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The metastable β-Ti alloy, Ti-5553, has been heat-treated to permit a determination of the role of the ω phase on non-classical precipitation of the α phase. Thus, samples containing ...the athermal ω phase have been up-quenched to 600°C. The observations made are consistent with the activation of the non-classical, pseudo-spinodal, nucleation mechanism, with the ω phase playing essentially no direct role. The α/β interfacial structures have been shown to exhibit significant degrees of coherency compared with those in conventional α/β alloys, and implications of this on lowering the interfacial energy, and so the activation for nucleation, are discussed.
A metastable nano-scale disordered precipitate with orthorhombic symmetry has been identified using high resolution scanning transmission electron microscopy. The phase, termed O′, is metastable, ...formed by a shuffle mechanism involving a 110<11̅0> transverse phonon wave in samples of Ti–26Nb–2Zr (at.%) quenched from the β phase. The addition of 2% Zr to Ti–26Nb appears to suppress significantly the stability of both the 112̅ shear and 2/3 longitudinal phonon wave but promotes the 110<11̅0> transverse shuffle. This results in the nano-size O′ phase being homogeneously formed in the parent β phase matrix rather than the massive α″ phase.
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Non-conventional transformation pathways have been exploited to determine the degree of refinement of the microstructure of a metastable β-Ti alloy. This has involved a set of heat-treatments where ...samples, initially quenched to room temperature (RT) from above the β transus are slowly heated to 350 °C, then isothermal annealed at 350 °C for various times prior to up-quenching to 600 °C and again isothermally annealed at that temperature, followed by quenching to RT. Interrupted treatments have also been performed so that the factors influencing the development of the final microstructure could be determined. Microstructures have been characterized by scanning and transmission electron microscopy, and atom probe tomography. It has been found that a distribution of the α phase may be formed whose scale is intermediate between the “super-refined” and “refined” versions observed in previous research. The results are consistent with precursory isothermal ω precipitates playing an indirect role in the formation of the refined distribution of the α phase via a non-conventional transformation pathway.
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A stable sol of cerium oxide nanoparticles forms spontaneously when cerium(iv) ammonium nitrate (CAN) is dissolved in room-temperature water at mM concentrations. Electron microscopy experiments ...reveal the formation of highly crystalline cerium oxide particles several nm in diameter and suggest that they are formed from amorphous particles that are similar in size. Under the low pH conditions of the experiments, the nanoparticles form a stable dispersion and show no evidence of aggregation, even many months after synthesis. The absence of particles large enough to scatter light significantly makes it possible to observe the crystallization kinetics through dramatic changes in the UV-visible absorption spectra that occur during solution aging. Measurements show that the cerium oxide nanocrystals are formed roughly an order of magnitude more slowly in D
O than in H
O solution. This large solvent kinetic isotope effect (k
/k
∼ 10), which is reported here for the first time for the crystallization of a solid metal oxide phase, indicates a rate-determining proton transfer reaction, which is assigned to the conversion of hydroxy to oxo bridges. In D
O solution, the absorption per mole of cerium ions increases by over 400% at 290 nm as the weakly absorbing precursor phase is transformed into nanocrystalline cerium oxide. An isosbestic point is detected at 368 nm, and the absorption spectra can be modeled throughout aging by the sum of spectra of just two interconverting species. Preliminary ultrafast transient absorption experiments confirm that the optical properties of the amorphous precursors differ greatly from those of the final, nanocrystalline phase. Crystallization of CeO
from CAN in water has much in common with the crystallization of iron oxides from iron(iii) salts, including the importance of non-classical nucleation and growth pathways. It is an outstanding system for studying the poorly understood events that cause molecularly solvated ions to self-assemble into nanocrystals, following hydrolysis. At the same time, the strong susceptibility of CAN to spontaneously form CeO
nanocrystals under the mildest of reaction conditions indicates that caution is needed when working with this common sacrificial oxidant.
A previously unidentified, nanometer scale ordered precipitate has been characterized in the metastable β titanium alloy Ti-5Al-5Mo-5V-3Cr (Ti-5553) using conventional transmission electron ...microscopy (TEM) and high-angle annular dark field (HAADF) high-resolution scanning transmission electron microscopy (HRSTEM). The phase, termed Oʺ, is metastable, the ordering transformation occurring over an intermediate temperature range (e.g., 375°C), and this ordered face centered orthorhombic precipitate may be observed co-existing with isothermal ω and α phases in the alloy. It is also noted that this transient phase may provide the diffracted intensities observed from another phase, identified tentatively in the literature as orthorhombic martensite, αʺ 1.
