Composition and lattice parameters of silicide and matrix in cast Ti–Si–Al–Zr alloys

Abstract

The silicide chemistry, i.e. the type, composition, and lattice parameters of the silicide in as cast titanium based Ti–Si–Al–Zr alloys, has been studied. It has been shown that the stoichiometry of the silicide in the alloys can be expressed as (Ti_1?x , Zr_x)₅(Si_l?y, Al_y) _2·76?3·04(0≤X< 0·2, 0≤y<0·1). The presence of Al and Zr in the silicide increases its lattice parameters. Addition of Al coarsens the eutectic silicide and slows the formation of secondary silicide precipitates by solid state reaction. Addition of zirconium refines the eutectic silicide and promotes secondary silicide precipitation. The silirides are low in Al and rich in Zr, whereas the Ti matrix is rich in Al and low in Zr. The lattice parameters of the Ti matrix are decreased by Al and increased by Zr.

MST/1427     

Deformation and fracture in Al–Li-base alloys

Abstract

Aluminium–lithium-base alloys are of considerable interest because of their low density and high modulus. However, they have been shown to have low ductility and poor fracture toughness. This has been attributed to a variety of factors, including intense shear band formation, segregation to grain boundaries, and weakened grain boundaries due to precipitation and precipitate-free zones. The authors have investigated the deformation structures observed in binary and more complex commercial alloys. As would be expected, considering the microstructure of the alloys, extensive strain localization and shear band formation occurs in these alloys. However, it is shown that the commercial alloys are less sensitive to strain localization than the model binary alloy systems investigated. The stresss–train behaviour has been investigated. The alloys exhibit jerky flow, which is indicative of negative strain rate sensitivity, and strain rate change tests showed this to be the case. This is consistent with the deformation structures observed. The effect of weakened grain boundaries due to precipitation and precipitate-free zones has been studied by comparing the fracture characteristics of aged and unaged material. It is shown that the mode of failure is identical under appropriate conditions. It is concluded that segregation to grain boundaries is the major cause of the lower ductility and toughness of Al–Li alloys. This possibility has been investigated using in situ fracture surface analysis techniques. Results are presented on grain boundary segregation, and methods of reducing its influence on fracture behaviour are indicated.

MST/570     

Pearlite growth rate in Fe–C and Fe–Mn–C steels

The kinetic theory for the growth of pearlite in binary and ternary steels is implemented to ensure local equilibrium at the transformation front with austenite, while accounting for both boundary and volume diffusion of solutes. Good agreement is on the whole observed with published experimental data, although the reported growth rate at the lowest of temperatures is much smaller than predicted. To investigate this, experiments were conducted to replicate the published data. It is found that the cooperation between cementite and ferrite breaks down at these temperatures, and surface relief experiments are reported to verify that the resulting transformation product is not bainite.     

Broadband down-conversion in Bi–Yb-codoped yttrium and yttrium–aluminum oxides

Comparative study of the broadband down-conversion processes in Bi³⁺–Yb³⁺-codoped yttrium oxide (Y₂O₃) and various yttrium–aluminum oxides (Y₃Al₅O₁₂, YAlO₃ and Y₄Al₂O₉) has been performed from the point of view of search for materials suitable for enhancement of efficiency of silicon solar cells. The studied materials in the form of nanopowders have been synthesized by sol–gel method and characterized by X-ray powder diffraction, scanning electron microscopy and luminescence techniques. Relative down-conversion efficiency for studied materials has been estimated. It was shown that optimal concentration of Yb³⁺ ions should be 2–4 at.% simultaneously with Bi³⁺ ions in the amount of about 1 at.%. Such dopants content provides the highest emission intensity of Yb³⁺ ions in near-infrared when excited into Bi³⁺ ions absorption in ultraviolet. Perspectives of the studied materials for enhancement of silicon solar cells are discussed.     

Microstructure and stresses in a keatite solid–solution glass–ceramic

The microstructure of a translucent keatite solid–solution glass–ceramic (keatite s.s.) of the LAS-system (Li₂O–Al₂O₃–SiO₂) has been analyzed with SEM, AFM, XRF, XRD, and TEM. The glass–ceramic consists mainly of keatite s.s. with minor secondary phases such as zirconium titanate, gahnite and probably rutile. Furthermore the resistance to temperature differences (RTD) of this glass–ceramic was investigated. It is shown that, in spite of the relatively high coefficient of thermal expansion (CTE) of about 1 × 10⁻⁶ K⁻¹, an improved RTD can be achieved by special ceramization treatment. With this, compressive stresses in the first 100 μm to 150 μm are induced. These stresses can presumably be contributed to a difference in CTE between the surface-near zone and the bulk. Said CTE difference is caused by chemical gradients of CTE-relevant elements, such as Zn, K, and supposedly additional alkali elements such as Li. These stresses are useful to increase the strength and application range of glass–ceramics based on keatite s.s.

