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.
How do engineering materials deform when bearing mechanical loads? To answer this crucial question, the book bridges the gap between continuum mechanics and materials science. The different kinds of ...material deformation are explained in detail.
Titanium alloys are ideally suited for use in implant or osteosynthesis applications due to their good mechanical properties, corrosion resistance, and biocompatibility. In terms of higher strength ...applications, Ti–6Al–4V and Ti–6Al–7Nb are frequently used. However, both alloys contain the alloying elements aluminum and, in the former case, vanadium, which could have toxic effects on the human body. Therefore, in the present study, two novel, medium- to high-strength titanium alloys, Ti–0.44O–0.5Fe–0.08C–0.4Si–0.1Au and Ti–0.44O–0.5Fe–0.08C–2.0Mo, have been developed on the basis of CP-Titanium Grade 4. They only contain alloying elements, which are either already present in the human body or which are biocompatible. Dedicated thermo-mechanical treatments were developed for both alloys and the resulting mechanical properties were evaluated by tensile and (partly) fatigue tests with subsequent fracture surface analysis. Results reveal that these new alloys show excellent mechanical properties and, therefore, might be a possible alternative for Ti–6Al–4V for use in medical applications.
Graphical abstract
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.
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 3
-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.
By coarsening of the γ’-precipitates and selective extraction of one of the two existing phases, porous structures can be produced from nickel-based superalloys. There are two basic approaches to ...achieve a bicontinuous γ/γ’-microstructure—directional and incoherent coarsening. Single crystalline superalloy membranes are produced by the so-called rafting of the microstructure, i.e., directional coarsening. Unlike this process, incoherently coarsened membranes lack a detailed understanding of the mechanisms leading to cross-linking of the precipitates. In this paper, the growth and coalescence of precipitates during initial slow cooling from above the γ’ solvus temperature was studied. In addition to the three-dimensional morphological changes of the precipitates, it is also shown that only little coalescence of the particles occurs despite the high γ’ content and, therefore, their very small distance. The loss of coherency that occurs during this part of coarsening must first advance through further aging before a bicontinuous microstructure is formed.
Colloidal emulsions for lipophilic drugs can be fabricated using premix membrane emulsification. The state of the art is the application of membranes made from, for example, polycarbonate or ...polyester, which, however, are prone to fouling and cause waste, due to the low number of cycles. With the use of metallic membranes made from the nickel based single crystalline superalloy CMSX-4, these key disadvantages are eliminated. However, instead, the pore size and the resulting droplet size distribution need to be adjusted and improved. This can be realized by tailoring the size of the γ'-particles, which is controllable by the time and temperature used during precipitation heat treatment and the quenching method after homogenization heat treatment. Therefore, we utilized different heat treatment protocols, varying the cooling rate (water quenching and air cooling) after homogenization heat treatment and the holding time and temperature during precipitation heat treatment. Then, we investigated the γ/γ'-microstructure, including the γ'-morphology and γ'-particle size. We show that water quenching has a significant impact on the γ/γ'-microstructure and often leads to irregular-shaped and poorly aligned γ'-particles after precipitation heat treatment. In comparison, air cooling, followed by a subsequent precipitation heat treatment, results in well-aligned and cubic shaped γ'-particles and is, therefore, favorable for membrane fabrication. A reduction in precipitation temperature leads to morphology changes to the γ'-particles. A reduction of the holding time during precipitation heat treatment diminishes the γ'-particle growth, resulting in smaller γ'-particles. Additionally, a suitable heat treatment protocol for membrane fabrication was identified with a γ'-edge length of 224 ± 52 nm and well-aligned, cubic shaped γ'-particles.
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/mm
2
, 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 Y
2
O
3
formation are necessary.
In the framework of the CRC 880 “Fundamentals of high-lift for future civil air craft” methods for the reduction of aircraft noise are investigated. An important method for this noise reduction is ...the usage of porous material as low noise trailing edges. To improve the aeroacoustic properties of porous materials, an innovative rolling process was established by Tychsen et al. (Metals 8:598, 2018). Here, the rolling process is described as it is used as an important method for the production of samples. The influence of cold rolling on two different porous materials namely porous aluminum 80–110 (PA 80–110) and PA 120–150 is investigated. Important characteristics studied are the porosity, mechanical properties and the dependence of flow resistivity from the degree of deformation. The flow resistivity is of particular interest as the aeroacoustic performance is significantly influenced by it. The results are then compared to the findings for PA 200–250, which was investigated in Tychsen et al. (Metals 8:598, 2018). Lastly, experimental trailing edges made out of cold rolled porous aluminum with a gradient in thickness reduction are shown. The characterization of the aeroacoustic behavior is not part of this study. Reference is made to Rossignol et al. (Int J Aeroacoust 19:365–384, 2020), where trailing edges shown here are characterized aeroacoustically. The findings shown here demonstrate that different porous materials can be tailored by cold rolling without negative impact on the mechanical behavior. It is proven that the new rolling process is a versatile tool for the production of gradient porous material.
Nickel-base superalloys such as VDM 780 may possess a high content of Cr and Co. This influences solution energies of phase-forming elements such as Al and Ta (γ′-phase), Nb (γ″- and δ-phase), and Ti ...(η-phase). We perform density functional theory studies of a nickel matrix at 0 K with high concentrations of either Co and Cr and calculate the influence of these elements on solution energies. In the case of Co, the solution energy can be predicted well by the nearest-neighbor interaction in the Co-rich matrix. For Cr, the effect is more complicated because Cr has a larger ionic radius and changes the magnetic state of the material. The effect of a Cr-rich matrix on the energy of Co is dominated by magnetic effects and interactions with the other elements by elastic deformation of the lattice. A high content of Co or Cr will thus increase the solvus temperature of the strengthening phase in nickel-base superalloys, in agreement with the literature and thermodynamic calculations.
PDF
