Investigation of TaC and TiC for Particle Strengthening of Co-Re-Based Alloys

Cobalt-Rhenium (Co-Re)-based alloys are currently investigated as potential high-temperature materials with melting temperatures beyond those of nickel-based superalloys. Their attraction stems from the binary Co-Re phase diagram, exhibiting complete miscibility between Co and Re, whereby the melting temperature steadily increases with the Re-content. Thus, depending on the Re-content, one can tune the melting temperature between that of pure Co (1495 °C) and that of pure Re (3186 °C). Current investigations focus on Re-contents of about 15 at.%, which makes melting with standard equipment still feasible. In addition to solid solution strengthening due to the mixture of Co- and Re-atoms, particle strengthening by tantalum carbide (TaC) and titanium carbide (TiC) precipitates turned out to be promising in recent studies. Yet, it is currently unclear which of the two particle types is the best choice for high temperature applications nor has the strengthening mechanism associated with the monocarbide (MC)-precipitates been elucidated. To address these issues, we perform compression tests at ambient and elevated temperatures on the particle-free base material containing 15 at.% of rhenium (Re), 5 at.% of chromium (Cr) and cobalt (Co) as balance (Co-15Re-5Cr), as well as on TaC- and TiC-containing variants. Additionally, transmission electron microscopy is used to analyze the shape of the precipitates and their orientation relationship to the matrix. Based on these investigations, we show that TiC and TaC are equally suited for precipitation strengthening of Co-Re-based alloys and identify climb over the elongated particles as a rate controlling particle strengthening mechanism at elevated temperatures. Furthermore, we show that the Re-atoms are remarkably strong obstacles to dislocation motion, which are overcome by thermal activation at elevated temperatures.

Laser powder bed fusion (LPBF) of commercially pure titanium and alloy development for the LPBF process

Laser powder bed fusion (LPBF) of titanium or titanium alloys allows fabrication of geometrically more complex and, possibly, individualized implants or osteosynthesis products and could thus improve the outcome of medical treatments considerably. However, insufficient LPBF process parameters can result in substantial porosity, decreasing mechanical properties and requiring post-treatment. Furthermore, texturized parts with anisotropic properties are usually obtained after LPBF processing, limiting their usage in medical applications. The present study addresses both: first, a design of experiments is used in order to establish a set of optimized process parameters and a process window for LPBF printing of small commercially pure (CP) titanium parts with minimized volume porosity. Afterward, the first results on the development of a biocompatible titanium alloy designed for LPBF processing of medical implants with improved solidification and more isotropic properties are presented on the basis of conventionally melted alloys. This development was performed on the basis of Ti-0.44O-0.5Fe-0.08C-0.4Si-0.1Au, a near-α alloy presented by the authors for medical applications and conventional manufacturing, with yttrium and boron additions as additional growth restriction solutes. In terms of LPBF processing of CP titanium grade 1 powder, a high relative density of approximately 99.9% was obtained in the as-printed state of the volume of a small cubical sample by using optimized laser power, scanning speed, and hatch distance in combination with a rotating scanning pattern. Moreover, tensile specimens processed with these volume settings and tested in the as-printed milled state exhibited a high average yield and ultimate tensile strength of approximately 663 and 747 N/mm2, respectively, combined with a high average ductility of approximately 24%. X-ray diffraction results suggest anisotropic mechanical properties, which are, however, less pronounced in terms of the tested specimens. Regarding alloy development, the results show that yttrium additions lead to a considerable microstructure refinement but have to be limited due to the occurrence of a large amount of precipitations and a supposed higher propensity for the formation of long columnar prior β-grains. However, phase/texture and microstructure analyses indicate that Ti-0.44O-0.5Fe-0.08C-0.4Si-0.1Au-0.1B-0.1Y is a promising candidate to achieve lower anisotropy during LPBF processing, but further investigations on LPBF printing and Y2O3 formation are necessary.

