Internal friction of γ-TiAl alloys at high temperature

Intermetallic γ-TiAl based alloys of engineering interest with respect to high temperature applications are two-phase alloys consisting of γ-TiAl (ordered face-centered tetragonal structure) and ₂-Ti₃Al (ordered hexagonal structure). For this investigation a γ-TiAl based alloy with a composition of Ti-46.4 at%Al-4 at% (Cr, Nb, Ta, B) was studied utilizing a low frequency subresonance apparatus at frequencies between 0.01 and 10 Hz. Above 1000 K, the damping increases strongly up to Q⁻¹=0.2. The frequency and temperature dependence of the high-temperature background was analysed by applying a Maxwell rheological model for viscoelastic relaxation including a distribution of relaxation times. The activation enthalpy of H=3.9 eV agrees well with that from creep experiments (H=3.6 eV) carried out at temperatures between 973 and 1073 K at 200 MPa. The results are discussed in terms of diffusion controlled dislocation climb.     

Photoluminescence in solid films of C60

Photoluminescence has been measured for solid films of nominally pure C₆₀ and C₆₀/C₇₀ mixtures, as a function of temperature and excitation energy. The photoluminescence spectrum shows vibronic structure, and there is evidence for the involvement of three modes. Trace amounts (<0.1%) of C₇₀ appear to be clearly visible in the spectrum of the nominally pure C₆₀ film.     

Internal friction of the glassy tektites

Tektites are naturally occurring glasses, perhaps of meteoritic origin. They occur all over the world. They range in size from μm to decimeter. They are based on the compounds silica and alumina, with smaller concentrations of alkali metal and transition metal oxides. Their origin is in vigorous dispute; some scientists think they are of terrestrial earth origin, others believe they come from interplanetary space, as is true of meteorites. We have secured seven specimens from various regions of the earth and have measured the internal friction spectrum of several in the kilocycle frequency range above room temperature, where internal friction peaks of synthetic glasses have been observed. A well defined loss peak is observed around 500 K in a number of specimens. We have also measured the loss spectrum of obsidian and of blast furnace slag. From comparison of these measurements, we conclude that tektites are probably of volcanic origin, the loss peaks being associated with the reorientation of Fe–oxygen complexes.     

First-principles calculations of elemental crystalline boron phases under high pressure: Orthorhombic B28 and tetragonal B48

The structural, electronic, mechanical properties and hardness of orthorhombic B₂₈ and tetragonal B₄₈ boron phases have been studied by first-principles of pseudopotential calculations. The results indicated that both boron phases are energetically and also mechanically stable. In addition to electronic properties of highly directional covalent bonds, mechanical properties, and also the Debye temperatures of structures support that both are superhard materials. Calculated electronic band structures and density of states revealed that orthorhombic B₂₈ crystal is a semiconductor, and the tetragonal B₄₈ is metallic. The pressure-dependent behaviors of both structures are different, and both are ultra-incompressible and anisotropic materials. The physical parameters of the structures such as lattice parameters, bond lengths, and also energy gaps between valance and conduction bands are closely sensitive to applied external pressures. By means of pressure-volume graphs, obtained EOSs for α-rhombohedral B₁₂, orthorhombic B₂₈ and tetragonal B₄₈ boron phases are compared with available data. However, energetically possible pressure-induced phase transitions among the purposed structures are predicted on the pressure range of 0-460 GPa. Furthermore, our calculations showed that for the pressures from 0 GPa to 24 GPa energetically the more stable elemental boron phase is α-rhombohedral B₁₂, and from 24 GPa to 106 GPa is orthorhombic B₂₈, and from 106 GPa to 460 GPa is α-Ga-type boron.     

Thermodynamics of intercalation in C60 fullerite and stability of cubic close-packed fulleride compounds

Stability of known or hypothetical fullerene C₆₀ intercalation compounds was investigated through a donor-acceptor scheme. A thermodynamic cycle was employed, requiring evaluation of energy costs related with charge transfer and formation of the intercalated solid. Strongly exothermic reactions are predicted in several cases, especially with some elements of the rare-earth series, which should consequently form stable, low dimensional phases. These latter might be interesting candidates for superconductivity, as alkali-doped C₆₀.     

