The Influence of Cobalt on the Microstructure of the Nickel-Base Superalloy MAR-M247

The influence of cobalt on the microstructure of MAR-M247, a cast nickel-base superalloy, was investigated. Nickel was substituted for Co to produce 0, 5, and the standard 10 pct Co versions of MAR-M247. The microstructures of the alloys were examined using optical and electron microscopy, X-ray diffraction, phase extraction, and differential thermal analysis. Samples were examined in as-cast, heat treated, long-time aged, and stress-rupture tested conditions. As Co was removed from MAR-M247, the γ′ volume fraction decreased, the mean γ′ particle size increased, the W and Ti concentrations in the γ′ increased, the Cr and Al concentrations in the γ phase decreased, and the amount of carbides increased. This increase in carbide precipitation caused a change from discrete grain boundary carbides to a grain boundary film as Co level decreased. M. V. NATHAL, formerly of Case Western Reserve University, R. D. MAIER, formerly of Case Western Reserve University,     

Zur Kinetik der Sauerstoffreduktion bei der Korrosion von Nickel in sauerstoffhaltiger Schwefelsäure

Kinetics of oxygen reduction during the corrosion of nickel in oxygen containing sulfuric acid The rate of oxygen reduction was determined for the corrosion of nickel in sulphuric acid containing oxygen. The rate depends upon the oxygen concentration and the acidity of the electrolyte. For a second order reaction with a preceding diffusion of oxygen and protons the values of the rate constant and the thickness of the diffusion layer were calculated. It could be shown that in In sulphuric acid the reduction rate is control led by the oxygen diffusion while in the weaker acids the rate controlling step is a proton transfer to the oxygen, adsorbed at the metal surface. Measurements of the dependance of the corrosion in In sulphuric and hydrochloride acid confirm these results.     

Carbon Nanotubes for Microelectronics?

Despite all prophecies of its end, silicon-based microelectronics still follows Moore's Law and continues to develop rapidly. However, the inherent physical limits will eventually be reached. Carbon nanotubes offer the potential for further miniaturization as long as it is possible to selectively deposit them with defined properties.     

Time Horizon: Implications for Aggregate Scheduling Effectivness1

This performance of intuitive aggregate scheduling is compared with the scheduling performance of a mathematical model. A controlled experiment was designed to illustrate one scheduling problem characteristic which accounts for the superiority of model decisions over intuitive decisions. This relative superiority increases as the time-horizon complexity increases. The horizon complexity is a manifestation of the cost structure of the decision setting.     

The development of γ-γ′ lamellar structures in a nickel-base superalloy during elevated temperature mechanical testing

Changes in the morphology of the γ′ precipitate were examined during creep and tensile testing at temperatures between 927 and 1038 °C in [001]-oriented single crystals of a model Ni-Al-Mo-Ta superalloy. In this alloy, the γ′ particles link together to form lamellae, or rafts, which are aligned with their broad faces perpendicular to the applied tensile axis. The dimensions of the γ and γ′ phases were measured as the lamellar structure developed and were related to time and strain in an attempt to trace the changing γ-γ′ morphology. The results showed that directional coarsening of γ′ began during primary creep, and the attainment of a fully developed lamellar structure did not appear to be directly related to the onset of steady-state creep. The rate of directional coarsening during creep increased as the temperature was raised and also increased as the stress level was raised at a given testing temperature. The raft thickness remained equal to the initial γ′ size from the start of the creep test up through the onset of tertiary creep for all testing conditions. It was found that extensive rafts did not develop during the shorter testing times of the tensile tests, and that tensile testing of pre-rafted structures did not alter the morphology of the rafts. The overall behavior of the alloy was a clear indication of the stability of the finely-spaced γ-γ′ lamellar structure.     

Elevated temperature creep-rupture behavior of the single crystal nickel-base superalloy NASAIR 100

The creep and rupture behavior of [001] oriented single crystals of the nickel-base superalloy NASAIR 100 was investigated at temperatures of 925 and 1000 °C. In the stress and temperature ranges studied, the steady state creep rate, time to failure, time to the onset of secondary creep, and the time to the onset of tertiary creep all exhibited power law dependencies on the applied stress. The creep rate exponents for this alloy were between seven and eight, and the modulus-corrected activation energy for creep was approximately 350 kjoule/mole, which was comparable to the measured activa-tion energy for Ostwald ripening of the γ′ precipitates. Oriented γ′ coarsening to form lamellae perpendicular to the applied stress was very prominent during creep. At 1000 °C, the formation of a continuous γ-γ′ lamellar structure was completed during the primary creep stage. Shear through the γ-γ ' interface is considered to be the rate limiting step in the deformation process. Gradual thickening of the lamellae appeared to be the cause of the onset of tertiary creep. At 925 °C, the fully developed lamellar structure was not achieved until the secondary or tertiary creep stages. At this temperature, the γ-γ′ lamellar structure did not appear to be as beneficial for creep resistance as at the higher temperature.     

Metabolic Glycoengineering in hMSC-TERT as a Model for Skeletal Precursors by Using Modified Azide/Alkyne Monosaccharides

Metabolic glycoengineering enables a directed modification of cell surfaces by introducing target molecules to surface proteins displaying new features. Biochemical pathways involving glycans differ in dependence on the cell type; therefore, this technique should be tailored for the best results. We characterized metabolic glycoengineering in telomerase-immortalized human mesenchymal stromal cells (hMSC-TERT) as a model for primary hMSC, to investigate its applicability in TERT-modified cell lines. The metabolic incorporation of N-azidoacetylmannosamine (Ac₄ManNAz) and N-alkyneacetylmannosamine (Ac₄ManNAl) into the glycocalyx as a first step in the glycoengineering process revealed no adverse effects on cell viability or gene expression, and the in vitro multipotency (osteogenic and adipogenic differentiation potential) was maintained under these adapted culture conditions. In the second step, glycoengineered cells were modified with fluorescent dyes using Cu-mediated click chemistry. In these analyses, the two mannose derivatives showed superior incorporation efficiencies compared to glucose and galactose isomers. In time-dependent experiments, the incorporation of Ac₄ManNAz was detectable for up to six days while Ac₄ManNAl-derived metabolites were absent after two days. Taken together, these findings demonstrate the successful metabolic glycoengineering of immortalized hMSC resulting in transient cell surface modifications, and thus present a useful model to address different scientific questions regarding glycosylation processes in skeletal precursors.