Reassessment of the Matrix Composition of Co-Re-Cr-Based Alloys for Particle Strengthening in High-Temperature Applications and Investigation of Suitable MC-Carbides

This study reassesses the optimum matrix composition of Co-Re-Cr-based alloys for strengthening by MC-type carbides. It is found that the composition of Co-15Re-5Cr is ideally suited for this purpose as it allows the solution of the carbide-forming elements such as Ta, Ti, Hf, and C within a matrix consisting entirely of fcc-phase (typically at 1450 °C), having a high solubility for these elements, while precipitation heat treatment (typically at 900-1100 °C) occurs in a hcp-Co matrix, displaying a much lower solubility. In the case of the monocarbides TiC and HfC, this was investigated and achieved for the first time in Co-Re-based alloys. TaC and TiC emerged as suitable particles in Co-Re-Cr alloys for creep applications due to a large population of nano-sized particle precipitation, which is not the case for the mainly coarse HfC. Both Co-15Re-5Cr-xTa-xC and Co-15Re-5Cr-xTi-xC exhibit a formerly unknown maximum solubility close to x = 1.8 at.%. Therefore, further research on the particle-strengthening effect and the governing creep mechanisms of carbide-strengthened Co-Re-Cr alloys should focus on alloys with the following compositions: Co-15Re-5Cr-1.8Ta-1.8C and Co-15Re-5Cr-1.8Ti-1.8C.

New Approach in the Determination of a Suitable Directionally Coarsened Microstructure for the Fabrication of Nanoporous Superalloy Membranes Based on CMSX-4

The pore size of nanoporous superalloy membranes produced by directional coarsening is directly related to the γ-channel width after creep deformation, since the γ-phase is removed subsequently by selective phase extraction. The continuous network of the γ'-phase thus remaining is based on complete crosslinking of the γ'-phase in the directionally coarsened state forming the subsequent membrane. In order to be able to achieve the smallest possible droplet size in the later application in premix membrane emulsification, a central aspect of this investigation is to minimize the γ-channel width. For this purpose, we use the 3w₀-criterion as a starting point and gradually increase the creep duration at constant stress and temperature. Stepped specimens with three different stress levels are used as creep specimens. Subsequently, the relevant characteristic values of the directionally coarsened microstructure are determined and evaluated using the line intersection method. We show that the approximation of an optimal creep duration via the 3w0-criterion is reasonable and that coarsening occurs at different rates in dendritic and interdendritic regions. The use of staged creep specimens shows significant material and time savings in determining the optimal microstructure. Optimization of the creep parameters results in a γ-channel width of 119 ± 43 nm in dendritic and 150 ± 66 nm in interdendritic regions while maintaining complete crosslinking. Furthermore, our investigations show that unfavorable stress and temperature combinations favor undirectional coarsening before the rafting process is completed.

Etiology of recurrent large vessel occlusions treated with repeated thrombectomy

BACKGROUND

Repeated mechanical thrombectomy for acute stroke treatment in individual patients has been proven feasible. However, less is known about the etiology of recurrent vessel occlusions after prior thrombectomy. We aimed to understand if the etiology of such recurrent events differs from the first stroke.

METHODS

Retrospectively, we identified all patients at our center who received a repeated mechanical thrombectomy between 2007 and 2019. Clinical data were retrieved from medical records. Etiology of stroke was evaluated retrospectively, and angiographic studies were revisited.

RESULTS

Twenty-three patients (1.5%) were identified. Median age was 68 years (IQR 56-77). Median NIHSS at first admission was 11 points (IQR 5-15). In nine cases (39.1%), the recurrent vessel occlusion was located exactly at the same position as the prior occlusion. Overall, five (21.7%) patients had a remarkable extracranial pathology as likely cause of stroke recurrence. In 16 patients (69.6%), the etiology of the first stroke and its recurrence was considered as likely being the same, mostly of cardioembolic or unknown origin. In the seven remaining patients (30.4%), the cause of stroke possibly differed from the first event, with five patients (21.7%) having a postinterventional intracranial intimal lesion as possible cause of stroke.

CONCLUSION

Incidence of repeated thrombectomy was low. However, the high number of patients with known origin of stroke etiology raises the question how their monitoring may be optimized. The number of patients with remarkable extracranial pathologies or intracranial endothelial lesions supports current clinical practice to pay attention to final angiographic series.

