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Journal of materials science & technology 102, 89 - 96 (2022). doi:10.1016/j.jmst.2021.06.041, In modern ��-solidified TiAl alloys, the decomposition of ��$_2$ phase is frequently observed during heat treatment or high-temperature deformation of the alloys. In this study, high-temperature deformation and decomposition mechanisms of ��$_2$ phase in a Ti-45Al-8.5Nb-0.2B-0.2W-0.02Y alloy are investigated. In a sample deformed at 800 ��C, the precipitation of ��$_o$(��$_o$) phase is observed within the equiaxed ��$_2$ phase. The nucleation of ��$_o$ particles within the ��$_o$ matrix indicates the ��$_2$ ��� ��$_o$ ��� ��$_o$ transformation. In addition, numerous �� phase precipitates form within the ��$_o$(��) areas. The ��$_2$ lamellae decompose into ultrafine (��$_2$+��) lamellae and coarsened �� lamellae via ��$_2$ ��� ��$_2$ + �� and ��$_2$ ��� �� transformation, respectively. Moreover, the ��$_o$ phase nucleates within the ultrafine lamellae via ��$_2$ ��� ��$_o$ transformation. However, in a sample deformed at 1000 ��C, the nucleation of ��$_o$ particles is sluggish, which is caused by the efficient release of the internal stress via dynamic recrystallization (DRX). These results indicate that complex phase transformations can be introduced by the decomposition of ��$_2$ phase in TiAl alloys with a high amount of ��-stabilizing elements., Published by Ed. Board, Journal of Materials Science & Technology, Shenyang

The transformations between the phases α2 (Ti3Al) and ωo were investigated in a lamellar multiphase titanium aluminide alloy based on γ (TiAl). The paper complements an earlier investigation performed on the same material in which the importance of deformation-induced twin structures for the α2→ωo transformation was demonstrated. The present study shows that the reverse transformation ωo→α2 can also occur during high-temperature deformation. The transformation is probably triggered by constraint stresses, which exist between the different constituents due to the crystalline mismatch. The combined operation of mechanical twinning of the α2 phase and the reversible transformation fully converts the α2 lamellae into a mixture of α2 and ωo. This conversion greatly reduces the mechanical anisotropy existing in former α2 lamellae. Regarding the technical use of the alloy, the stability of the converted structure with respect to further annealing was also examined. The reported processes occur at the nano-meter and sub nano-meter scale, thus, advanced characterization techniques were applied, such as high-resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT).

The nitriding of forging tools is an industrially established standard used to increase the hardness of the tool surface layer and reduce wear. However, this modification of the tool surface layer, as well as the microstructural changes that occur during this operation due to the thermo-mechanical load, cannot be considered during wear calculations with the widely used Archard wear model in the context of FE simulations. Based on previous work, this study further develops two tempering tests for the investigation of the hardness evolution of two nitride profiles based on H11 tool steel. Here, significant tempering effects could be observed depending on temperature, mechanical stress superposition and time. The results are used for setting up a new material model that is implemented in an existing numerical wear model. The validation is carried out in two laboratory forging test series. The evaluation shows that the hardness development in terms of tempering effects of a nitrided forging tool can be numerically predicted, especially for high forging cycles. However, due to the unexpected occurrence of adhesion effects, only limited applicability of the wear prediction then carried out is achieved.

The paper describes the microstructure evolution by high-temperature compression of a high Nb-containing TiAl alloy. The paper extends a previous publication [Song, et al. Intermetallics 109 (2019) 91–96], in which a unique twin-like morphology in the α2 (Ti3Al) phase was reported. However, the origin of these structures could not be clarified without doubt. The present study is focused on phase transformations that in this multiphase alloy can be associated with deformation. Particular attention is paid to local transformations of the α2 phase into O phase or ω-related phases, which, because of structural and chemical similarity of these phases with α2, can easily occur and could mistakenly be considered as a twin structure. The details of the atomic processes involved are elucidated by electron microscopy. Given the large shufflings and the atomic site interchanges required for the operation of this twinning system, it is concluded that twinning of the α2 phase is a diffusive-displacive process. Within the α2 phase, ωo is heterogeneously nucleated. The nucleation sites are defect-rich areas, which are subjected to high local stresses. The study strongly emphasizes the close relationship between high-temperature deformation and phase transformations in multiphase titanium aluminide alloys.

