Microstructural stability of Co-Re-Cr-Ta-C alloy strengthened by TaC precipitates

It is becoming increasingly clear that new materials that can operate at substantially higher temperatures than Ni-base superalloys are needed for future gas turbines. High melting Co-Re-Cr based alloy strengthened by carbides, particularly the MC type carbide, shows promise [1]. A fine dispersion of globular TaC precipitate is exploited for this purpose. Additionally Cr, which is mainly added to improve oxidation resistance, also stabilizes lamellar M23C6 type Cr carbide. The microstructure of a Co-Re-Cr-Ta-C alloy with the two types of carbides is seen in Fig. 1. Please click Additional Files below to see the full abstract

Antibiotic Conjugates with an Artificial MECAM-Based Siderophore Are Potent Agents against Gram-Positive and Gram-Negative Bacterial Pathogens

The development of novel drugs against Gram-negative bacteria represents an urgent medical need. To overcome their outer cell membrane, we synthesized conjugates of antibiotics and artificial siderophores based on the MECAM core, which are imported by bacterial iron uptake systems. Structures, spin states, and iron binding properties were predicted in silico using density functional theory. The capability of MECAM to function as an effective artificial siderophore in Escherichia coli was proven in microbiological growth recovery and bioanalytical assays. Following a linker optimization focused on transport efficiency, five β-lactam and one daptomycin conjugates were prepared. The most potent conjugate 27 showed growth inhibition of Gram-positive and Gram-negative multidrug-resistant pathogens at nanomolar concentrations. The uptake pathway of MECAMs was deciphered by knockout mutants and highlighted the relevance of FepA, CirA, and Fiu. Resistance against 27 was mediated by a mutation in the gene encoding ExbB, which is involved in siderophore transport

Additional Phases at High Boron Content in High-Temperature Co–Re–Cr Alloys

Boron largely increases the ductility of polycrystalline high-temperature Co–Re–Cr alloys. Therefore, the effect of boron addition on the alloy structural characteristics is of large importance for the stability of the alloy at operational temperatures. Along with the Co-solid solution matrix phase transformation from hcp to fcc structure, additional structural effects were observed in situ at very high temperatures (up to 1500 °C) using neutron diffraction (ND) in boron-containing Co–17Re–23Cr alloys. Increasing boron content up to 1000 wt. ppm lowers the temperature at which sublimation of Co and Cr from the matrix occurs. As a result, the composition of the matrix in the surface region is changed leading to the formation of a second and a third matrix hcp phases at high temperatures. The consideration on the lattice parameter dependence on composition was used to identify the new phases appearing at high temperatures. Energy-dispersive spectroscopy and ND results were used to estimate the amount of Co and Cr which sublimated from the surface region of the high-boron sample. In the sense of alloy development, the sublimation of Co and Cr is not critical as the temperature range where it is observed (≥1430 °C) is significantly above the foreseen operation temperature of the alloys (1200 °C)

Creep deformation of Co-Re-Ta-C alloys with varying C content – investigated in-situ by simultaneous synchrotron radiation diffraction

The creep deformation of precipitation hardened Co-Re-Ta-C alloys is investigated during in-situ synchrotron diffraction experiment at 1373 K. At room temperature, the alloys have a structure consisting of ϵϵ-Co (hcp) and metastably retained γγ-Co (fcc) and are strengthened by precipitates of the mono-carbide of Ta, which are finely dispersed in the alloy matrix. The alloy exhibits an allotropic ϵ→γϵ→γ-Co phase transformation when heating to View the MathML source>1173K. A lower C content in the alloy generally promotes this transformation. It is shown that this transformation is strongly influenced by application of compressive load. The transformation ϵ→γϵ→γ-Co at high temperature under load leads to microstructure refinement and subsequently to dissolution of hardening precipitates. This results in a considerable acceleration of the creep rate. Further, the equilibrium ratio of γγ/ϵϵ-Co phase is significantly altered under compressive load. This behavior is attributed to a volume relaxation as the ϵϵ- and γγ-Co phase have different unit cell volumes

TaC Precipitation Kinetics During Cooling of Co−Re‐Based Alloys

Cobalt−rhenium (Co−Re) alloys are developed for high-temperature applications at ≈1200 °C and are strengthened by dispersion of nanosized tantalum carbide (TaC) precipitates. Herein, the precipitation behavior during cooling from supersolution depending on the cooling rate and the addition of chromium is presented. The phase composition (matrix phases and TaC) is analyzed from the wide-angle neutron diffraction patterns measured in situ during temperature cycling. The precipitation of nanosized TaC particles is measured by in situ and ex situ neutron and X-ray small-angle scattering. The in situ measurements are used to extract the temperature-dependent volume fraction of the precipitates; the final size distribution after cooling is extracted from the ex situ measurements. A Kampmann−Wagner's numerical (KWN) model is adapted to isochronal cooling processes. The in situ measurements give the unique possibility to calibrate the model parameters, whereas the ex situ measurements are used to assess the model predictions