Stability of TaC precipitates in a Co–Re-based alloy being developed for ultra-high-temperature applications

Co–Re alloys are being developed for ultra-high-temperature applications to supplement Ni-based superalloys in future gas turbines. The main goal of the alloy development is to increase the maximum service temperature of the alloy beyond 1473 K,i.e.at least 100 K more than the present single-crystal Ni-based superalloy turbine blades. Co–Re alloys are strengthened by carbide phases, particularly the monocarbide of Ta. The binary TaC phase is stable at very high temperatures, much greater than the melting temperature of superalloys and Co–Re alloys. However, its stability within the Co–Re–Cr system has never been studied systematically. In this study an alloy with the composition Co–17Re–23Cr–1.2Ta–2.6C was investigated using complementary methods of small-angle neutron scattering (SANS), scanning electron microscopy, X-ray diffraction and neutron diffraction. Samples heat treated externally and samples heatedin situduring diffraction experiments exhibited stable TaC precipitates at temperatures up to 1573 K. The size and volume fraction of fine TaC precipitates (up to 100 nm) were characterized at high temperatures within situSANS measurements. Moreover, SANS was used to monitor precipitate formation during cooling from high temperatures. When the alloy is heated the matrix undergoes an allotropic phase transformation from the ∊ phase (hexagonal close-packed) to the γ phase (face-centred cubic), and the influence on the strengthening TaC precipitates was also studied within situSANS. The results show that the TaC phase is stable and at these high temperatures the precipitates coarsen but still remain. This makes the TaC precipitates attractive and the Co–Re alloys a promising candidate for high-temperature application.

Stress evolution in thermal barrier coatings for rocket engine applications

Background

Thermal barrier coatings are a promising concept to improve the lifetime of the copper liner of a rocket engine. Due to the high heat fluxes and the large thermal conductivity of copper, coatings have to be designed especially for this application.

Methods

In this paper, we perform fully thermo-mechanically coupled finite element analyses of a small section of a combustion chamber with a coating system comprising a NiCuCrAl bond coat and a NiCrAlY top coat.

Results

Heat fluxes are calculated to determine reasonable coating thickness values. Elastic and plastic deformation in the materials is considered to study the stress evolution. A crack model serves to estimate the possibility of vertical cracks propagating through the coating system.

Conclusions

Several design guidelines are developed from these results that will aid future development of thermal barrier coatings.

Neutron and synchrotron probes in the development of Co–Re-based alloys for next generation gas turbines with an emphasis on the influence of boron additives

Nickel-based superalloys are the materials of choice in the hot section of current gas turbines, but they are reaching temperature limits constrained by their melting temperature range. Co–Re alloy development was prompted by a search for new materials for future gas turbines, where the temperature of application will be considerably higher. Addition of the very high melting point refractory metal Re to Co can increase the melting range of Co alloys to much higher temperatures than the commercial Co alloys in use today. The alloy development strategy is first discussed very briefly. In this program, model ternary and quaternary compositions were studied in order to develop a basic understanding of the alloy system.In situneutron and synchrotron measurements (small and wide angle) at high temperatures were extensively used for this purpose and some selected results from thein situmeasurements are presented. In particular, the effect of boron doping in Co–Re–Cr alloys and the stability of the TaC precipitates at high temperatures were investigated. A fine dispersion of TaC precipitates strengthens some Co–Re alloys, and their stability at the application temperature is critical for the long-term creep properties.

Nanoforging - Innovation in three-dimensional processing and shaping of nanoscaled structures

BACKGROUND

This paper describes the shaping of freestanding objects out of metallic structures in the nano- and submicron size. The technique used, called nanoforging, is very similar to the macroscopic forging process.

RESULTS

With spring actuated tools produced by focused ion beam milling, controlled forging is demonstrated. With only three steps, a conical bar stock is transformed to a flat- and semicircular bent bar stock.

