Microstructural evolution and the effect on hardness of Sanicro 25 welded joint base metal after creep at 973 K

The microstructural evolution and the hardness of Sanicro 25 welded joint base metal after creep at 973 K were investigated, aiming to determine which precipitate is the most important to affect the change in hardness. The precipitates in as-received specimen consist of primary NbCrN and NbC. Creep at 973 K results in the precipitation of secondary NbCrN and Cu-rich particles which grow slightly and M₂₃C₆ which coarsens noticeably. The precipitation of secondary NbCrN, Cu-rich particles and M₂₃C₆ greatly increases the hardness in the early stage of creep. However, when the creep rupture time falls in the range from 582 to 4265 h, the hardness is reduced mainly owing to the growth of Cu-rich particles. With further creep, the growth of both secondary NbCrN and Cu-rich particles decreases the hardness furthermore. It is thus concluded that both secondary NbCrN and Cu-rich particles are the key precipitates to affect the hardness change in Sanicro 25 welded joint base metal after creep at 973 K.     

Effect of frequency on high-temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers, Si₃N₄SiC_W, has been undertaken at elevated temperature during static and dynamic loading at increasing K and ΔK respectively. It is shown that cyclic sub-critical crack growth rates are lower than static crack growth rates. The increased crack growth rate during static far field loading is attributed to the stress relaxation of the inter-granular glass phase which allows time-dependent processes to occur ahead of the crack tip which lead to enhanced sub-critical crack growth rates. During cyclic fatigue the glass phase has insufficient time to relax and glassy ligaments are able to bridge the crack wake thereby shielding the crack tip from the full force of the applied load. Also, at particular temperatures, bridging between the surfaces of the crack wake by the inter-granular glass phase results in increased strength and fatigue retardation. The extent of ‘crack wake healing’ is shown to be time and temperature dependent. The viscosity of the glass phase is directly related to the temperature and the bonding force associated with glass phase bridging is observed to reduce with increasing temperature. The results from a previous study at room temperature are compared to those found during this investigation.     

Spatial variations of optoelectronic properties in single crystalline CuGaSe2 thin films studied by photoluminescence

Single crystal CuGaSe₂ (CGSe) thin films were grown epitaxially on GaAs substrates with different compositions and studied with spatially resolved photoluminescence with micrometer resolution (μPL). Polycrystalline counterparts grown on glass were studied for comparison. μPL performed at room temperature is used to analyze spatial variations of the band gap (?Eg) and the splitting of quasi-Fermi levels (?(E_Fn − E_Fp)) of the absorber. In contrast to earlier studies on Cu(In,Ga)Se₂ (CIGSe) we have concentrated on inhomogeneities occurring in the absence of alloying effects due to the In and Ga mixture.The epitaxially grown specimen exhibited a significantly smaller amount of variations than the polycrystalline counterparts. Cu-rich samples showed higher variation of ?(E_Fn − E_Fp) compared to the Cu-poor counterparts. It is suggested that this is related to formation of a secondary phase Cu_xSe under Cu-rich conditions giving rise to spatially fluctuating Cu-excess. The observed band gap variations could be attributed to strain effects in the absorber layer, and do not indicate any variations of the electronic structure of the absorber.     

FATIGUE CRACK PROPAGATION CHARACTERISTICS OF GLASS FIBRE REINFORCED POLYCARBONATE IN HOT DISTILLED WATER

The fatigue crack propagation behaviour of polycarbonate and glass fibre reinforced polycarbonate was studied in hot distilled water. The effects of temperature, distilled water and glass fibre content on fatigue crack growth rate were determined. In distilled water at 333 K, the fatigue crack growth rate decreased with increasing glass fibre content. A melting was observed of the adhesive bond between the glass fibres and the matrix. It was evident that the fracture morphology closely relates to the fatigue crack growth rate which depends on the ΔK parameter rather than the K_max parameter when the materials are tested in hot distilled water.     

Growth of Cu2ZnSnS4 thin films using sulfurization of stacked metallic films

We fabricated Cu₂ZnSnS₄ (CZTS) thin films through sulfurization of stacked metallic films. Three types of Cu-Zn-Sn metallic films, i.e., Cu-rich, Cu-correct and Cu-poor precursor films were sputtered onto Mo-coated glass. The sulfurization of stacked Cu-Zn-Sn alloy films was performed at a relatively high temperature, 570 °C, with S-powder evaporation. CZTS films from Cu-rich and Cu-correct precursors showed a Cu_2 − xS phase on the film surface, while CZTS films from Cu-poor precursors didn't show the Cu_2 − xS phase. However, all films didn't exhibit any extra secondary phase and exhibited good crystalline textures even with Cu-ratio differences in metallic precursor films. Fabricated CZTS films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and Raman scattering measurements. SEM cross-section images of CZTS films showed that Cu-poor CZTS films were grown with more smooth film surface compared with other types of CZTS films.     

