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.     

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.     

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.     

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.     

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.     

Magnetic held dependence of elastic modulus and resistance in La2/3Ca1/3MnO3

We have measured the magnetic field dependence of the anelastic modulus and the electric resistance of a ceramic sample of the magnetoresistant perovskite La_2/3Ca_1/3MnO₃ around the metal–insulator transition by the vibrating reed technique (2.5 KHz). Previous work [1] showed that the modulus becomes higher while the internal friction has a peak at the transition temperature (T_c=262 K). In this work, improvements made on the equipment allowed us to measure at constant deformations (<10⁻⁵) and magnetic fields up to 4500 Gauss. We made isothermal measurements of internal friction, modulus and resistance as a function of an applied magnetic field. We found that most of the changes induced by the magnetic field take place in a few degrees (almost 5 K) near the transition temperature where the changes in resistance are more important. Up to the highest magnetic field applied, we found 3% maximum variations of the modulus and no hysteresis while cycling the magnetic field. We suppose that the experiment is placed in the linear response of the inverse Wiedemann effect [2], due to the small deformations used, and that the ferromagnetic domain structure is responsible for the observed effects. Some additional measurements are needed (magnetic hysteresis loops) to be compared with our results.     

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.     

Synthesis and properties of CaCd2Sb2 and EuCd2Sb2

High density polycrystalline CaCd₂Sb₂ and EuCd₂Sb₂ intermetallics are synthesized by Spark Plasma Sintering and their thermoelectric properties are investigated. X-ray diffraction measurements reveal both materials have a structure in space group, containing a small amount of CdSb as a second phase. Thermoelectric measurements indicate both are p-type conductive materials. The figure of merit value of CaCd₂Sb₂ is 0.04 at 600 K and that of EuCd₂Sb₂ is 0.60 at 617 K. Theoretical calculations show that CaCd₂Sb₂ is a degenerate semiconductor with a band gap of 0.63 eV, while EuCd₂Sb₂ is metallic with DOS of 13.02 electrons/eV. For deeper understanding of the better thermoelectric properties of EuCd₂Sb₂, its low temperature magnetic, transport and heat capacity properties are investigated. Its Nèel temperature is 7.22 K, convinced by heat capacity anomaly at 7.13 K. Hall effect convinced that it is a p-type conductive material. It has high Hall coefficient, high carrier concentration and high carrier mobility of +1.426 cm³/C, 4.38 × 10¹⁸/cm³ and 182.40 cm²/Vs, respectively. They are all in the magnitude of good thermoelectric materials. The Eu 4f level around Fermi energy and antiferromagnetic order may count for the better thermoelectric properties of EuCd₂Sb₂ than that of CaCd₂Sb₂.     

Effect of WS2 particle size on mechanical properties and tribological behaviors of Cu-WS2 composites sintered by SPS

The mechanical properties and tribological behaviors of Cu-WS₂ composites fabricated by spark plasma sintering (SPS) using two different WS₂ particle sizes of 0.6 and 5.0 µm and Cu powders as raw materials were investigated. The results indicate that the bending strength and tribological behavior of Cu-WS₂ composites are greatly affected by the size of WS₂ particles. The bending strength of Cu-WS₂ composites with the WS₂ particle size of 5.0 μm is 292.2 MPa. As the size of WS₂ particle decreases to 0.6 µm, the bending strength also decreases to 181.5 MPa. Moreover, as the WS₂ particle size decreases from 5.0 to 0.6 µm, the wear rate of Cu-WS₂ composite sharply increases from 2.99×10^?14 to 6.13×10^?14 m³/(N·m) and its friction coefficient increases from 0.158 to 0.172. The size of WS₂ particle (5.0 μm) plays an important role in forming transfer film formed on the counter-face. The sample with 5.0 μm WS₂ particle forms smoother and more continuous transfer film, which results in a low wear rate and friction coefficient of the Cu-WS₂ composites.     

Towards understanding anelasticity of the β1′ martensitic phase of Cu–Al–Ni

The present work continues the series of experimental investigations undertaken in order to elucidate the mechanisms controlling elastic and anelastic properties of the β₁′ martensitic phase of Cu-based shape memory alloys. The paper reports an attempt to distinguish between ‘dislocation’ and ‘interface’ mechanisms of the internal friction in the β₁′ martensitic phase of Cu–Al–Ni single crystals. Two types of experiments have been performed. First, the ultrasonic strain amplitude-independent and amplitude-dependent internal friction (ADIF) of a monovariant specimen for temperatures 90–300 K is carefully re-examined. Second, in situ measurements of the ADIF and of the influence of ultrasonic oscillations on the plastic deformation (acoustoplastic effect) were carried out during quasistatic deformation of a quenched polyvariant specimen. Experimental results support a dislocation rather than an interface mechanism of anelasticity, at least at ultrasonic frequencies and moderate strain amplitudes.     

Novel conjugated polymers employing the binding of polyfluorene derivatives and C60

New polyfluorene (PF)-based conjugated polymers, PClBEHF, PClBEHF-co-PDEHF, PN₃BEHF, PN₃BEHF-co-PDEHF, PEHPBF, PC₆₀BEHF, PC₆₀BEHF-co-PDEHF, and PEHPBF⁺PCBA⁻ containing chloro, azido, piperidine, and fullerene (C₆₀) groups as pendants, were synthesized. In the case of PC₆₀BEHF and PC₆₀BEHF-co-PDEHF, the C₆₀ moiety was introduced by using covalent bond formation between the azido group and C₆₀. PEHPBF⁺PCBA⁻ was synthesized by using ionic bond formation between the piperidinium cation of PEHPBF and carboxylate anion of PCBA. The PL and EL spectra of PClBEHF, PClBEHF-co-PDEHF, PN₃BEHF, PN₃BEHF-co-PDEHF, and PEHPBF exhibited red-shifted and broad peaks caused by the aggregation which was induced by the increasing interchain interaction due to the polar pendants. After binding with C₆₀, covalently and ionically, the photovoltaic properties of PC₆₀BEHF, PC₆₀BEHF-co-PDEHF, and PEHPBF⁺PCBA⁻ were studied. PL was quenched completely, which indicates photo-induced charge transfer from PF to pendant C₆₀. Even though photovoltaic effects did not occur in short circuit condition, blending with MEH-PPV as the ratio of 1:1 generated the photovoltaic effects.