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The interfaces between the phase separated regions in the dendritic grains of laser-deposited samples of the high entropy alloy CoCrCuFeNiAl have been studied using aberration-corrected analytical ...(scanning) transmission electron microscopy ((S)TEM). The compositional variations have been determined using energy dispersive x-ray spectroscopy (EDS) in (S)TEM. It was found that between B2, consisting mainly of Al, Ni, Co, and Fe, and disordered bcc phase, consisting mainly of Cr and Fe, there is a transition region, approximately 1.5nm in width, over which the chemical composition changes from the B2 to that of the bcc phase. The crystal structure of this interfacial region is also B2, but with very different sublattice occupancy than that of the adjacent B2 compound. The structural aspects of the interface between the ordered B2 phase and the disordered bcc phase have been characterized using high angle annular dark-field (HAADF) imaging in STEM. It has been determined that the interfaces are essentially coherent, with the lattice parameters of the two B2 regions and the disordered bcc phase being more or less the same, the uncertainty arising from possible relaxations from the proximity of the surfaces of the thin foils used in imaging of the microstructures. Direct observations show that there is a planar continuity between all three constituent phases.
•In the dendritic grains, there are two dominant phases, one with the ordered B2 structure, and the other disordered bcc.•From the intensity ratios in HAADF, the B2 phase appears to have a stoichiometry of the form Al(Ni, Co, and Fe).•Energy dispersive x-ray spectroscopy reveals the presence of an ordered interface transition region between the two phases.•Nanodiffraction in the Titan shows that the interface region is also ordered with the B2 crystal structure based on C.
Abstract Body mass results from a complex interplay between genetics and environment. Previous studies of the genetic contribution to body mass have excluded repetitive regions due to the technical ...limitations of platforms used for population scale studies. Here we apply genome-wide approaches, identifying an association between adult body mass and the copy number (CN) of 47S-ribosomal DNA (rDNA). rDNA codes for the 18 S, 5.8 S and 28 S ribosomal RNA (rRNA) components of the ribosome. In mammals, there are hundreds of copies of these genes. Inter-individual variation in the rDNA CN has not previously been associated with a mammalian phenotype. Here, we show that rDNA CN variation associates with post-pubertal growth rate in rats and body mass index in adult humans. rDNA CN is not associated with rRNA transcription rates in adult tissues, suggesting the mechanistic link occurs earlier in development. This aligns with the observation that the association emerges by early adulthood.
The early stages of super-refined α phase precipitation, aided by the presence of precursory isothermal cuboidal ω precipitates, in a Ti-20V alloy, has been investigated by coupling conventional ...transmission electron microscopy (TEM), high resolution scanning transmission electron microscopy (HRSTEM), including energy dispersive spectroscopy (EDS), and atom probe tomography (APT). These complementary techniques, reveal for the first time, conclusive experimental evidence of nucleation of α precipitates at the ω/β interface and the attendant partitioning of elements.
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The effects of two transition metal oxide dopants, Co and Cr, on the densification properties of Ce0.9Gd0.1O1.95 (CGO) have been examined. While a low concentration of Co oxide was found to reduce ...the sintering temperature of CGO by over 150 degree C, the addition of Cr oxide, at concentrations greater than 0.1cat.%, was found to be detrimental to the densification of the material. Energy-dispersive X-ray (EDX) spectrum imaging was used to examine the elemental evolution in both materials. At 70% relative density, a continuous transition-metal-oxide-rich film was located around the primary crystallites in both materials. With increasing relative density, the concentration of both dopants was shown to decrease without there being a considerable change in the apparent grain boundary layer thickness. All specimens examined revealed considerable grain boundary Gd enrichment and Ce depletion, with this effect becoming more pronounced with increasing relative density in the Cr-doped CGO. EDX spectrum imaging performed on the high density specimens revealed the presence of discrete second phases in both materials, which were subsequently analysed by electron energy loss spectroscopy. Using the current set of results, mechanisms responsible for the modified densification behaviour in Co-doped and Cr-doped CGO are suggested.
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