Microstructure and creep resistance of Ti-rich Mo + Mo5Si3 + Mo5SiB2 alloys

New materials are necessary to increase efficiency of power generation and aircraft engines by higher combustion temperatures. Recently, it was found that alloying with high amounts of Ti replaces Mo3Si by Mo5Si3 and stabilizes the phase field Mo + Mo5Si3 + Mo5SiB2, which could be beneficial for higher oxidation resistance, since Mo5Si3 is more oxidation resistant than Mo3Si. Additional, Ti-rich Mo-Si-B alloys show an increased creep resistance compared to Ti-free Mo-Si-B alloys and a significant reduction in the alloy density. However, using the compositions reported in literature to stabilizing Mo5Si3 does not lead to reproducible results. This is most likely due to segregation effects and the formation of metastable phases like Ti5Si3. The addition of minor elements can be an option to widen the phase field Mo + Mo5Si3 + Mo5SiB2 as a function of Ti concentration. In this study we want to show the efficiency of such minor alloying additions on the stability of Ti-rich Mo + Mo5Si3 + Mo5SiB2 alloys. This was done by CALPHAD calculations using the commercial Pandat software for several elements such as Al, Cr, Fe, Hf and Zr. Fe seems to be the most promising candidate for stabilizing Mo5Si3. Experimental evaluation was exemplified with Mo-12.5Si-8.5B-xTi-2Fe and Mo-9Si-8B-xTi-2Fe model alloys to determine possible concentration ranges of Ti. Those alloys were produced by repetitive arc-melting of high-purity metals, Si and B in a Zirconium gettered high-purity argon atmosphere, followed by homogenization treatment at different temperatures for various times in a high-purity argon atmosphere. The identification of the resulting phases was done by XRD, SEM and EDS/ESMA analysis. Additionally, the creep resistance of those alloys was determined at temperatures ranging from 1100 to 1300°C and correlated to their microstructural features

Characterisation of the oxidation and creep behaviour of novel Mo-Si-Ti alloys

The oxidation and creep behaviour of novel eutectic-eutectoid Mo-Si-Ti alloys were studied and compared to previously investigated entirely eutectic Mo-20Si-52.8Ti (at%) and eutectoid Mo-21Si-34Ti reference alloys [Schliephake et al. in Intermetallics 104 (2019) 133-142]. While the latter reference alloys showed either outstanding oxidation behaviour in the temperature range of 800 to 1200 °C (eutectic alloy) or reasonable creep resistance (eutectoid alloy), a combination of both was successfully achieved in a Ti-rich alloy variant (Mo-21Si-43.4Ti). The ubiquitous catastrophic oxidation (“pesting”) of Mo-based alloys at 800 °C is suppressed in this alloy and reasonable oxidation resistance at higher temperatures is observed. For the first time, the unexpected oxidation resistance of the alloys exhibiting eutectic volume fractions of more than 50 vol% is rationalised by a systematic deconvolution of mass gain by scale formation and mass loss by evaporation of volatile species. Furthermore, creep is revealed to be based on similar creep mechanisms throughout the alloy series. Therefore, the observed improvement in creep resistance of the pesting-resistant Ti-rich alloy variant over the eutectic alloy is attributed to the decreasing homologous temperature when testing both at 1200 °C

Occupational Therapy After Myocardial or Cerebrovascular Infarction: Which Factors Influence Referrals?