Effect of Pd additions on phase stability and hydrogen-induced internal friction of Ti34Zr11Cu47Ni8 glassy alloys

The effects of additions of Pd below 10 at.% on the stability and hydrogen-induced internal friction behavior of Ti₃₄Zr₁₁Cu₄₇Ni₈ glassy alloys have been investigated. Thermal analyses indicate that the supercooled liquid region decreases distinctly with increasing Pd content. It was found that internal friction peak temperatures of Ti₃₄Zr₁₁Cu₄₇Ni₈–Pd hydrogenated glassy alloys (HGAs) in the hydrogen content range below approximately 30 at.% H were higher than those of the original Ti₃₄Zr₁₁Cu₄₇Ni₈ HGAs, especially at hydrogen contents below 10 at.% H. It was also found that the internal friction peaks of Ti₃₄Zr₁₁Cu₄₇Ni₈–Pd HGAs increased with increasing hydrogen content below 15 at.% H, after which they tended to saturate. These results are in contrast to the effects of Si addition as previously reported. The effects of Pd are discussed from the viewpoint of the interstitial site distribution for hydrogen and local atomic structure of the glassy alloy.     

Internal friction of Cu–CuAlO alloys

Polycrystalline internal oxidize copper alloy with CuAlO particles has been studied by isothermal mechanical spectrometry, above room temperature. The frequency range was 10⁻⁴ to 30 Hz and the maximum strain 2×10⁻⁵. Experiments were carried out on a sample after 18% cold-rolling and successive annealings at various temperatures. The internal friction spectra obtained between 290 and 400 K exhibit only a low frequency background superimposed to a non thermally activated maximum. After annealing at 400 K, the background decreases, a relaxation peak appears and its maximum rises progressively as temperature increases. The relaxation parameters, obtained from the frequency shift of the maximum with temperature, leads to E_a=0.9 eV, and τ₀=6.9×10⁻¹⁰ s. These values are similar to those described in the literature for a relaxation peak assigned to dislocation motion inside grains.     

Electrodeposition of nickel-fullerene C60 composition coatings

A method of preparing composition electrodeposits (CED) nickel-fullerene C₆₀ is proposed. The kinetics of electrodeposition of CED under potentiostatic conditions and the deposit structure are studied. It is shown that fullerene species are hydrogenated during the deposition. The corrosion and electrochemical behavior of CED in 0.5 M H₂SO₄ is studied. Original Russian Text ? V.N. Tseluikin, N.D. Solov’eva, I.F. Gun’kin, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 4, pp. 418–420.     

Correlation between the surface state and optical properties of S6 site and C2 site in nanocrystalline Eu:Y2O3

Eu³⁺ doped Y₂O₃ nanocrystals are synthesized via precipitation and hydrothermal methods. With the increase of hydrothermal temperature, the number of surface states decreases when nanorods are formed. The decrease of surface states induces the increase in the ratio of S₆ site to C₂ site, and meanwhile results in the spectral red-shift of charge transfer band as well as the enhancement of red emission. The results indicate that the surface states and optical properties have close correlations.     

Elastic and anelastic properties of Marval 18 steel

Marval 18 is a precipitation hardened steel with particularly high hardness and low creep, presently used for constructing parts of the interferometers for the detection of gravitational waves in the experiments VIRGO and LIGO. The elastic moduli have been measured in samples subjected to the same treatments as the parts of the interferometer VIRGO. In addition, the anelastic spectra of samples subjected to different thermal treatments have been measured between 50 and 350 K. It is found that, in the absence of plastic deformation, the elastic energy loss coefficient under flexural vibrations around 1 kHz can vary by more than one order of magnitude depending on the thermal treatments, and is dominated by the thermoelastic effect. The main reason for such strong variations is supposed to be the dependence of the thermal conductivity on the average sizes and distances between the precipitate particles.     