CONCLUSION

Compared with other forming techniques in the reduced scale, nanoforging represents a beneficial approach in forming freestanding metallic structures, due to its simplicity, and supplements other forming techniques.

Sequential decisions on FAS sequencing guided by biomarkers in patients with lymphoproliferation and autoimmune cytopenia

Clinical and genetic heterogeneity renders confirmation or exclusion of autoimmune lymphoproliferative syndrome difficult. To re-evaluate and improve the currently suggested diagnostic approach to patients with suspected FAS mutation, the most frequent cause of autoimmune lymphoproliferative syndrome, we prospectively determined 11 biomarkers in 163 patients with splenomegaly or lymphadenopathy and presumed or proven autoimmune cytopenia(s). Among 98 patients sequenced for FAS mutations in CD3(+)TCRα/β(+)CD4(-)CD8(-) "double negative" T cells, 32 had germline and six had somatic FAS mutations. The best a priori predictor of FAS mutations was the combination of vitamin B12 and soluble FAS ligand (cut-offs 1255 pg/mL and 559 pg/mL, respectively), which had a positive predictive value of 92% and a negative predictive value of 97%. We used these data to develop a web-based probability calculator for FAS mutations using the three most discriminatory biomarkers (vitamin B12, soluble FAS ligand, interleukin-10) of the 11 tested. Since more than 60% of patients with lymphoproliferation and autoimmune cytopenia(s) in our cohort did not harbor FAS mutations, 15% had somatic FAS mutations, and the predictive value of double-negative T-cell values was rather low (positive and negative predictive values of 61% and 77%, respectively), we argue that the previously suggested diagnostic algorithm based on determination of double-negative T cells and germline FAS sequencing, followed by biomarker analysis, is not efficient. We propose vitamin B12 and soluble FAS ligand assessment as the initial diagnostic step with subsequent decision on FAS sequencing supported by a probability-calculating tool.

The High Temperature Behaviour of TiAl Containing Carbide Reinforcements

The mechanical behaviour of γ-TiAl reinforced with Ti2AIC platelets is investigated in the temperature range between 293K and 1255K. Comparison with the particle-free material of same matrix composition reveals a pronounced strengthening effect of the platelike reinforcements up to about 1000K. However, under creep loading the composite is remarkably weak. It is shown that this weakness is caused by diffusional creep along the particle/matrix interface. Modelling of this process indicates that the dimensions of the Ti2AlC particles (thickness d=lμm, length l=20μm) are too small to avoid this unwanted deformation mechanism. It is concluded that thicknesses well above 10±m are needed to maintain reinforcement strengthening at high temperature in this system.

Nanoporous membranes produced from polycrystalline Ni-based superalloys

Porous metal membranes are produced from a two phase system in which the discrete cubic gamma'-precipitates connect during self assembly. In the so called rafting process the cubic particles start to coarsen and finally create a network within the gamma-matrix. In a following electrochemical leaching process one of the phases can be removed leaving the nanoporous membrane. So far, single crystalline alloys have been used for producing thin nanoporous membranes. Now research is in progress to produce the nanoporous membranes from polycrystalline alloys in a creep process. A modification of the commercially available alloy Nimonic 115 was used for these membranes. The permeability of these membranes was proven in a gas-flow test.

Coating of meso-porous metallic membranes with oriented channel-like fine pores by pulsed laser deposition

There is increasing demand to functionalize meso- and nanoporous materials by coating and make the porous substrate biocompatible or environmentally friendly. However, coating on a meso-porous substrate poses great challenges, especially if the pore aspect ratio is high. We adopted the pulsed laser deposition (PLD) method to coat Ni(3)Al-based meso-porous membranes, which were fabricated from a single-crystal Ni-based superalloy by a unique selective phase dissolution technique. These membranes were about 250 µm thick and had channel-like pores (∼200 nm wide) with very high aspect ratio. Two different coating materials, i.e. diamond-like carbon (DLC) and titanium, were used to coat these membranes. High energy C or Ti ions, produced in the plasma plume by the PLD process, penetrated the channel-like pores and deposited coatings on the pore walls deep inside the membrane. The thickness and the quality of coatings on the pore walls were examined using the dual-beam system. The coating thickness, of the order of 50 nm, was adherent to the pore walls and was quite uniform at different depths. The carbon and the Ti deposition behaved quite similarly. The preliminary experiments showed that the PLD is an adequate method for coating fine open cavities of complex geometry. Simulations based on stopping and the range of ions in matter (SRIM) calculations helped in understanding the deposition processes on pore walls at great depths.