ZrCo2 and HfCo2 Laves phases were found and characterized in γ-γ′ Co-base superalloys using high-energy X-ray diffraction, energy dispersive X-ray spectroscopy and transmission electron microscopy. They formed during casting and are stable during long-term annealing at 900 °C. The ZrCo2 phase with a cubic MgCu2-type structure exhibits numerous nano-twins and stacking faults, while HfCo2 has a hexagonal MgNi2-type structure instead of MgCu2-type in contrast to literature. Their chemical compositions strongly deviated from stoichiometry and significant amounts of Al and W are contained in these phases. These could affect the atomic radius ratio and valence electron concentration and thus change the structure of the Laves phases.

It is commonly agreed that the α2 phase in γ(TiAl)-based alloys is quite difficult to deform plastically especially by order twinning. In the present study, for the first time we report deformation twinning in the α2 phase in a nearly-lamellar two-phase γ-TiAl alloy investigated by electron microscopy. Diffraction pattern and high-resolution images indicate that the twinning plane is ( 20 2 ¯ 1 ) and the twinning direction is [ 10 14 ¯ ] , which is essentially analogous to what has been observed in disordered hexagonal structures. The twinning and anti-twinning mechanisms are interpreted. The twinning is a combined result of superpartial dislocation glide activated by the dissociation of type superdislocation, accompanied by atomic shuffling and short-range diffusion which is facilitated by the anti-structural bridges diffusion mechanism during high-temperature compression. The appearance of deformation twinning in the α2 phase can play an important role for the excellent mechanical properties of high Nb containing TiAl alloys, which is of great importance for the further development of γ-TiAl alloys.

The phase stability, γ′ precipitate morphology and element partitioning behavior in dependence on temperature and alloying composition have been systematically investigated in Co-9Al-9W-2X alloys using electron microscopy, energy-dispersive X-ray spectroscopy (EDX) and high-energy X-ray diffraction (HEXRD). The results show that the decomposition of the γ′ phase is promoted by adding some quaternary elements which increase the γ′ solvus temperature (except V). A larger lattice misfit in the Mo, V, Ti, Nb or Ta containing alloys can explain that coarser γ′ particles with a more cuboidal morphology were found in these alloys. Additional alloying elements and the annealing temperature show a stronger influence on the partitioning behavior of W between the γ and γ′ phases compared to that of Co and Al. The partitioning tendency towards the γ′ phase decreases with increasing temperature for all investigated alloying elements except Cr.

Knowledge about the sublattice site preference of alloying elements in the L12-γ′ phase of novel Co-base superalloys is a necessary pre-requisite to understand their influence on the properties of the alloys in general and the γ′ phase in particular. In the present study, the atomic site occupancy of the alloying elements in the L12-γ′ structure in Co-9Al-9W-2X quaternary alloys after long-term annealing at 900 °C for 5000 h was determined using the atom location by channeling enhanced microanalysis (ALCHEMI) technique in combination with energy-dispersive X-ray spectroscopy (EDX) composition analysis in a transmission electron microscope (TEM). The experimental ALCHEMI data were evaluated by comparing them with those calculated by the program ‘Inelastic Cross Section Calculator’ (ICSC). The results show that Co mainly occupies one sublattice site and Al/W are located at the other sublattice site in the L12 unit cell in the ternary alloy. The additional elements Ti, V, Mo and Ta which partition strongly to the γ′ phase tend to occupy the Al/W sublattice site, and Cr which partitions more to the γ phase also favors the Al/W sublattice site, while Ni weakly partitions into the γ′ phase and favors the Co sublattice site. The results of this study can provide evidence to the predictions on the site preference in literature based on the phase composition or on theoretical studies.