Modelling and simulation of bulk metallic glass production process with suction casting

Abstract

An integrated model, which coupled nucleation and crystalline growth with the heat transfer process, is presented in the present paper. The temperature, temperature gradient, cooling rate and the crystalline fraction of Zr₆₅Al_7·5Cu_17·5Ni₁₀ in suction casting have been calculated with this model. The results show that the metallic glass can be obtained at the bottom and the radial boundary of the rod sample, and that the crystalline phase precipitates in the centre of the sample. The crystalline fraction reaches the highest value of 0·0128 in the centre of the sample. Comparatively lower nucleation rate, as well as the higher viscosity and the cooling rate account for the formation of the bulk metallic glass.     

Crystallization of (Fe,Mn)-based nanoparticles in sodium-silicate glasses

In this investigation, glasses from the system Na₂O/MnO/SiO₂/Fe₂O₃ are prepared using a conventional glass-melting technique. During annealing the glass, a nanocrystalline (Fe, Mn)-based spinel phase is precipitated. The phase composition and microstructure of the formed glass–ceramics are studied using X-ray diffraction and electron microscopy. Anomalous small-angle X-ray scattering experiment is used to gather information on the size, composition and element distribution for the precipitated (Fe, Mn)-based nanocrystals. The sizes of the formed spinel crystals, as determined by scanning electron microscopy and anomalous X-ray scattering, are in the range from 12 to 50 nm for annealing temperatures in the range from 550 to 700 °C. Annealing for a longer period of time at temperatures ≥600 °C results in the formation of a second crystalline phase, NaFe(SiO₃)₂ (aegirine). The ASAXS data show the formation of core–shell structure for the (Fe, Mn)-based crystals with core consisting mostly of iron oxide and a shell, depleted of Fe and Mn. The growth of the spinel crystals is assumed to be kinetically self-constrained.     

The effect of pressure on the growth rate of trans-1,4-polyisoprene crystals at pressures of 1 bar to 3.5 kbar

The growth rates of both low-melting form and high-melting form trans-1, 4-polyisoprene crystals have been measured in thin films by transmission electron microscopy over a range of pressures up to 3.5 kbar. The effect of pressure is to shift the growth rate versus temperature curves to higher temperatures and to cause a continuous decrease in the maximum with increasing pressure. The variation of the crystal growth rate with temperature can be represented, at all pressures, by an equation derived from secondary nucleation theory. The variation of the crystal growth rate with increasing pressure can be largely accounted for by the measured increase in the equilibrium melting temperature of 15 K kbar^–1 and the calculated increase in the glass transition temperature of 20 K kbar^–1. The decrease in the maximum growth rate with increasing pressure results from the continuous decrease in (T _m ⁰ -T _g). It is suggested that crystals of the same form, grow by the same basic mechanism as at atmospheric pressure and that the major effect of pressure is on the melt rather than on the crystals.     

Nanostructured SiC ceramics prepared by a new process—Crystallization of interfacial glass

Large bulk and fully dense SiC based nanoceramics with average grain size of 50 nm and 20–30 wt.% nanometer sized α-Sialon, Al_xSi_{3 − x}O₆ and α-SiO₂ interfacial phases were prepared by a new process, crystallization of interfacial glass, using LMAS glass-coated SiC powder as starting material. The process involves two major steps: densification by hot pressing, and crystallization of interfacial glass by annealing treatment. The densification was controlled by interfacial glass content, hot pressing temperature, and hot pressing pressure; density 99.8% theoretical being reached for SiC/30 wt. % glass nanoceramics hot-pressed at 1520 °C and 22 MPa for 30 min. The crystallization was complete and nearly all the interfacial glass was transformed into nanocrystalline phases after 800 °C and 900 °C for 5 h annealing treatments. Plastic flow and rearrangement of particles and interfacial glass infiltration are densification mechanisms. A large number of nanometer sized SiC powder particles serve as nucleating agents, e.g. hetero-nucleation, and are responsible for interfacial glass crystallization. A characteristic of the present process is that there is no SiC grain growth during densification and interfacial glass crystallization.     