Background: Cardiovascular diseases remain the number one cause of death worldwide, and many survivors suffer lasting disabilities. Occupational therapy can help such patients regain as much function as possible. However, little is known about the factors influencing referrals to occupational therapy after stroke or myocardial infarction (MI). Method: Data from the IMS Disease Analyzer® database were observed for a three-year period. The study population included 7,440 patients who were examined by a cardiologist due to stroke or MI. In addition to baseline characteristics, the presence of certain cardiovascular risk factors or comorbidities was recorded. Cox regression analyses were performed and the Charlson Comorbidity Index (CCI) was utilized. Results: Occupational therapy was received by 1,779 patients; 88.5% had suffered an MI and 11.5% a stroke. In the group without referral (n = 5,661), 60.7% had experienced an MI and 39.3% a stroke. No significant gender-related differences were observed. Younger age, an MI diagnosis, and the presence of hypertension positively influenced referral rate and time, while risk factors, such as adiposity, delayed therapy. The CCI was higher in the group with occupational therapy. Conclusion: The chance of being offered occupational therapy increased with younger age, history of MI, and the presence of hypertension. Future studies should also consider severity of ischemic lesion to account for the degree of remaining impairment

Comparison of the Internal Fatigue Crack Initiation and Propagation Behavior of a Quenched and Tempered Steel with and without a Thermomechanical Treatment

Previous studies have shown that a thermomechanical treatment (TMT) consisting of cyclic plastic deformation in the temperature range of dynamic strain aging can increase the fatigue limit of quenched and tempered steels by strengthening the microstructure around non-metallic inclusions. This study considers the influence of a TMT on the shape, size and position of crack-initiating inclusions as well as on the internal crack propagation behavior. For this, high cycle fatigue tests on specimens with and without TMT were performed at room temperature at a constant stress amplitude. The TMT increased the average lifetime by about 40%, while there was no effect of the TMT on the form or size of critical inclusions. Surprisingly, no correlation between inclusion size and lifetime could be found for both specimen types. There is also no correlation between inclusion depth and lifetime, which means that the crack propagation stage covers only a small portion of the overall lifetime. The average depth of critical inclusions is considerably higher for TMT specimens indicating that the strengthening effect of the TMT is more pronounced for near-surface inclusions. Fisheye fracture surfaces around the critical inclusions could be found on all tested specimens. With increasing fisheye size, a transition from a smooth to a rather rough and wavy fracture surface could be observed for both specimen types

Regulation der Akkumulation des Transkriptionsfaktors Hypoxie-induzierbarer Faktor-1 alpha in humanen Ovarialkarzinom-Zelllinien

Der Transkriptionsfaktor Hypoxie-induzierbarer Faktor-1 (HIF-1) und sein Zielgen Vascular endothelial growth factor (VEGF), ein wesentlicher Induktor der Gefäßneubildung in hypoxischen Geweben, spielen eine entscheidende Rolle für das Wachstum und die Metastasierung von Tumoren. In der vorliegenden Arbeit konnte gezeigt werden, dass die Aktivierung von HIF-1 und Expression von VEGF als HIF-1-Zielgen sowohl durch einen verminderten Sauerstoffpartialdruck als auch durch den Einfluss des Zytokins Interleukin-1β (IL-1β) gefördert wird. Obwohl sich im Electrophoretic-mobility-shift-assay (EMSA) zeigte, dass das unter Zytokin-Einfluss vermehrt gebildete HIF-1α im Kern als HIF-1-Komplex an die DNA bindet, wurde im Reportergenassay mit dem HIF-spezifischen Vektor pH3SVL eine verminderte Transaktivität des HIF-1-Komplexes nach IL-1β Behandlung festgestellt. Da die Transaktivität des HIF-1 in der C-terminalen Transaktivierungsdomäne (TAD-C) des Proteins reguliert wird, wurde ein chimeres Aktivator/Reporter-System angewandt, bei dem die TAD-C von HIF-1 mit dem Transkriptionsfaktor Gal4 der Hefe fusioniert wurde. Im anschließend durchgeführten Assay mit dem Reportergen Luciferase war bei den Kontrollproben eine Zunahme der Luciferaseaktivität unter Hypoxie sichtbar. Demnach scheint die TAD-C auszureichen, um die Transaktivität von HIF-1 zu vermitteln. Die mit IL-1β versetzten Zellen zeigten sowohl unter Normoxie als auch unter Hypoxie eine deutlich verminderte Luciferaseaktivität. Aus diesen Ergebnissen lässt sich schließen, dass IL-1β die TAD-C von HIF-1 in der Art modifiziert, dass zwar die Bindung von HIF-1 an die DNA möglich ist, die Transkriptionsaktivität aber gehemmt wird. Daher muss angenommen werden, dass die unter Zytokin-Einfluss vermehrte VEGF-Expression unabhängig von dem Transkriptionsfaktor HIF-1α stattfindet. Durch das genaue Verständnis der durch IL-1β hervorgerufenen Veränderungen wird erhofft, entscheidende Verbesserungen in den Therapiemöglichkeiten bei Erkrankungen zu erlangen, die mit einem reduzierten Sauerstoffpartialdruck und/oder einer gesteigerten Zytokinsekretion einhergehen