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Solid state phase transformations in Ti–Al–C and Ti–Al–Nb–C alloys

Abstract

Results are reported of an investigation of solid state transformations in a series of α₂ based alloys having an aluminium content of 26 at.-% with carbon up to 3 at.-%; two α₂ basedquaternary Ti–Al–Nb–C alloys with 5 and 12 at.-%Nb and 3 at.-%C were also studied. Ordering occurs in the ternary Ti–Al–C alloys and also in the 23Al–5Nb–3C alloy on quenchingfrom 1250°C. Additional carbide precipitation was not observed in the ternary Ti–Al–C alloys on reheating to 750°C. Additions of niobium resulted in the presence of the β phase at 1050°C in the 5%Nb alloy and at 1050 and 750°C in the 12%Nb alloy. In the quaternary Ti–Al–Nb–C alloys, (Ti, Nb)₃AlC was found to be the primary phase and was present in the microstructure over the temperature range studied. In the 21Al–12Nb–3C alloy, the ordered β phase transformed to α″₂ martensite on quenching from 1250;amp;#x00B0;C.

MST/1306     

Microstructural change and precipitation hardening in melt–spun Mg–X–Ca alloys

Mg–Al–Si–Ca and Mg–Zn–Ca base alloys were rapidly solidified bymelt spinning at the cooling rate of about a million K/s. The melt-spun ribbons were aged in the range 100–400%C for 1 h. The effect of additional elements on microstructural change and precipitation hardening after heat treatment was investigated using TEM, XRD and a Vickers microhardness tester. Age hardening occurred after aging at 200%C in the Mg–Al–Si–Caalloys mainly due to the formation of Al₂Ca and Mg₂Ca phases, whereas in the Mg–Zn–Ca alloys mostly due to the distribution of Mg₂Ca. TEM results revealed that spherical Al₂Ca precipitate has the coherent interface with the matrix. Considering the total amount of additional elements, Mg–Zn–Ca alloys showed higher hardness and smaller size of precipitates than Mg–Al–Si–Ca alloys. With the increase of Ca content, the hardness values of the aged ribbons were increased. Among the alloys, Mg–6Zn–5Ca alloy showed the maximum value of age hardening peak(H_v:180) after aging at 200%C for 1h.     

Kinetic and electrical phenomena in gas–liquid systems

Disperse systems consisting of a liquid and gas bubbles located in it are considered. Two possible versions of evolution of bubbles under the conditions studied are assessed. In simple liquids, contact between two bubbles causes them to merge, as the separating film breaks. In the case of complex organic liquids, amphiphilic film is formed on the surface of bubbles, and the lifetime of bubbles in contact increases with their size. Under an external electric field, chains of bubbles are formed, lined up along the electric field potential lines. The presence of bubbles in liquid greatly lowers the breakdown threshold, as the critical parameters of the breakdown field in liquids are two to three orders of magnitude higher than those in gases at atmospheric pressure. Various breakdown mechanisms in liquids are discussed from the viewpoint of formation of the gas phase during the passage of an electric current through a liquid medium. The character of propagating a streamer in separate bubbles is studied with their random distribution in liquid and in the case of formation of some structures of bubbles; the critical parameters of disperse systems, that can lead to their electrical breakdown, are presented. Along with the general concepts of electrical breakdown in dispersed systems, experimental studies of these processes are considered, and the nature of electrical breakdown in liquid dielectrics, including transformer oil, is discussed.     

