Hemispherical total emissivity and specific heat capacity of deeply undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 melts

High-temperature high-vacuum electrostatic levitation (HTHVESL) and differential scanning calorimetry (DSC) were combined to determine the hemispherical total emissivity epsilon T, and the specific heat capacity cp, of the undercooled liquid and throughout the glass transition of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass forming alloy. The ratio of cp/epsilon T as a function of undercooling was determining from radiative cooling curves measured in the HTHVESL. Using specific heat capacity data obtained by DSC investigations close to the glass transition and above the melting point, epsilon T and cp were separated and the specific heat capacity of the whole undercooled liquid region was determined. Furthermore, the hemispherical total emissivity of the liquid was found to be about 0.22 at 980 K. On undercooling the liquid, the emissivity decreases to approximately 0.18 at about 670 K, where the undercooled liquid starts to freeze to a glass. No significant changes of the emissivity are observed as the alloy undergoes the glass transition

Developments in the Ni–Nb–Zr amorphous alloy membranes

Most of the global H2 production is derived from hydrocarbon-based fuels, and efficient H2/CO2 separation is necessary to deliver a high-purity H2 product. Hydrogen-selective alloy membranes are emerging as a viable alternative to traditional pressure swing adsorption processes as a means for H2/CO2 separation. These membranes can be formed from a wide range of alloys, and those based on Pd are the closest to commercial deployment. The high cost of Pd (USD *31,000 kg-1) is driving the development of less-expensive alternatives, including inexpensive amorphous (Ni60Nb40)100-xZrx alloys. Amorphous alloy membranes can be fabricated directly from the molten state into continuous ribbons via melt spinning and depending on the composition can exhibit relatively high hydrogen permeability between 473 and 673 K. Here we review recent developments in these low-cost membrane materials, especially with respect to permeation behavior, electrical transport properties, and understanding of local atomic order. To further understand the nature of these solids, atom probe tomography has been performed, revealing amorphous Nb-rich and Zr-rich clusters embedded in majority Ni matrix whose compositions deviated from the nominal overall composition of the membrane

Phonon engineering in isotopically disordered silicon nanowires

The introduction of stable isotopes in the fabrication of semiconductor nanowires provides an additional degree of freedom to manipulate their basic properties, design an entirely new class of devices, and highlight subtle but important nanoscale and quantum phenomena. With this perspective, we report on phonon engineering in metal-catalyzed silicon nanowires with tailor-made isotopic compositions grown using isotopically enriched silane precursors ²⁸SiH, ²⁹SiH, and ³⁰SiH with purity better than 99.9%. More specifically, isotopically mixed nanowires ²⁸Si ³⁰Si with a composition close to the highest mass disorder (x ∼ 0.5) were investigated. The effect of mass disorder on the phonon behavior was elucidated and compared to that in isotopically pure Si nanowires having a similar reduced mass. We found that the disorder-induced enhancement in phonon scattering in isotopically mixed nanowires is unexpectedly much more significant than in bulk crystals of close isotopic compositions. This effect is explained by a nonuniform distribution of ²⁸Si and ³⁰Si isotopes in the grown isotopically mixed nanowires with local compositions ranging from x = ∼0.25 to 0.70. Moreover, we also observed that upon heating, phonons in ²⁸Si ³⁰Si nanowires behave remarkably differently from those in ²⁹Si nanowires suggesting a reduced thermal conductivity induced by mass disorder. Using Raman nanothermometry, we found that the thermal conductivity of isotopically mixed ²⁸Si Si nanowires is ∼30% lower than that of isotopically pure ²⁹Si nanowires in agreement with theoretical predictions. (Figure Presented)

Observation of Precipitation Evolution in Fe-Ni-Mn-Ti-Al Maraging Steel by Atom Probe Tomography

We describe the full decomposition sequence in an Fe-Ni-Mn-Ti-Al maraging steel during isothermal annealing at 550 °C. Following significant pre-precipitation clustering reactions within the supersaturated martensitic solid solution, (Ni,Fe)3Ti and (Ni,Fe)3(Al,Mn) precipitates eventually form after isothermal aging for ~60 seconds. The morphology of the (Ni,Fe)3Ti particles changes gradually during aging from predominantly plate-like to rod-like, and, importantly, Mn and Al were observed to segregate to these precipitate/matrix interfaces. The (Ni,Fe)3(Al,Mn) precipitates occurred at two main locations: uniformly within the matrix and at the periphery of the (Ni,Fe)3Ti particles. We relate this latter mode of precipitation to the Mn-Al segregation

Metastable phase formation during the decomposition of Fe-20 at.% Mo

The early stages of the isothermal decomposition of an Fe-20 at.% Mo alloy at 500 degrees C have been studied by means of atom-probe (AP) and field-ion microscopy (FIM), as well as high-resolution (HREM) and conventional (CTEM) transmission electron microscopy. CTEM reveals a characteristic modulated structure oriented along the (100)-type directions of the b.c.c. matrix. Electron diffraction patterns show satellite-like intensities close to the fundamental reflections in (100)-type directions, indicating an approximate 6 nm characteristic length scale of the decomposition microstructure. FIM and HREM reveal precipitates about 2 nm in size with a b.c.c. structure formed coherently within the matrix. AP analyses show these precipitates to consist of almost pure Mo. The size misfit between the Mo-rich precipitates and the Fe-rich matrix causes large coherency strains, resulting in precipitate alignment along (100)-type directions. The Mo-rich b.c.c. solid solution precipitates in a metastable equilibrium with the Fe-rich b.c.c. matrix, whereas the formation of the equilibrium intermetallic phases is kinetically suppressed. A coherent metastable miscibility gap between the Fe-rich and the Mo-rich b.c.c. solid solution is assessed by incorporating a continuum elasticity strain-energy term into the Gibbs free energy. (C) 2000 Acta Metallurgica me. Published by Elsevier Science Ltd. All rights reserved