Enhanced oxidation resistance of Ti-rich Mo-Si-B alloys by pack-cementation process

To increase efficiency by higher combustion temperatures of aircraft engines and energy generation, new high temperature materials are inevitable. Mo-Si-B alloys for example satisfy several requirements such as good oxidation and creep resistance. Recently, novel Ti-rich Mo-Si-B alloys have shown an increased creep resistance compared to Ti-free Mo-Si-B alloys by the formation of Ti-silicide precipitates during processing. However, due to the formation of a duplex SiO2 – TiO2 oxide layer, where fast inwards diffusion of oxygen takes place, the oxidation resistance is poor. In this study we show that oxidation resistance of Mo-Si-B-Ti alloys can be enhanced drastically at temperatures ranging from 800 to 1200°C for several hundreds of hours by pack-cementation application of a borosilica based coating. The Mo-12.5Si-8.5B-27.5Ti (in at.%) substrate was produced by repetitive arc-melting of high-purity metals, Si and B in a high-purity argon atmosphere. After homogenization treatment at 1600°C for 100h slices of this alloy were prepared for pack-cementation. The pack-cementation was done in an atmosphere of high-purity argon at 1000°C for 40h, followed by a conditioning step at 1400°C for 10h in air. The resulting layer consists of an outer borosilica layer followed by an inner MoSi2 and Mo5Si3 layer. To study the oxidation behavior, both isothermal and cyclic oxidation tests were carried out. After an initial mass loss during the first hours of oxidation, a steady state is reached for tests up to 1000 hours. To demonstrate the high stability of the outer borosilicate layer SEM cross-sections were prepared after different times of oxidation

Microstructural Investigations of Novel High Temperature Alloys Based on NiAl-(Cr,Mo)

Apart from the reported transition from the fibrous morphology in NiAl-34Cr to lamellae by adding 0.6 at.% Mo, further morphology transformations along the eutectic trough in the NiAl-(Cr,Mo) alloys were observed. Compositions with at least 10.3 at.% Cr have lamellar morphology while the first tendency to fiber formation was found at 9.6 at.% Cr. There is a compositional range, where both lamellae and fibers are present in the microstructure and a further decrease in Cr to 1.8at.% Cr results in fully fibrous morphology. Alongside these morphology changes of the (Cr,Mo)$_{ss}$ reinforcing phase, its volume fraction was found to be from 41 to 11 vol.% confirming the trend predicted by the CALPHAD approach. For mixed morphologies in-situ X-ray diffraction experiments performed between room and liquidus temperature accompanied by EDX measurements reveal the formation of a gradient in composition for the solid solution. A new Mo-rich NiAl-9.6Cr-10.3Mo alloy clearly shows this effect in the as-cast state. Moreover, crystallographic orientation examination yields two different types of colonies in this composition. In the first colony type, the orientation relationship between NiAl matrix and (Cr,Mo)$_{ss}$ reinforcing phase was (100)$_{NiAl||}$ (100) $_{Cr,Mo}$ and ⟨100⟩ $_{NiAl||}$ ⟨100⟩ $_{Cr,Mo}$. An orientation relationship described by a rotation of almost 60° about ⟨111⟩ was found in the second colony type. In both cases, no distinct crystallographic plane as phase boundary was observe