ASAXS study of the copper enriched precipitation in the 15-5PH alloy

The 15-5 PH alloy (composed of Fe, Cr, Ni and Cu), aged at 450 and 500°C has been mainly studied by anomalous small angle X-ray scattering (ASAXS), as well as by transmission electron microscopy, X-ray microanalysis and Mössbauer spectrometry. The two phase model (practically pure Cu precipitates embedded in a Cu depleted matrix) accounts for our results. From the ASAXS curves recorded at three absorption edges (Cr, Cu and Fe), the partial structure functions PSF_ij (with ij = Fe, (Cr+Ni) and Cu) have been determined. The ΔC_Fe/ ΔC_(Cr+Ni) and ΔC_Fe/ΔC_Cu ratios calculated at each and/or both edges are in satisfactory agreement with those determined from X-ray microanalysis carried out on extraction replicas.     

Crystallization of phosphate glasses

The effects of refining time on the transformation temperature, viscosity, and rate of crystallization of a simple lead phosphate glass of nominal composition 60 P₂O₅, 30 PbO, 10 K₂O have been determined. As the hydroxyl content is reduced and the cross-link density increased by continued refining, the transformation temperature and melt viscosity rise. At corresponding temperatures above T _g the viscosity increases and the rate of crystallization decreases with cross-link density, but crystallization rates at isoviscous temperatures are shown to be independent of cross-link density.     

Hot-stage microscopy of film detachment: Vapor-deposited arsenosilicate glasses on silicon

Direct observation of bubble formation in arsenosilicate glass films deposited on silicon from an AsCl₃ source was carried out by hot-stage microscopy. The growth of such detachment bubbles was followed at a controlled temperature and was plotted against time. After an initial fast growth occurring within the first minute, the bubble size increased at a slower rate. Films prepared from an AsH₃ source exhibited superior heat treatment characteristics. Residual chlorine in the glass appeared to be responsible for the poorer heat treatment properties of arsenosilicate films deposited from AsCl₃.     

Spray pyrolytic deposition of CuBiS2 thin films

Thin films of CuBiS₂ have been deposited on glass substrates using spray pyrolysis technique. The effect of substrate temperature on the growth of CuBiS₂ thin films is studied in the range of 150 to 400°C. The best quality films are grown at substrate temperature 250°C with 0·1 M composition. Other preparative parameters like spray rate, pressure, height of the solution, etc are optimized with respect to substrate temperature. Some optical and electrical properties of CuBiS₂ films are also studied and reported.     

The crystallization mechanism of high-cordierite glass

The crystallization mechanism of high-cordierite (2MgO·2Al₂O₃·5SiO₂) from a bulkglass containing B₂O₃ and P₂O₅ as nucleating agents was studied using X-ray diffraction, differential thermal analysis and a polarizing microscope. Thin glass specimens, with a mirror surface, were heated rapidly in an electric furnace in the temperature range 888 to 1363° C for a desired time and then rapidly quenched to room temperature. The normal rate of growth of precipitated cordierite crystals was measured and their morphological change was observed photographically. The growth rate increased with temperature, and the maximum rate occurred at about 1250° C. The crystal morphology was hexagonalprismatic, elongated along thec-axis. Faceted interface morphology was observed when the range of undercooling was from 7 to 174° C. Judging from the relationship between the reduced growth rate and the degree of undercooling, the crystallization mechanism in the range of lower undercooling was governed by a layer growth depending on the surface nucleation mechanism. In the range of higher undercooling, continuous growth was seen and at intermediate undercooling a transition range from a layer growth to a continuous growth was evident.     

Phase formation kinetics in SrO-Al2O3-SiO2-B2O3 glass

Phase formation in Sr-celsian glass, containing 3 wt% B₂O₃ (SA2SB), was investigated by using non-isothermal and isothermal kinetic analyses. While stoichiometric celsian (SA2S) glass showed two-stage crystallization of glass, from glassy state to hexacelsian and from hexacelsian to monocelsian, SA2SB glass showed direct crystallization of glass to monocelsian, resulting in considerable decrease in the temperature of monocelsian formation. The activation energy for monocelsian formation in SA2SB glass was 390 kJ/mol, which is lower than those for hexacelsian and monocelsian formation respectively in SA2S glass. The Avrami exponent of SA2SB glass was 2.0, indicating two-dimensional crystal growth with interface-controlled mechanism at a zero nucleation rate.     