Investigation on microstructure and internal friction for low density TiB2/ZL114 composites

TiB₂/ZL114 composites with the density of 2.733 g/cm³ were fabricated through reaction of K₂TiF₄ and KBF₄ (LSM method). The composites were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The internal friction measurements were performed on DTM-II-J dynamic modulus damping analyzer and the mechanisms were investigated. Experimental results indicate that reinforced particles are well-distributed in the matrix and the internal friction value of TiB₂/ZL114 composites is up to a maximum of 9.04x10^-3, almost twice that of ZL114. The internal friction results form dislocation vibration within the material, the sliding of grain boundary and phase interface, and together with the micro-plastic deformation caused by difference in coefficients of thermal expansion and elasticity modulus of various phases. The average internal friction values of samples with the sizes of 40 mmx4 mmx2 mm, 40 mmx8 mmx2 mm and 40~mm$\times 25 mmx2 mm are 8.83x10^-3, 8.89x10^-3, and 8.93x10^-3, respectively. Thus, the developed composites are of low density, high internal friction, and the sizes of samples have no relation to the internal friction behavior.     

Monoclinic phases and stress-relief conditions in (1 − x)Pb(Mg1/3Nb2/3)TiO3–xPbTiO3 solid solutions

A comparative study on heterophase states in perovskite-type solid solutions of (1 − x)Pb(Mg_1/3Nb_2/3)TiO₃–xPbTiO₃ is carried out for compositions near the morphotropic phase boundary. The conditions for mechanical stress relief at elastic matching of phases are analysed at x = const in a wide temperature range. The heterophase states concerned with the presence of the intermediate monoclinic phase are interpreted using the domain state–interface diagrams calculated for x = 0.28, 0.32 and 0.34. It is shown that optimum volume fraction parameters of the domains in the monoclinic phase of the B type are varied in relatively wide ranges and promote complete stress relief with cubic–monoclinic phase coexistence. Two scenarios of stress relief at x = 0.32 are considered in connection with different heterophase states (either tetragonal–monoclinic of the B type or tetragonal–monoclinic of the C type) in a wide temperature range. Possibilities of elastic matching of two polydomain phases (tetragonal–monoclinic of the B type) with almost equal relative widths of the domains in these phases are shown for x = 0.34. The active role of domains of the monoclinic phases in stress relief and forming the planar unstrained interfaces is discussed.     

High mobility n-channel organic field-effect transistors based on soluble C60 and C70 fullerene derivatives

We report on n-channel organic field-effect transistors (OFETs) based on the solution processable methanofullerenes [6,6]-phenyl-C₆₁-butyric acid ester ([60]PCBM) and [6,6]-phenyl-C₇₁-butyric acid methyl ester ([70]PCBM). Despite the fact that both derivatives form glassy films when processed from solution, their electron mobilities are high and on the order of 0.21 cm²/V s and 0.1 cm²/V s, for [60]PCBM and [70]PCBM, respectively. Although the derived mobility of [60]PCBM is comparable to the best values reported in the literature, the electron mobility of [70]PCBM is the highest value reported to date for any C₇₀ based molecule. We note that this is the only report in which C₆₀ and C₇₀ methanofullerenes exhibit comparable electron mobilities. The present findings could have significant implications in the area of large-area organic electronics and organic photovoltaics where C₆₀ derivatives have so far been the most widely used electron acceptor materials.     

La2O3含量对搅拌摩擦加工制备Ni /Al复合材料组织和性能的影响

采用搅拌摩擦加工方法在Al基体中添加不同La₂O₃含量的混合粉末(Ni+La₂O₃),制备 (Ni+La₂O₃)/Al复合材料。采用SEM、EDS、 EPMA及XRD对复合区微观结构及相组成进行分析,采用室温拉伸试验对 (Ni+La₂O₃)/Al复合材料力学性能进行了测试。结果表明,随着La₂O₃含量的增加,(Ni+La₂O₃)/Al复合材料的组织和性能先变好后变差。当La₂O₃添加量达到5%时,复合材料中Al₃Ni增强颗粒分布均匀、颗粒数量最多,块状的Ni粉团聚减少,其抗拉强度达到最大值215MPa,相比Ni/Al复合材料(抗拉强度176MPa),其抗拉强度提高了22%;当La₂O₃的添加量为7%时,复合材料中Al₃Ni增强颗粒含量减少,块状Ni粉团聚重新出现,抗拉强度下降至201MPa。     

Electroabsorption studies of undoped C60 thin films

Electroabsorption spectra have been measured for undoped C₆₀ thin films at 80 and 300 K in the spectral range 1.76 to 5 eV. Absorption and thermal modulation spectra have also been obtained. The electronic energy levels of C₆₀ are reviewed, and the electroabsorption spectra are discussed in terms of the Stark effect on energy levels of A_g, H_g and T_u symmetries.     

