shows that g/g¢ Co-base superalloys have a great potential as wrought alloys since the solvus temperature of the intermetallic compound is comparatively low, which gives a large processing window, and because a high volume fraction of the L12 phase at temperatures up to 900°C can be achieved. Please click Additional Files below to see the full abstract

High temperature indentation creep and nanoindentation testing of superalloys and TiAl alloys

Measuring of the high temperature mechanical behaviour of materials by local testing has become a key task in the field of nanomechanics. However, gaining access to the application temperature of many metallic high temperature materials, which is in the range of 600°C - 1100°C, is quite difficult. In addition, creep parameters can only be determined by long time measurements, where drift influences become a severe challenge. Here we present a new approach of indentation creep testing with a flat punch indenter. For this, a thermo mechanical analyzer with very precise temperature control is used, which allows testing at temperatures up to 1200°C. A flat punch indenter with a diameter of around 10 µm allows for example local investigations of the creep properties on the dendritic scale of superalloys. This approach is also interesting to study the creep properties along the gradient of diffusion couples. Here, first test measurements on superalloys and other materials are presented and discussed. For comparison also high temperature nanoindentation measurements will be shown. Such measurements have been conducted on a multiphase titanium aluminide alloy from room temperature up to 600°C. The results show, that the hardness of the (β0+ω0)-composite phase is the highest among all phases and remains constant up to the service temperature. Both approaches of high temperature testing are compared and the prospect of these methods will be discussed

Evaluation of Co-based thermodynamic databases with respect to own and literature experimental data

The development of Ni-based alloys proved the importance of dedicated Gibbs energies databases constructed following the CALPHAD method. Validated databases for Co-based and Ni/Co-based alloys are therefore imperative. These databases are being constructed concurrently with the development of new alloys in an interactive mode: databases anticipate quantities, new measurements are done which validate the database results or demand for changes. In this work we collect several thermodynamic assessments of ternaries and quaternaries systems, relevant for Co-based alloys, published recently in the literature and compare the calculated results with the obtained by using TCNI8 (which can also be used for Co-based alloys). We also compare calculated results to Liquidus and solidus temperatures experimentally determinate for several alloys in development in Erlangen. A comparison between First Principles calculated formation enthalpies of several TCP (topologically close packed) phases with the values calculated from the databases is also presented. As a result of this analysis necessary changes in the databases are pointed out as well as the regions of composition and temperature where more experimental data is required

Revealing atomic-scale vacancy-solute interaction in nickel

Imaging individual vacancies in solids and revealing their interactions with solute atoms remains one of the frontiers in microscopy and microanalysis. Here we study a creep-deformed binary Ni-2 at.% Ta alloy. Atom probe tomography reveals a random distribution of Ta. Field ion microscopy, with contrast interpretation supported by density-functional theory and time-of-flight mass spectrometry, evidences a positive correlation of tantalum with vacancies. Our results support solute-vacancy binding, which explains improvement in creep resistance of Ta-containing Ni-based superalloys and helps guide future material design strategies.Comment: Submitted to Physics Review Lette

Imaging individual solute atoms at crystalline imperfections in metals

Directly imaging all atoms constituting a material and, maybe more importantly, crystalline defects that dictate materials\u27 properties, remains a formidable challenge. Here, we propose a new approach to chemistry-sensitive field-ion microscopy (FIM) combining FIM with time-of-flight mass-spectrometry (tof-ms). Elemental identification and correlation to FIM images enabled by data mining of combined tof-ms delivers a truly analytical-FIM (A-FIM). Contrast variations due to different chemistries is also interpreted from density-functional theory (DFT). A-FIM has true atomic resolution and we demonstrate how the technique can reveal the presence of individual solute atoms at specific positions in the microstructure. The performance of this new technique is showcased in revealing individual Re atoms at crystalline defects formed in Ni–Re binary alloy during creep deformation. The atomistic details offered by A-FIM allowed us to directly compare our results with simulations, and to tackle a long-standing question of how Re extends lifetime of Ni-based superalloys in service at high-temperature

Influence of Rhenium and Ruthenium on the Microstructure and the High Temperature Deformation Behaviour of 4th Generation Nickel-base Superalloys