Morphological changes and hardness evolution in Cu–30Ni–5Cr and Cu–45Ni–15Cr spinodal alloys

Abstract

The development of increased strength in Cu–Ni–Cr alloys, compared with binary Cu–Ni alloys, is dependent upon heat treatment. These alloys have compositions which permit them to be solution treated at elevated temperature and then aged at a lower temperature, in a two phase field, to produce hardening. Decomposition into two phases may occur by nucleation and growth or by a spinodal reaction, depending on alloy composition and heat treatment temperature. As part of a more extensive study of ternary Cu–Ni–Cr alloys, the decomposition of Cu–30Ni–5Cr and Cu–45Ni–15Cr (wt-%) has been studied in the spinodal range. The evolution of microstructure has been determined together with the coarsening kinetics for the modulated spinodal decomposition products. Specimens rapid quenched from 1050°C, were aged in the temperature range 300–800°C. The progress of spinodal decomposition was followed via hardness measurements, X-ray diffraction, and scanning and transmission electron microscopy. Modulation wavelengths were measured from both X-ray diffraction patterns and electron micrographs. It was found that during the early stages of aging the modulation wavelength remained constant while the hardness increased continuously. After a certain period of aging, the hardness remained constant at its peak value, while the modulation wavelength increased continuously. The results are consistent with current theories of spinodal decomposition and hardening.

MST/1733     

Synthesis and characterization of MeO–PEG–PLGA–PEG–OMe copolymers as drug carriers and their degradation behavior in vitro

The objective of this study was to characterize the methylpoly (ethylene glycol)-poly (lacticacid-co-glycolicacid)-poly (ethylene-glycol) (MeO-PEG-PLGA-PEG-OMe, abbreviation as PELGE) copolymers as intravenous injection drug delivery carriers and their degradation behavior in vitro. A series of MeO-PEG-PLGA-PEG-OMe copolymers with various molar ratios of lactic to glycolic acid and various molecular weights and different MeO-PEG contents were synthesized by ring-opening polymerization in the presence of MeO-PEG with molar masses of 2000 and 5000, using stannous octoate as the catalyst. The hydrophilicity of PELGE copolymers, evaluated by contact angle measurements, was found to increase with an increase in their MeO-PEG contents. Methylpoly (ethylene glycol)-poly (lacticacid-co-glycolicacid) (MeO-PEG-PLGA, abbreviation as PELGA) nanoparticles and PELGE nanoparticles were prepared using the emulsion-solvent evaporation technique (o/w) with Pluronic F68 (Poloxamer 188 NF) as emulsifier in the external aqueous phase. The degradation behavior of the nanoparticles was evaluated by the lactate generation with time upon their in vitro incubation in PBS (pH 7.4). The rate of in vitro degradation of the PELGE or PELGA nanoparticles depended on their composition, increasing with an increase in the proportion of MeO-PEG or LA in the copolymer chains. The degradation rate was slower at higher lactide: glycolide ratio. The lower the molecular weight of PELGE; the higher the degradation rate of the nanoparticles.     

Analyzing fault and severity in pedestrian–motor vehicle accidents in China

The number of pedestrian–motor vehicle accidents and pedestrian deaths in China surged in recent years. However, a large scale empirical research on pedestrian traffic crashes in China is lacking. In this study, we identify significant risk factors associated with fault and severity in pedestrian–motor vehicle accidents. Risk factors in several different dimensions, including pedestrian, driver, vehicle, road and environmental factors, are considered. We analyze 6967 pedestrian traffic accident reports for the period 2006–2010 in Guangdong Province, China. These data, obtained from the Guangdong Provincial Security Department, are extracted from the Traffic Management Sector-Specific Incident Case Data Report. Pedestrian traffic crashes have a unique inevitability and particular high risk, due to pedestrians’ fragility, slow movement and lack of lighting equipment. The empirical analysis of the present study has the following policy implications. First, traffic crashes in which pedestrians are at fault are more likely to cause serious injuries or death, suggesting that relevant agencies should pay attention to measures that prevent pedestrians from violating traffic rules. Second, both the attention to elderly pedestrians, male and experienced drivers, the penalty to drunk driving, speeding, driving without a driver's license and other violation behaviors should be strengthened. Third, vehicle safety inspections and safety training sessions for truck drivers should be reinforced. Fourth, improving the road conditions and road lighting at night are important measures in reducing the probability of accident casualties. Fifth, specific road safety campaigns in rural areas, and education programs especially for young children and teens should be developed and promoted. Moreover, we reveal a country-specific factor, hukou, which has significant effect on the severity in pedestrian accidents due to the discrepancy in the level of social insurance/security, suggesting that equal social security level among urban and rural people should be set up. In addition, establishing a comprehensive liability distribution system for non-urban areas and roadways will be conducive to both pedestrians’ and drivers’ voluntary compliance with traffic rules.     