Internal friction study of environmental effects on metals and alloys

The internal friction method was applied for studying structural changes occurring in bulk metal due to its interaction with the environment. It was shown that the active dissolution of metal or its oxidation is accompanied by the generation of non-equilibrium vacancies. Diffusive transfer of vacancies into the bulk causes considerable rearrangements of the defect structure of the crystal and, therefore, changes in its anelastic response. Environmental effects have been studied in pure copper subjected to oxidation in a 0.3 N solution of NaNO₂. Such an interaction resulted in the formation of Hasiguti peaks. A similar effect was found in admiralty brass after oxidation in hot tap water, resulting in selective dissolution of zinc. Low-temperature internal friction peaks were also found in the range 150–200 K in Al–5 Mg industrial alloy subjected to active dissolution in 0.05 N KOH solution. The results obtained are discussed in terms of the applicability of the internal friction method for predicting the susceptibility of metals and alloys to environmentally assisted cracking.     

Effect of thermal cycling through the martensitic transition on the internal friction and Young’s modulus of a Ni50.8Ti49.2 alloy

The internal friction (IF) and Young’s modulus (E) of a Ni_50.8Ti_49.2 is affected by thermal cycling. With increasing the number n of thermal cycles, the IF peak P_AM (P_MA) occurring at the austenite/martensite transition temperature decreases to disappear almost completely. Meanwhile the associated E(T) minimum at the beginning (n<250) deepens and then becomes progressively shallower and wider (3×10³≤n<18×10³). The strong sensitivity of P_AM (P_MA) to thermal cycling and to impurity (hydrogen) contents suggests that this peak is predominantly associated with stress-assisted collective motions of twin boundaries located inside the martensite platelets embedded within the austenite phase, rather than with the martensitic transition itself. However, as the high temperature tail of P_AM starts at temperatures appreciably higher than the martensite start-temperature M_s, a premartensitic contribution to damping is also present. The widening of the E(T) minimum indicates that, for n≥3×10³, the direct transformation is to some extent hindered by the dislocation network introduced by thermal cycling. A not thermally activated IF peak P_TWM, which is believed to be due to stress-assisted motions of (001) compound twin boundaries in the homogeneous martensite state, grows with increasing n.     

Solubility of YbTe in Sb2Te3 and thermodynamic properties of the solid solution

The solubility of YbTe in Sb₂Te₃ is investigated by a combination of DTA, XRD, SEM, and EMF methods. The fragment of the T-x phase diagram of the YbTe-Sb₂Te₃ system is constructed for 0-25 mol.% YbTe. It is shown that the solubility limit for YbTe in Sb₂Te₃ is achieved at 15 mol.% YbTe at 300 K and at 17.5 mol.% YbTe at 855 K. From the EMF measurements with an YbTe electrode the partial thermodynamic functions of the YbTe pseudo-component are calculated for the alloys of different compositions. Also, the standard integral thermodynamic functions of the YbTe dissolution in Sb₂Te₃ as well as the standard thermodynamic functions of formation and the standard entropy of the solid solution are calculated from the experimental data.     

Internal friction of Ti–Ni alloys

The low-frequency internal friction and the modulus have been measured on Ti–Ni alloys of varying compositions, from Ni 49.89 to 51.0 at. %, in a wide range of temperatures covering the martensitic and the reverse transformations. The effect of heat treatment on the internal friction has also been investigated. The internal friction shows a sharp peak around the martensitic transformation and/or the formation of pre-martensitic state ‘R-phase’. The height and temperature of the peak depend strongly on the composition and on the heat treatment. Corresponding to the internal friction peak a remarkable depression of the modulus has been observed. At temperatures lower than 250 K two types of temperature dependences were observed; one is a step-type that changes slowly with temperature, and the other is a peak-type around 200 K. An annealing at 873 K has enhanced the low-temperature internal friction, while the solution heat treatment at 1273 K has appreciably decreased it. In all cases the damping of the parent phase is very low; the internal friction is lower than 10·10⁻⁴. The electrical resistivity and the thermoelectric power have been measured.