Nano-structured materials produced from simple metallic alloys by phase separation

A method which is able to produce different types of nano-structured materials, namely nano-particles, nano-structured surfaces and nano-porous membranes, from two-phase metallic alloys is reviewed. The new process first establishes nano-structures in the bulk alloy and then separates them by selective phase dissolution. Variation in processing makes it possible to produce different types of nano-structure even from the same alloy. The process can be applied to many different alloy systems. An overview is presented emphasizing the versatility of the process with examples of different nano-structure types that can be produced. Further, the new method is discussed in relation to similar processes (specifically, electrochemical processes) which have been used for nano-structure synthesis.

Interstitial pulmonary fibrosis after severe exposure to welding fumes

BACKGROUND

Interstitial pulmonary fibrosis (IPF) is reported after long term, severe exposure to welding fumes in poorly ventilated workplaces.

METHODS

Fifteen welders with IPF were examined--13 in our outpatient clinic--from 1990 to 1997. Occupational histories and examinations, lung function analyses, symptoms and clinical findings, histological analyses in 13 patients partly including SEM/EDX-analyses, chest X-rays, chest computed tomographies were conducted.

RESULTS

Duration of work as welders was 28 years and the cumulative dose of welding fumes 221 mg/m(3) x years (median). Lung function studies found pattern of restriction or combined restriction-obstruction, lower diffusion capacity, and reduced blood oxygen tension at exercise. Histologically, patchy interstitial fibrosis was noted. Accumulations of particulate matter typically for welding fume were detected. EDX showed increase of iron load and close topographical relationship to welding fume particles embedded in areas of scattered fibrosis.

CONCLUSION

While epidemiological data are limited, it is reasonable to conclude that a causal relationship exists between IPF in welders with long term exposure to high concentrations of welding fumes.

The effect of anatomical positional relationships on kinetic parameters after total hip replacement

The purpose of this study was to evaluate the significance of positional relationships of the components on gait after total hip replacement (THR). Gait was analysed using multicomponent force platforms integrated into a treadmill, in conjunction with an optoelectric measuring system. 26 patients, after undergoing total hip replacement, were classified according to the vertical and horizontal positions of the centre of rotation, the vertical position of the femur and the functional leg length, and compared with a control group. Deterioration in parameters of gait was observed following cranialisation of the centre of rotation or of the femur, whereas neither medial movement of the centre of rotation nor leg lengthening by up to 1 cm had any effect. This study emphasises the importance of considering, at the time of preoperative planning, the effect of the position of the prosthesis on the functional parameters of gait.

Cloning of a cDNA encoding a 3-dehydroquinate synthase from a higher plant, and analysis of the organ-specific and elicitor-induced expression of the corresponding gene

cDNA clones for all enzymes of the prechorismate pathway of higher plants have previously been cloned, with the exception of the second enzyme of the pathway, 3-dehydroquinate synthase. Here we describe the isolation of a cDNA encoding a 3-dehydroquinate synthase from tomato which was identified by complementing a 3-dehydroquinate synthase-deficient Escherichia coli strain with a tomato cDNA library. The deduced amino acid sequence contains a putative N-terminal plastid-specific transit peptide, and the sequence of the mature enzyme resembles those of the corresponding bacterial enzymes more than of the fungal enzymes. Sequence identity was even higher between the tomato and E. coli sequences than between the E. coli and other known bacterial sequences. The abundance of 3-dehydroquinate synthase transcripts differ in the organs of tomato plants analyzed. In cultured tomato cells, the abundance of 3-dehydroquinate synthase transcripts increased 9-fold within 4 to 5 h of elicitor treatment.