Ruthenium und Rhenium-haltige Nickelbasis-Superlegierungen der neuesten Generation besitzen eine deutlich höhere Hochtemperaturfestigkeit bei sehr hohen Temperaturen im Vergleich zu Legierungen früherer Generationen. In dieser Arbeit wurden Experimentallegierungen mit unterschiedlichen Gehalten an Ruthenium und Rhenium untersucht um die Einflüsse dieser Elemente, insbesondere Ruthenium als neues Legierungselement, zu charakterisieren. Dabei wurden die mikrostrukturellen Parameter, die die Hochtemperaturfestigkeit beeinflussen, in den verschiedenen Legierungen charakterisiert, und der Einfluss von Ruthenium und Rhenium auf diese bewertet. Im Einzelnen sind dies zum einen die Solvustemperatur, der Volumenanteil und die Morphologie der y'-Ausscheidungsphase und deren Entwicklung bei hoher Temperatur. Zum anderen wurde das Verteilungsverhalten der Legierungselemente zwischen der y- und y'-Phase und der damit verbundene Einfluss der Elemente auf die Mischkristallhärtung und die Gitterfehlpassung zwischen der y- und y'-Phase untersucht. Einige der in der Literatur genannten möglichen Ursachen für die erhöhte Phasenstabilität wurden in die Diskussion mit einbezogen um zum grundlegenden Verständnis der Wirkungsweise dieser Elemente beizutragen. Druckkriechversuche bei hohen Temperaturen und niedrigen Spannungen (T = 1100°C, S = 137 MPa) wurden durchgeführt, um den Einfluss der zuvor untersuchten Parameter und deren Auswirkungen auf die Kriechfestigkeit zu bestimmen. Die vorliegende Arbeit konnte zu einem verbesserten Verständnis des Zusammenspiels der verschiedenen Effekte, die die Hochtemperaturfestigkeit der Legierung beeinflussen, beitragen. Außerdem wurde deutlich, dass bei sehr hohen Anwendungstemperaturen refraktäre Legierungselemente und Parameter wie die Gitterfehlpassung von großer Bedeutung sind. Aufgrund der positiven Eigenschaften von Ruthenium erschließen sich weitere Möglichkeiten das Einsatzgebiet von Nickelbasis-Superlegierungen zu höheren Temperaturen zu verschieben. Aufgrund der erhöhten Phasenstabilität durch Ruthenium kann auch ein höherer Anteil an refraktären Legierungselementen hinzulegiert werden. In weiteren Arbeiten ist dann zu klären, bis zu welchem Wert es von Vorteil ist die Gitterfehlpassung durch geeignete Legierungselemente wie Rhenium und Ruthenium zu erhöhen, so dass eine weitere Festigkeitssteigerung erzielt werden kann, ohne dass die y'-Teilchen schon nach einer Standardwärmebehandlung inkohärent in der y-Matrix vorliegen. Außerdem ist zu klären, auf welchem Mechanismus die erhöhte Phasenstabilität durch Ruthenium zurückzuführen ist, so dass in der Zukunft Nickelbasis-Superlegierungen gezielt optimiert werden können, um die Effizienz zukünftiger Turbinen deutlich zu steigern und die Emissionen zu verringern.Ruthenium and rhenium containing 4th generation nickel-base superalloys possess an increased high temperature strength compared to former nickel-base superalloys. In the present work several experimental alloys with an identical base composition possessing different contents of rhenium and ruthenium were investigated to characterise the influence of both alloying elements on nickel-base superalloys. In order to obtain an exact understanding of the role of rhenium and ruthenium a variety of experiments were performed. The solvus temperature, the volume fraction, the morphology and the evolution of the y' precipitate phase were investigated since the y' phase is crucial for the high temperature strength. Additionally the partitioning behaviour of the alloying elements between the y matrix and the y' phase were determined by measuring the element concentration in both phases. Differences in the partitioning behaviour affect the solid solution hardening and the lattice misfit between y- and y' which again influence the mechanical properties. The four experimental alloys under closer investigation were stable against the formation of TCP phases, so that the direct influence of the alloying elements on the mechanical properties could be studied. Creep experiments under compression were performed at T = 1100°C and S = 137 MPa to relate the aforementioned factors on the high temperature creep strength. This work could contribute to the understanding of the interaction between various factors which influence the high temperature strength of nickel-base superalloys. Alloying elements like rhenium and ruthenium are of great importance as they enable the usage of nickel-base superalloys at even higher application temperatures. Further investigations should focus on the optimal lattice misfit and initial microstructure of highly alloyed nickel-base superalloys and the reasons for the enhanced phase stability so that future nickel-base superalloys can be optimised specifically to increase the efficiency of future turbines