Crystallization kinetics of selenium-tellerium glasses

Bulk glasses of the compositions Se₇₀Te₃₀ and Se₈₀Te₂₀ were prepared by the melt quenching technique. Differential thermal analyses were performed at different heating rates. The values of the glass transition temperature,T _g, the crystallization temperature,T _c, and the peak temperature of crystallization,T _p, were found to depend on the composition and the heating rate. The activation energy for the glass transition,E _i, as well as the activation energy for crystallization,E _c, were evaluated from the heating rate dependence ofT _g andT _p. The crystallization mechanism was examined through analysis of the data under non-isothermal conditions. The results indicated that surface crystallization is dominant for both compositions.     

Ultrafast Growth of High‐Quality Monolayer WSe2 on Au

The ultrafast growth of high‐quality uniform monolayer WSe₂ is reported with a growth rate of ≈26 µm s^?1 by chemical vapor deposition on reusable Au substrate, which is ≈2–3 orders of magnitude faster than those of most 2D transition metal dichalcogenides grown on nonmetal substrates. Such ultrafast growth allows for the fabrication of millimeter‐size single‐crystal WSe₂ domains in ≈30 s and large‐area continuous films in ≈60 s. Importantly, the ultrafast grown WSe₂ shows excellent crystal quality and extraordinary electrical performance comparable to those of the mechanically exfoliated samples, with a high mobility up to ≈143 cm² V^?1 s^?1 and ON/OFF ratio up to 9 × 10⁶ at room temperature. Density functional theory calculations reveal that the ultrafast growth of WSe₂ is due to the small energy barriers and exothermic characteristic for the diffusion and attachment of W and Se on the edges of WSe₂ on Au substrate.     

Microstructure and grain growth in Li2O-Al2O3-SiO2 glass ceramics

Microstructure and grain growth were studied in two glass ceramics of the Li₂O-Al₂O₃-SiO₂ system, one an experimental material of basic composition Li₂O-Al₂O₃-4SiO₂ and the second a commercial material of approximately 0.7Li₂O-Al₂O₃-6SiO₂ composition with small amounts of other oxides. There was evidence from transmission electron microscopy that the commercial material contained residual glass at grain-boundary triple points and glass layers at some but not all grain boundaries. No definite evidence was found for residual glass in the experimental material. The composition of secondphase regions in the commercial material was studied by STEM microanalysis. Al-rich regions of irregular morphology were found but there was no evidence that residual glass was SiO₂-rich, as has been previously suggested for this type of glass-ceramic. Grain growth showed a fairly similar time dependence in the two materials with growth more rapid in the commercial material at a given temperature. It is suggested that grain growth is controlled by precipitate particles rather than by residual glass phase.     

Effect of glass additions on the sintering behaviors and electrical microwave properties of BaO–Nd2O3–Sm2O3–TiO2 ceramics

The effect of Al₂O₃-doped silica glass (AS-glass) addition on the structure and dielectric properties of the BaO–Nd₂O₃–Sm₂O₃–TiO₂ (BNST) microwave materials was investigated. This BNST material has a dielectric constant (k) of 80 and a quality factor (Q×f) of 6000 GHz when sintered at 1350 °C for 2 h. Both the microstructure and the microwave dielectric characteristics were determined as a function of the sintering temperature and glass content. A Q×f as high as 8700 GHz was achieved in ceramics added with 20 wt % glass, however, the k value drops to 30. The high Q×f value is attributed to the improved densification of the dielectric when glass is added. Results of X-ray diffraction experiments indicate that glass addition enhances the growth in the longitudinal direction of the columnar crystal and a preferred (0 0 2) orientation. The presence of columnar structure plays an important role in the improvement of the microwave dielectric properties.     

DSC study of some Ge-Sb-S glasses

Differential scanning calorimetric analysis was made on three glasses of the Ge-Sb-S system in order to obtain insight into the kinetics of glass transition and of the inherent relaxation processes occurring in the glass transition region. The heat capacity of the supercooled liquid referred to as the glass was measured. The value of the heat capacity jump at the glass transition, C_p, has been obtained for each glass. These values are in good agreement with those found for similar chalcogenide glasses. The relaxation process in the glassy alloy Ge₃₀Sb₁₀S₆₀ was investigated by measuring the excess heat capacity of the annealed glass in the glass transition region. A relaxation enthalpy of 2.7 meV for annealing at 595 K for 17 h was determined. A kinetic study of the glass transition in the Ge₂₀Sb₁₀S₇₀ glass was done. From the change in the glass transition temperature with scanning rate, an apparent activation energy of 3.9 eV was obtained. This value agrees with those measured for the apparent activation energy of the shear viscosity in similar glasses.