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The microstructural development of a forged Ti-43Al-4Nb-1Mo-0.1B (in at%) alloy during two-step heat-treatments was investigated and its impact on the tensile ductility at room temperature was analyzed. The investigated material, a so-called TNM™ gamma alloy, solidifies via the β-route, exhibits an adjustable β/B2-phase volume fraction and can be forged under near conventional conditions. Post-forging heat-treatments can be applied to achieve moderate to near zero volume fractions of β/B2-phase allowing for a controlled adjustment of the mechanical properties. The first step of the heat-treatment minimizes the β/B2-phase and adjusts the size of the α-grains, which are a precursor to the lamellar γ/α2-colonies. However, due to air cooling after the first annealing step, the resulting microstructure is far from thermodynamic equilibrium. Therefore, a second heat-treatment step is conducted below the eutectoid temperature which brings the microstructural constituents closer to thermodynamic equilibrium. It was found that temperature and duration of the second heat-treatment step critically affect the solid-state phase transformations and, thus, control the plastic fracture strain at room temperature. Scanning and transmission electron microscopy studies as well as hardness tests have been conducted to characterize the multi-phase microstructure and to study its correlation to the observed room temperature ductility.
TNM™ alloys are novel γ-TiAl based alloys which exhibit a high concentration of β-stabilizing elements such as Nb and Mo. Due to the high volume fraction of disordered β-phase these alloys can be hot-die forged under near conventional conditions. In this study, solid-state phase transformations and phase transition temperatures in Ti-(41-45)Al-4Nb-1Mo-0.1B (in at%) alloys were analyzed experimentally and compared to thermodynamic calculations. Results from scanning electron microscopy, conventional and high-energy X-ray diffraction as well as differential scanning calorimetry were used for the characterization of the prevailing phases and phase transformations. For the prediction of phase stabilities and phase transition temperatures thermodynamic calculations were conducted. ThermoCalc® was applied using a commercially available TiAl database. Combining all results a stable as well as a metastable phase diagram for Ti-(41-45)Al-4Nb-1Mo-0.1B alloys is proposed.
The effect of transportation and lairage on the faecal shedding and post-slaughter contamination of carcasses with Escherichia coli O157 and O26 in young calves (4–7-day-old) was assessed in a cohort study at a regional calf-processing plant in the North Island of New Zealand, following 60 calves as cohorts from six dairy farms to slaughter. Multiple samples from each animal at pre-slaughter (recto-anal mucosal swab) and carcass at post-slaughter (sponge swab) were collected and screened using real-time PCR and culture isolation methods for the presence of E. coli O157 and O26 (Shiga toxin-producing E. coli (STEC) and non-STEC). Genotype analysis of E. coli O157 and O26 isolates provided little evidence of faecal–oral transmission of infection between calves during transportation and lairage. Increased cross-contamination of hides and carcasses with E. coli O157 and O26 between co-transported calves was confirmed at pre-hide removal and post-evisceration stages but not at pre-boning (at the end of dressing prior to chilling), indicating that good hygiene practices and application of an approved intervention effectively controlled carcass contamination. This study was the first of its kind to assess the impact of transportation and lairage on the faecal carriage and post-harvest contamination of carcasses with E. coli O157 and O26 in very young calves.
We report some preliminary results from the Massachusetts - Stony Brook CO survey of the first galactic quadrant using the 14-meter millimeterwave telescope of the Five College Radio Astronomy Observatory. The survey contains approximately 50 000 observations spaced every 3 arcminutes in 1 and b between longitudes 0° and 90° and latitudes −1° and 1°. We have mapped emission from giant molecular clouds (GMC) which we identified, in earlier more limited strip surveys of the galactic plane (Solomon, Sanders and Scoville 1979; Sanders 1981), on size scales from a few parsecs to hundreds of parsecs, in order to determine the degree of clustering and organization into large-scale features. In addition to the characteristic size of 20 – 60 pc for individual GMC, we find clustering of clouds on a scale of from 100 to 300 pc.
The primate malaria Plasmodium knowlesi has a long-standing history as an experimental malaria model. Studies using this model parasite in combination with its various natural and experimental non-human primate hosts have led to important advances in vaccine development and in our understanding of malaria invasion, immunology and parasite–host interactions. The adaptation to long-term in vitro continuous blood stage culture in rhesus monkey, Macaca fascicularis and human red blood cells, as well as the development of various transfection methodologies has resulted in a highly versatile experimental malaria model, further increasing the potential of what was already a very powerful model. The growing evidence that P. knowlesi is an important human zoonosis in South-East Asia has added relevance to former and future studies of this parasite species.