Titanium aluminides based on the L10 ordered g-phase are promising structural light-weight materials for applications in aircraft engines. Typical compositions for γ-TiAl alloys lie in the range Ti-(44-48)Al (at.-%). For high creep resistance, a two-phase microstructure based on lamellar (α2+γ)-colonies is desirable that may be tuned towards better ductility by introducing pure γ-grains (near lamellar or duplex microstructure).γ-TiAl alloys are often alloyed with niobium for increased oxidation resistance and improved mechanical properties. HEXRD and TEM studies of the alloy Ti-42Al-8.5Nb revealed that the orthorhombic O-phase forms during annealing at 500-650°C. This orthorhombic phase has been known in Nb-rich, Al-lean, α2-based Ti-aluminides since the late 1980ies (Nb> 12.5 at.-%, Al< 31 at.-%) but the finding in γ-based alloys is new.TEM imaging showed that the O-phase is located within α2 lamellae of lamellar (α2+γ)-colonies. O-phase domains and α2 phase form small columnar crystallites based in the α2/γ interface. The columnar crystallites grow parallel to the [0001] direction of the α2 phase and appear as facets when observed along this direction. The evolution of domains and facets with annealing time and the chemical homogeneity of the phases are investigated.The results of STEM imaging show that O-phase domains form during annealing at 550 °C for 8hours or 168 hours. After 168 hours of annealing Nb segregations are observed by EDX mapping within O-phase domains. In comparison, no segregation of niobium is detected after 8 hours of annealing.

The influence of alloying elements on the formation of the intermetallic phases B2 and D019 via L12 decomposition during long-time annealing was investigated in Co-9Al-9W-2X alloys using electron microscopy. It is found that the type, shape and location of these intermetallic phases strongly depend on the alloying elements, which could be mainly attributed to different formation energies. Ti promotes B2 phase formation contrary to Mo, Nb and Ta which stabilize three-phase γ + D019 + B2 domains. Nb greatly destabilized the γ′ phase and is not suggested for alloy design in the Co-9Al-9W system.

Intermetallics 100, 70 - 76 (2018). doi:10.1016/j.intermet.2018.06.006, The addition of carbon in TiAl alloys can improve the mechanical properties by precipitate hardening through the perovskite Ti$_3$AlC carbide. Usually precipitates coarsen during continuous annealing. However, in the Ti-45Al-5Nb-0.75C alloy a splitting of the perovskite carbides was observed in later stages of annealing. By investigation with transmission electron microscopy the details of this splitting process are revealed after annealing at 900 °C. The results show that the re-orientation of the $γ$ phase regions between sub-particles is associated with the splitting step from carbide needles into small sub-particles. $γ$ domains with a different orientation with respect to the $γ$ matrix nucleate and gradually replace the $γ$ matrix phase in regions between the carbide sub-particles. The progress of the splitting process is locally different in different carbides and also in one individual carbide. By increasing the temperature from 800 to 900 °C the growth of the emerging carbide conglomerates and the splitting process of the carbides are greatly accelerated. It is found that both are diffusion-controlled processes., Published by Elsevier Science, Amsterdam [u.a.]

The rafting of γ′ precipitates was investigated in a single crystal of Co-9Al-9W(at%) superalloy after compression creep tests at 850 °C. Scanning electron micrographs show that the γ′ precipitates raft perpendicular to the compressive stress axis, due to their positive lattice parameter misfit. The rafting of γ′ precipitates occurred after the minimum strain rate was reached. The dislocation structure and stacking faults were investigated by transmission electron microscopy. Dislocations preferentially moved in the horizontal γ matrix channels where they can relieve the coherency stress at horizontal γ/γ′ interfaces. After extended periods of creep, rafting of the γ′ precipitates perpendicular to the external compressive stress axis occurred. The merging of γ′ precipitates during rafting initiated at the precipitates corners, leaving pockets of matrix phase in the vertical γ channels between adjacent γ′ precipitates. The necessary diffusion of alloying elements during the rafting process between the two orientations of matrix channels should be decelerated as diffusion is slower in the ordered L12 γ′ phase than in the disordered γ phase.

Three cast alloys with compositions of Co-9Al-8W-0.1B, Co-9Al-9W-0.1B and Co-9Al-11W-0.1B (all atomic percent) were aged at 900 °C for 200 h, 500 h, 1000 h, 2000 h, 5000 h and 10000 h to investigate the thermal stability of L12 type γ′-Co3(Al,W) precipitates. The microstructure during the extended ageing process was observed by Scanning and Transmission Electron Microscopy in order to investigate the evolution of the γ/γ′ two-phase microstructure and the formation of any additional phases. It was found after 5000 h ageing that four phases, γ, γ′, D019 and B2, were present within the microstructure as identified by X-ray Diffraction combined with Transmission Electron Microscopy and Electron Backscattered Diffraction. The γ′ precipitates coarsened according to modified Lifshitz, Slyozov and Wagner theory generally. The fraction of γ′ phase decreased while that of D019 phase increased contrarily. The phase transformation process from γ′-L12 to D019 was illustrated by Transmission Electron Microscopy. Stacking faults were identified as the initial step of the decomposition of the γ′ phase.