Correlation of microstructure and magnetic properties in Cu–Co–Ni alloys

The present study concerns correlation of microstructure and magnetic properties of nanocrystalline binary 50Cu–50Co and ternary 50Cu–25Co–25Ni (wt%) alloys prepared by ball milling and subsequent isothermal annealing of the ball milled alloys. High resolution transmission electron microscopic (HR-TEM) investigation has shown deformation-induced microstructural features. Field emission scanning electron microscopy (FE-SEM) has revealed a distinct change in morphology of as-milled CuCoNi alloys after annealing. Differential scanning calorimetric (DSC) and X-ray diffraction (XRD) analysis have revealed that annealing of the CuCoNi alloy above 350 °C results into precipitation of nanocrystalline Co (fcc) in the CuNi matrix by spinodal decomposition. It is also demonstrated that isothermal annealing of the ball milled alloys in the temperature range between 350 and 650 °C significantly influence the magnetic properties, e.g. coercivity (H_c), remanence (M_r) and magnetic saturation (M_s) due to annihilation of defects such as stacking and twin fault along with dissolution and/or precipitation of magnetic phases in the Cu-rich matrix.     

Microstructural change and precipitation hardening in melt-spun Mg–X–Ca alloys

Mg–Al–Si–Ca and Mg–Zn–Ca base alloys were rapidly solidified by melt spinning at the cooling rate of about a million K/s. The melt-spun ribbons were aged in the range 100–400°C for 1 h. The effect of additional elements on microstructural change and precipitation hardening after heat treatment was investigated using TEM, XRD and a Vickers microhardness tester. Age hardening occurred after aging at 200°C in the Mg–Al–Si–Ca alloys mainly due to the formation of Al₂Ca and Mg₂Ca phases, whereas in the Mg–Zn–Ca alloys mostly due to the distribution of Mg₂Ca. TEM results revealed that spherical Al₂Ca precipitate has the coherent interface with the matrix. Considering the total amount of additional elements, Mg–Zn–Ca alloys showed higher hardness and smaller size of precipitates than Mg–Al–Si–Ca alloys. With the increase of Ca content, the hardness values of the aged ribbons were increased. Among the alloys, Mg–6Zn–5Ca alloy showed the maximum value of age hardening peak(H_v:180) after aging at 200°C for 1 h.     

Prediction and characterization of growth temperatures in Al–Zn–Mg alloys

Growth temperatures of α-Al, intermetallic τ and eutectic α + τ phases in Al-12 wt.% Zn 6 wt.% Mg alloy has been determined as a function of growth velocity in the range of 3 × 10^? 5 to 1 × 10^? 3 m/s at a temperature gradient of 2500 K/m, using a directional solidification technique. The experimental results are found to be in good agreement with predictions of growth temperatures of competing constituents for multicomponent systems.     

Microstructure and precipitation in Al–Li–Cu–Mg–(Mn,Zr) alloys

Abstract

Hot rolled Al–6Li–1Cu–1Mg–0·2Mn (at.-%) (Al–1·6Li–2·2Cu–0·9Mg–0·4Mn, wt-%) and Al–6Li–1Cu–1Mg–0·03Zr (at.-%) (Al–1·6Li–2·3Cu–1Mg–0·1Zr, wt-%) alloys developed for age forming were studied by tensile testing, electron backscatter diffraction (EBSD), three-dimensional atom probe (3DAP), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). For both alloys, DSC analysis shows that ageing at 150°C leads initially to formation of zones/clusters, which are later gradually replaced by S phase. On ageing at 190°C, S phase formation is completed within 12 h. The precipitates identified by 3DAP and TEM can be classified into (a) Li rich clusters containing Cu and Mg, (b) a plate shaped metastable precipitate (similar to GPB2 zones/S″), (c) S phase and (d) δ′ spherical particles rich in Li. The Zr containing alloy also contains β′ (Al₃Zr) precipitates and composite β′/δ′ particles. The β′ precipitates reduce recrystallisation and grain growth leading to fine grains and subgrains.     

Mechanism of hopping conduction in Be–Fe–Al–Te–O semiconducting glasses and glass–ceramics

Journal of Materials Science - Electrical properties of beryllium-alumino-tellurite glasses and glass–ceramics doped with iron ions were studied using impedance spectroscopy. The conductivity...     