We present recent results from mid-infrared observations of a sample of nearby, infrared luminous starbursts and AGN made with the new mid-infrared instrument, “Michelle”, on UKIRT. Narrow band imaging in the 7-13 micron range with sub-arcsec resolution has been used to study the spatial distribution of polycyclic aromatic hydrocarbon (PAH) features. The comparison of these sub-arcsecond resolution data with radio continuum data at similar resolution can be used to determine, 1) the sources of excitation required for PAH emission, and 2) whether PAH features are a measure of the relative contribution of star formation and AGN to the bolometric energy output of a galaxy. Unlike the far-infrared emission from dust, that in the mid-infrared can be used to discriminate between different heating sources.
The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K-11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100-1000 seconds.
A subsample of the ZZ Ceti stars shows amplitude variations on time scales of the order of one month. These variations could be driven by nonlinear phenomena.
We report the analysis of 154 hours of nearly continuous high-speed photometric data on the pulsating DB white dwarf (DBV) GD 358 obtained during the Whole Earth Telescope (WET) run of May 1990. The Fourier transform (FT) of the light curve is dominated by power in the range from 1200 – 1700μHz with more than 180 significant peaks in the total transform. We also see significant power at the sums and differences of the dominant frequencies, indicating the importance of nonlinear behavior. We can use this data to obtain an accurate total stellar mass, and surface He layer mass. The implied surface He layer mass, if correct, provides a significant and surprising challenge to stellar evolution theory, as well as the theory of chemical mixing.
White dwarf stars provide important boundary conditions for the understanding of stellar evolution. An adequate understanding of even these simple stars is impossible without detailed knowledge of their interiors. PG1346+082, an interacting binary white dwarf system, provides a unique opportunity to view the interior of one degenerate as it is brought to light in the accretion disk of the second star as the primary strips material from its less massive companion (see Wood et at. 1987).
PG1346+082 is a photometric variable with a four magnitude variation over a four to five day quasi-period. A fast Fourier transform (FFT) of the light curve shows a complex, time-dependent structure of harmonics. PG1346+082 exhibits flickering – the signature of mass transfer. The optical spectra of the system contain weak emission features during minimum and broad absorption at all other times. This could be attributed to pressure broadening in the atmosphere of a compact object, or to a combination of pressure broadening and doppler broadening in a disk surrounding the compact accretor. No hydrogen lines are observed and the spectra are dominated by neutral helium. The spectra also display variable asymmetric line profiles.
The variability of CD-24 7599 (V=11.48 mag) was discovered by JCC during observing run XCOV7 of the Whole Earth Telescope (WET, Nather et al. 1990) network in February, 1992. The star was observed as an additional target and 117 hours of high-quality temporal spectroscopic observations were obtained.
Our analysis of these data revealed the presence of 7 independent pulsation modes between 27.0 and 38.1 cycles per day (313 – 441 μHz) with semiamplitudes of 2.1 – 10.2 milli-modulation amplitudes (mma). We showed that peaks at linear combination frequencies detected in the power spectra were not due to eigenmodes excited to visible amplitude by resonant mode coupling.
Intermetallic titanium aluminides solidifying via the disordered β-phase are of great interest for several high-temperature applications in automotive and aircraft industries. In this paper the thermocyclic oxidation behavior of three β-solidifying γ-TiAl-based alloys at 800°C and 900°C in air, with and without fluorine treatment, is reported for the first time. The behavior of the well-known TNM alloy (Ti-43.5Al-4Nb-1Mo-0.1B, in at.%) is compared with that of two Nb-free model alloys which contain different amounts of Mo (Ti-44Al-3Mo and Ti-44Al-7Mo, in at.%). During thermocyclic high-temperature exposure in air a mixed oxide scale develops on all three untreated alloys. Small additions of fluorine in the subsurface region of the alloys change the oxidation mechanism from mixed oxide scale formation to alumina at both temperatures. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated samples.
The high cost of single-crystal III–V substrates limits the use of gallium arsenide (GaAs) and related sphalerite III–V materials in many applications, especially photovoltaics. However, by making devices from epitaxially grown III–V layers that are separated from a growth substrate, one can recycle the growth substrate to reduce costs. Here, we show damage-free removal of an epitaxial single-crystal GaAs film from its GaAs growth substrate using a laser that is absorbed by a smaller band gap, pseudomorphic indium gallium arsenide nitride layer grown between the substrate and the GaAs film. The liftoff process transfers the GaAs film to a flexible polymer substrate, and the transferred GaAs layer is indistinguishable in structural quality from its growth substrate.