The plastic deformation mechanisms and the microstructure development during creep deformation of L12-hardened Co-base superalloys show a number of unique features. The preferred orientation of rafting is determined by their positive lattice mismatch. In addition, the regular interfacial dislocation networks often found in rafted specimens of other types of superalloys do not form. While the ordered γ′-L12 precipitates are supposed to harden the material, they are actually found to be frequently cut by partial dislocations generating stacking faults. In this work, specimens from creep tests interrupted at different strains were investigated using transmission and scanning electron microscopy. By this, it is possible to find out which of these processes take place in which stage of creep deformation. For a better understanding of creep deformation, the balance between γ′ cutting and dislocation activity within the matrix channels is of special interest.

This work elucidates the morphology evolution of perovskite Ti3AlC carbides in a Ti-45Al-5Nb-0.75C alloy during ageing at 800 °C. The carbides in the γ matrix are initially needle-shaped with their long axis parallel to the [001] lattice direction of the γ matrix. After extended annealing, they decompose into small carbide sub-particles. By combining different transmission electron microscopy characterization methods and atom probe tomography it has been verified that the carbides first split into several parallel needles that are aligned along the [001]γ lattice direction. Later these parallel needles further decompose into numerous small sub-particles, while the matrix phase region between the sub-particles crystallographically reorients. To the authors' knowledge this is the first work which demonstrates such a precipitate splitting process in a matrix with a tetragonal crystal structure. It is proposed that the decomposition into small sub-particles is energetically favourable owing to the elastic interaction energy between the split sub-particles.

Intermetallic titanium aluminides are often alloyed with niobium to improve oxidation resistance and mechanical properties. Depending on the overall alloy composition and on the solubility of the present phases for niobium, new phases may be introduced in the alloys' microstructure upon niobium addition. Here we describe the microstructure of the alloy Ti-42Al-8.5Nb produced by powder metallurgy after annealing at 550 °C and 650 °C. We identify the orthorhombic O-phase within α 2 laths of lamellar (α 2 + γ) colonies by single crystal electron diffraction and high-resolution imaging. Domains of O-phase variants and α 2 phase form columnar crystallites in α 2 (0001) planes having low indexed α 2 {11-20} and {1-100} interface planes. The nm-sized domains are rotated with respect to each other resulting in elastic strains across the domain boundaries. No elemental segregation of Nb was detected in α 2 /O-phase lamellae.

γ-TiAl alloys are intermetallic materials that are designed for applications in air craft engines. Oftentimes alloying elements are added to improve the corrosion resistance and mechanical properties. Here we investigate microstructural changes that take place when Nb is added to a binary γ-TiAl alloy. Previous studies of the alloy Ti-42Al-8.5Nb have shown that upon Nb addition the orthorhombic O-phase forms out of the parent α2 phase within the alloy’s microstructure. During annealing Nb segregates to the newly formed O-phase while Al and Ti partition to the remaining α2-like matrix phase. The changes these newly formed O-phase domains undergo with time at elevated temperature are not known. Here we use Transmission Electron Microscopy techniques to monitor the distribution of Nb and the size of O-phase domains during long-term annealing between 1 week and 4 weeks at 550°C. The results show that the size of O-phase domains shrinks with annealing time. At the same time the Nb concentration remains largely constant or possibly shows a small decrease.

Reinforcement Learning soll zielfuhrende und effiziente Agenten-Steuerungen automatisch generieren. In diesem Kapitel wird beschrieben, was ein Softwareagent ist und wie er mithilfe seiner „Taktik“ (engl. policy) in einer Umgebung mehr oder weniger intelligentes Verhalten erzeugt. Der Aufbau des Grundmodells des „Bestarkenden Lernen“ wird beschrieben und der Intelligenzbegriff im Sinne einer individuellen Nutzenmaximierung vorgestellt. Auserdem werden einige formale Mittel eingefuhrt. Es wird dargestellt, wie mithilfe der Bellmanschen Gleichung voneinander abhangige Zustande bewertet werden und welche Rolle die „optimale Taktik“ dabei spielt.