Liquid–Liquid Interfacial Tension in Ternary Monotectic Alloys Al–Bi–Cu and Al–Bi–Si

The liquid–liquid interfacial tension in the ternary monotectic alloys Al_34.5-x Bi_65.5Cu _x and (Al_0.345Bi_0.655)_100-x Si _x (mass%) has been determined as a function of its Cu (Si) content by a tensiometric technique. It is established that the interfacial tension gradually increases when either Cu or Si is added to Al–Bi alloys. The increase of can be related to the increase of the miscibility gap (both width and height) when Cu (Si) is added to the Al–Bi binary. The temperature dependences of the interfacial tension in binary Al_34.5Bi_65.5 and ternary Al_23.25Bi_65.5Cu_11.25 and (Al_0.345Bi_0.655)₉₅Si₅ monotectic alloys are well described by the power function with the critical-point exponent .     

Relations Between Transport Coefficients in Lennard–Jones Fluids and in Liquid Metals

Several simple approximate hard-sphere relations for transport coefficients are compared with the results of molecular dynamics (MD) simulations performed on Lennard–Jones (LJ) fluids. Typically the individual transport coefficients: self-diffusion coefficients, D, shear viscosity, _s, bulk viscosity, _B, and thermal conductivity, , agree within a factor of two of the exact results over the fluid and liquid parts of the phase diagram, which seems reasonable in view of the approximations involved in the models. We have also considered the ratio, /_s, and the product, D_s, for which simple analytic expressions exist in the hardsphere models. These two quantities also agree within a factor of two of the simulation values and hard sphere analytic expressions. Using time correlation functions, Tankeshwar has recently related the ratio /D to thermodynamic quantities, in particular, to the differences in specific heats, C _p – C _V, and to the isothermal compressibility, T. Using D and thermodynamic values taken solely from LJ MD simulations, his relation was tested and found to give typically better than ~20% agreement at liquid densities, deteriorating somewhat as density decreases into the gas phase. Finally liquid metals are considered. In this case, is dominated by its electronic contribution, which is related approximately to the electrical conductivity by the Wiedemann–Franz Law. Some theoretical results for the electrical conductivity of Na are referenced, which allow a semiquantitative understanding of the measured thermal conductivity of the liquid metal. Shear viscosity is also discussed and, following the work of Tosi, is found to be dominated by ionic contributions; Nevertheless, at the melting temperature of Na, a relation emerges between thermal conductivity, electrical resistivity and shear viscosity.     

Morphology and crystallographic phase of V–C particles formed in Fe–Cr–Ni–V–C alloys

Abstract

The morphology and crystallographic phase of V–C carbide particles formed in cast Fe–Cr–Ni–V–C alloys were investigated by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy (TEM). The combination of results obtained with these techniques revealed that cuboidal, cruciform and spherical carbide particles were formed, depending on the alloy composition, all having the cubic-VC_1?x structure (Fm-3m). Detailed TEM observations suggested that small carbide particles were initially cubic in shape and became spherical with increasing particle size. All cuboidal and spherical carbides were single crystallites with no grain boundary at any particle sizes, even after growing to 6 μm in diameter.     

Correlation Between Charge, Spin and Lattice in La–Eu–Sr Manganites

To understand the structural, electrical, magnetic, elastic and anelastic properties of La_0.67?x Eu _x Sr_0.33MnO₃ (0.30≤x≤0.39) manganites, a series of samples was prepared by citrate gel route. X-ray diffraction studies along with Rietveld analysis indicate the samples crystallize in single phase with Pnma space group. Studies on the variations of magnetization with temperature indicate that the Curie transition temperature (T _C) decreases with increasing Eu content. Furthermore, Eu substitution is found to increase the electrical resistivity and significantly enhances the colossal magnetoresistance effect, while it is found to decrease the characteristic metal–insulator transition temperature (T _P). On analyzing the electrical resistivity data, it has been concluded that the resistivity below T _P can be explained based on electron–electron, electron–phonon and two magnon scattering processes, while that above T _P, the adiabatic small polaron model is found to explain the observed behavior. Finally, the longitudinal modulus (L) and internal friction (Q ^?1) have been measured and a dramatic change in L (T) is observed at T _C, accompanied by a sharp peak in Q ^?1 (T). Simultaneous occurrence of magnetic, transport and lattice anomalous behavior at T _C indicate the presence of strong electron–phonon and spin–phonon interactions in these manganites.