The ternary system Fe - 25 at% Co - 9 at% Mo shows an age hardening behavior similar to aluminum alloys. After solution annealing followed by rapid quenching, the Fe-Co-matrix is hardened during subsequent aging through precipitation of the intermetallic µ-phase (Fe,Co)7Mo6. In aged condition the entire Mo content is present in coarse primary and fine µ-phase particles and, therefore, the matrix consists exclusively of 71 at% Fe and 29 at% Co. The binary system Fe-Co shows a transformation from the disordered bcc structure to the ordered B2 structure between 25 and 72 at% Co at a critical ordering temperature ranging from room temperature to 723°C. As a consequence, the remaining overaged matrix in the Fe - 25 at% Co - 9 at% Mo system should also show such a transition. However, an ordered phase is brittle and, thus, not wanted for many applications. Better mechanical properties in terms of ductility can be achieved with a partially or fully disordered phase. Such a state can be obtained by rapid quenching from temperatures above the critical ordering temperature. In this study such an approach was implemented on the ternary Fe - 25 at% Co - 9 at% Mo alloy. The effect of different cooling rates on the mechanical properties was investigated by means of hardness testing. The actual ordering transition of the Fe - 29 at% Co matrix was determined with differential scanning calorimetry and neutron diffraction.
Ensuring microstructural stability under technical relevant conditions is a determining criterion for the development of innovative high-temperature materials. In this work, the influ-ence of C and Si on the microstructural stability during creep exposure was investigated for a β-solidifying γ-TiAl based alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%), named TNM. With a two-step heat treatment a microstructure consisting of fine lamellar α2/γ-colonies, surrounded by βo-phase and areas of discontinuous precipitation, starting from the boundaries of the lamellar colonies, was adjusted. Creep tests were carried out to examine the potential of C and Si to prevent microstructural instability during creep and hence improving the creep properties. At 815 °C the discontinuous precipitation process of the TNM alloy continues during ensuing creep testing leading to a reduced creep resistance. In comparison, the minimum creep rate of the TNM-0.3C-0.3Si alloy was significantly decreased caused by the lower βo-phase content and average lamellar spacing within the α2/γ-colonies, the precipitation of p-Ti3AlC carbides and the retarded kinetics of discontinuous precipitation.
The distribution of alloying elements in the constituent phases of a C-containing γ-TiAl based alloy has been characterized locally by atom probe tomography. The major elements of the alloy under consideration – Ti, Al, Nb, and Mo – are distributed uniformly within each of the constituent phases. Furthermore, Mo is preferentially dissolved in the βo-phase, whereas Nb content is similar in all phases. The selected C concentration of the alloy is below the overall solubility limit as no precipitates have been observed. Therefore, C is enriched in the α2-phase, whereas the βo-phase is depleted of C. In addition, βo/γ-interfaces have been prepared by site specific sample preparation and characterized by atom probe tomography. Segregation of Mo and C into the interfaces and their close vicinity was observed.
Grain boundaries (GBs) in polycrystalline silicon (poly-Si) thin film solar cells are frequently found to be detrimental for device performance. Biaxiallytextured silicon with grains that are well-aligned in-plane and out-of-plane can possess fewer GB defects. In this work, we use TCAD Sentaurus device simulator and known experimental work to investigate and quantify the potential performance gains of biaxially-textured silicon. Simulation shows there can be performance gain from well-aligned grains when GB defects dominate carrier recombination or when grains are small. On the other hand, when intra-grain defects dominate recombination and grains are large, well-aligned grains do not lead to much performance gain. Another important result from our simulation is when intra-grain and GB defects are few, Jsc is almost independent of grain size while Voc drops with decreasing grain size.
Thin-film absorber layers for photovoltaics have attracted much attention for their potential for low cost per unit power generation, due both to reduced material consumption and to higher tolerance for defects such as grain boundaries. Cu2ZnGeSe4 (CZGSe) comprises one such material system which has a near-optimal direct band gap of 1.6 eV for absorption of the solar spectrum, and is made primarily from earth-abundant elements.
CZGSe metallic precursor films were sputtered from Cu, Zn, and Ge onto Mo-coated soda lime glass substrates. These were then selenized in a two-zone close-space sublimation furnace using elemental Se as the source, with temperatures in the range of 400 to 500 C, and at a variety of background pressures. Films approximately 1-1.5 µm thick were obtained with the expected stannite crystal structure.
Next, Cu2ZnSnSe4 (CZTSe), which has a direct band gap of 1.0 eV, was prepared in a similar manner and combined with CZGSe as either compositionally homogeneous or layered absorbers. The compositional uniformity of selenide absorbers made by selenizing compositionally homogeneous Cu-Zn-Ge-Sn precursor layers was determined and the band gap as a function of composition was investigated in order to demonstrate that the band gap is tuneable for a range of compositions. For layered Cu-Zn-Ge/Cu-Zn-Sn precursor films, the composition profile was measured before and after selenization to assess the stability of the layered structure, and its applicability for forming a band-gap-graded device for improved current collection.