In der Regel reichen die verfugbaren Ressourcen nicht aus, um Steuerung, Bewertungsfunktion oder Modell tabellarisch zu erfassen. Daher werden in diesem Kapitel parametrisierte Schatzer eingefuhrt, mit denen wir z. B. die Bewertung von Zustanden auch dann abschatzen konnen, wenn sie nicht in genau gleicher Form zuvor beobachtet worden sind. Im Besonderen wird auf die sogenannten „kunstliche neuronale Netze“ eingegangen. Wir werden auch Moglichkeiten kennenlernen, mit solchen Schatzern parametrisierte Policies zu erstellen, die bei einem gegebenen Zustand eine Wahrscheinlichkeitsverteilung uber die zur Verfugung stehenden Aktionen generieren.

In diesem Abschnitt wird beschrieben, wie eine optimale Handlungsstrategie fur eine Umwelt mit einer endlichen Anzahl von Zustanden und Aktionsmoglichkeiten berechnet werden kann. Sie lernen den Unterschied zwischen einer off-Policy und einer on-Policy Bewertung von Zustandsubergangen kennen. Es werden die Verfahren der Wertiteration und der iterativen Taktiksuche vorgestellt und in Ubungsszenarien mit dem Java Hamster angewendet und ausprobiert. Es wird die iterative Taktiksuche, als wechselseitige Verbesserung von Bewertung und Steuerung, als verallgemeinerungsfahige Strategie zur Suche von optimalem Verhalten vorgestellt. Daruber hinaus werden die Grundlagen der Berechnung von optimalen Zugen in einem uberschaubaren Brettspielszenario mit Gegenspielern beschrieben.

In diesem Kapitel geht es um einen verhaltensorientierten Begriff des maschinellen Lernens und die Einordnung des Reinforcement Learnings in das Gebiet des maschinellen Lernens allgemein. Es wird ein grober Uberblick uber die verschiedenen Prinzipien des Maschinellen Lernens gegeben und erklart wodurch sie sich vom technischen Ansatz her unterscheiden. Im Anschluss wird auf die Implementierung von Reinforcement Learning Algorithmen mit der Programmiersprache Java eingegangen.

Eine fruhe wegweisende Klassifizierung von Systemverhalten publizierten Arturo Rosenblueth, Norbert Wiener und Julian Bigelow bereits 1943 in einem Artikel der Zeitschrift „Philosophy of Science“ (Rosenblueth et al. 1943). Die Veroffentlichung der drei renommierten Wissenschaftler war dazu geeignet, eine Welle von Diskussionen unter Forschern in vielen Disziplinen anzuregen, darunter Philosophen, Biologen, Neurologen und spater auch unter Wissenschaftlern im Bereich der aufkommenden Computertechnologie.

In diesem Kapitel wird beschrieben, wie der Agent ein unbekanntes Umweltsystem, in das er gesetzt wurde erkunden kann. Dabei entdeckt er Zustande mit Belohnungen und muss zum einen die Pfade zu diesen Zielen optimieren („Performanz verbessern“), zum anderen aber auch neue Ziele erkunden („Kompetenzen erlangen“). Hierbei muss er einen trade-off zwischen Ausbeutung und Erkundung berucksichtigen. Einerseits muss er den moglichen Lohn bereits entdeckter Ziele kassieren, andererseits die Erkundung besserer Pfade bzw. die Entdeckung neuer Ziele bewerkstelligen. Hierbei gibt es verschiedene Ansatze, zum einen zielen sie darauf ab, gemachte Erfahrungen so zu verarbeiten, dass sich der Agent unter gleichen Bedingungen kunftig besser verhalt „modellfreie Methoden“, zum anderen gibt es Ansatze, die darauf abzielen, Modelle, die vorhersagen konnen, was bei der Auswahl bestimmter Aktionen passieren wurde, zu optimieren.

The effect of small amounts of Si and Cr on the high temperature oxidation of alloys based on Co-9Al-9W was investigated at 900 °C. The results show that the effect of Si additions on oxidation resistance was more pronounced than for Cr additions. An addition of 4 at.% Cr was insufficient to produce a protective layer, while 2 at.% Si led to the formation of a dense, continuous Al2O3 scale. The thermodynamics and kinetics for Al2O3 layer formation were analyzed by combining CALPHAD and Wagnerʼs theory.

Background:Acceptance has grown for single-needle dialysis when a vascular access is not suited for the insertion of two needles. Its impact on flow patterns was unknown. Methods:We produced transparent life-size models of arteriovenous(AV)fistulas and AV grafts that we placed into a pulsatile flow system to examine flow characteristics during simulated dialysis cycles. For this purpose we injected dye, and recorded and analyzed the resulting flow patterns using suitable software tools. Results:There is a definite flow reversal during the arterial phases in both AV fistulas and -less pronounced-in AV grafts leading to flow oscillations. Conclusion:Flow oscillations initiate subintimal hyperplasia possibly leading to stenoses in vivo. Reversal of the flow direction may also cause steal phenomena in the poorly perfused distal extremity. Single-needle dialysis should therefore be avoided if possible.

The lattice misfit between the γ matrix and coherently embedded γ′ precipitates in the ternary system Co–9Al-xW-0.1B(x = 8,9,11) was measured in-situ by synchrotron X-ray diffraction between room temperature and 1000 °C. The lattice parameters of both γ′ and γ phases increase with rising temperature. The thermal expansion of both phases is similar before the γ′ precipitates start to dissolve. When the temperature rises above 700 °C, the lattice parameter of the γ phase increases more rapidly due to a redistribution of W atoms from the dissolving γ′ precipitates. Thus, the misfit remains almost constant in the temperature range between 20 °C and 700 °C but decreases drastically above 700 °C. Nevertheless, the misfit in these Co–Al–W alloys remains positive up to the γ′ solvus.

This work monitors the nucleation, growth and coarsening of carbides in powder metallurgically processed, high Nb containing TiAl alloys. The effects of carbon content, annealing conditions and internal defects on the precipitation and stability of carbides were systematically investigated by high energy X-ray diffraction and transmission electron microscopy. In general, at 800 °C the carbide microstructure can still change significantly up to 1000 h of annealing. It is found that a higher carbon concentration promotes the carbide precipitation process and increases the thermal stability of carbides. Internal interfaces and other crystallographic defects act not only as heterogeneous nucleation sites for perovskite Ti 3 AlC carbides but also as carbon sinks. This retards the carbide nucleation in the interior of γ-TiAl grains. By homogenising the carbon distribution through solution heat treatment the nucleation of carbides in the γ-TiAl matrix is significantly accelerated as an effect of higher matrix carbon content.

Ternary Co–Al–W alloys with W contents between 8 and 11 at.% were investigated with respect to element partitioning, γ′ solvus, γ′ volume fraction, lattice mismatch and creep properties. W is enriched in the γ′ precipitates and an increasing W content stabilizes this phase. The lattice mismatch is positive up to 900 °C and rafting occurs during creep with a preferred direction in accordance with this positive lattice mismatch.

Wandern ist in - zwei Drittel der Deutschen wandern. Allgemein wird hierbei vor allem von individuellen Wanderungen gesprochen, während Wandern in der Gruppe eher altmodisch erscheint. Auch infolge demografischer Veränderungen oder der Fluktuation bei Veranstaltern überzeugen entsprechende Angebote für Gruppenwanderungen oft nur eine begrenzte Zielgruppe. Wie Wandervereine, Reiseveranstalter, Tourismusorganisationen und Regionen oder auch örtliche Wanderführer Gruppenwanderreisen professionell entwickeln und positionieren, erfahren Sie in diesem Buch aus unterschiedlichen fachlichen Perspektiven.

Journal of alloys and compounds 807, 151649 - (2019). doi:10.1016/j.jallcom.2019.151649, The microstructure evolution and carbide precipitation in a Ti-46Al-8Nb-0.7C alloy as well as its creep properties at intermediate temperatures are investigated by high-energy X-ray diffraction and electron microscopy. The alloy with a nearly fully lamellar microstructure exhibits excellent creep resistance, which could be attributed to the good microstructural stability and strengthening effects from both P- and H-carbides. It is also found that the creep parameters have different effects on the precipitation of the carbides. The overall volume fraction of the carbides shows a positive correlation with the creep temperature and time. However, the thermal stability of P-carbides in the γ grain interior decreases at a higher creep temperature. The creep stress hardly affects the precipitation and morphology development of the P-carbides. On the contrary, a higher stress can promote the H-carbide formation at the γ/α2 interfaces via α2 lath decomposition in lamellar colonies., Published by ScienceDirect, Amsterdam [u.a.]

In this study, new multilayer TiAl-based composites were developed and characterized. The materials were produced by spark plasma sintering (SPS) of elemental Ti and Al foils and ceramic particles (TiB2 and TiC) at 1250 &deg, C. The matrix of the composites consisted of &alpha, 2-TiAl and &gamma, TiAl lamellas and reinforcing ceramic layers. Formation of the &alpha, 2 + &gamma, structure, which occurred via a number of solid&ndash, liquid and solid&ndash, solid reactions and intermediate phases, was characterized by in situ synchrotron X-ray diffraction analysis. The combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis revealed that an interaction of TiC with Ti and Al led to the formation of a Ti2AlC Mn+1AXn (MAX) phase. No chemical reactions between TiB2 and the matrix elements were observed. The microhardness, compressive strength, and creep behavior of the composites were measured to estimate their mechanical properties. The orientation of the layers with respect to the direction of the load affected the compressive strength and creep behavior of TiC-reinforced composites. The compressive strength of samples loaded in the perpendicular direction to layers was higher, however, the creep resistance was better for composites loaded in the longitudinal direction. The microhardness of the composites correlated with the microhardness of reinforcing components.

Intermetallics 33, 44-53 (2013). doi:10.1016/j.intermet.2012.09.023, Published by Elsevier Science, Amsterdam [u.a.]

Intermetallics 89, 32 - 39 (2017). doi:10.1016/j.intermet.2017.05.006, Carbide precipitation in TiAl alloys with different alloying element additions and thermal history was investigated by transmission electron microscopy and high energy X-ray diffraction. The results reveal that Nb addition in TiAl alloys does not increase the carbon solubility in the γ matrix significantly. With increasing carbon concentration in the alloys, a splitting of the P-type carbides takes place earlier in the course of ageing at 800 °C and the formation of H-type carbides is promoted. For a nearly single γ phase alloy Ti-51Al-5Nb-0.5C, H-type carbides are the thermodynamically stable phase. However, with decreasing Al concentration, P-type carbides become thermodynamically stable at low carbon concentration (below 1%) in Ti-45Al and Ti-45Al-5Nb alloys. It is feasible to achieve high thermodynamical stability of the P-type carbides in TiAl alloys through controlling alloying elements., Published by Elsevier Science, Amsterdam [u.a.]

Diffusion brazing is a potential method to repair parts made from TiAl-alloys. Two different brazing materials with varying contents of titanium, iron and nickel were investigated. The phases present in the brazed zone were identified by high energy X-ray diffraction (HEXRD) at the material science beamline HEMS at the PETRA III synchrotron facility at DESY in Hamburg, Germany, and the microstructure was characterised by scanning electron microscopy (SEM). The braze zone itself is composed of one to two transitional layers from the substrate material to the middle of the joint. Near the substrate material the phase constitution reassembles a TiAl-alloy while the middle of the joint is similar to α/β-titanium alloys. Besides phases commonly encountered in TiAl-alloys such as γ, α2 and β, additional phases, which are related to the presence of nickel or iron as melting point depressing elements are present. The microstructure of the brazed zone changes significantly during a subsequent heat treatment.

The quality of wrought discs made from large Ti-45Al-5Nb-0.2B-0.2C (at. %) ingots has been assessed at various stages of processing. Such components suffer from different types of defects that can usually be traced back to structural and chemical inhomogeneities that are present in the starting ingot. In conventional manufacturing routes the imparted mechanical strain is often insufficient for complete dynamic recrystallisation, particularly for large parts. In an attempt to overcome these problems, a new multi-stage method has been developed, which has the advantages that: i) the strain is significantly higher than that imparted using conventional routes, ii) the flow direction of the material is inverted between the first and second processing stages in the sense that the length of the ingot is first increased via extrusion, and then the resulting extruded bar is shortened via axial forging in the subsequent processing stages, and iii) the geometrical constraints associated with the production of large components from ingots can be overcome. The reliability of the method has been assessed by metallographic and mechanical characterisation. The challenges that have to be addressed for the new technique to be employed for the manufacture of large components are discussed.