Internal Friction in Glass Fiber- and Carbon Fiber-Reinforced Composites with a T-107 Matrix

Technical Physics - The temperature and amplitude dependences of internal friction in hybrid composites consisting of unidirectional carbon fibers and glass tissues in a T-107 molten epoxy matrix...     

Internal friction in Cu6PS5Br superionic crystals and related composites

Mechanical properties of Cu₆PS₅Br single crystals and composites based on them have been investigated by the internal friction method. The measurements of the internal friction and the shear modulus have been performed in the temperature range of 80–300 K at deformation frequencies of 10–100 mHz in a mode of forced torsional vibrations. The maxima caused by the superionic and ferroelastic phase transitions have been found in temperature dependences of the internal friction. It has been shown that a more than two-fold decrease in the shear modulus with increasing temperature in the range of 150–230 K is caused by mobility unfreezing in the cation sublattice of the Cu₆PS₅Br single crystal during the superionic phase transition. An abrupt (more than threefold) increase in the shear modulus upon heating in the range of 260–270 K is caused by the ferroelastic phase transition of the Cu₆PS₅Br single crystal. Parameters of the internal friction of this single crystal in the course of mentioned phase transitions have been determined.     

The Effects of Copper and Polytetrafluoroethylene (PTFE) on Thermal Conductivity and Tribological Behavior of Polyoxymethylene (POM) Composites

The effects of copper and polytetrafluoroethylene (PTFE) on thermal conductivity and tribological behavior of polyoxymethylene (POM) composites were investigated by a hot disk thermal analyzer and an M-2000 friction and abrasion testing machine. The results indicated that the incorporation of 3 wt% copper particles into POM had little effect on the thermal conductivity of POM composites, but led to the decreased friction coefficient and wear rate of composites. As the copper content was increased, the thermal conductivity increased and reached 0.477 W m^?1 K^?1 for POM-25% Cu composite, an increase of 35.9% compared with that of unfilled POM, while the friction coefficient and wear rate of composites also increased. The incorporation of PTFE into POM-Cu composites had a negligible effect on the thermal conductivity of composites, but helped in the formation of a continuous and uniform transfer film and resulted in the reduction in the friction coefficient and wear rate of composites. The POM-15% Cu-10% PTFE composite, with a value of wear rate similar to unfilled POM possessed higher thermal conductivity and lower friction coefficient.     

Temperature dependence of the internal friction of polycrystalline indium

The temperature dependences of the internal friction and the elastic modulus of polycrystalline indium have been investigated in the temperature range 7–320 K at oscillatory loading frequencies of approximately 100 kHz. The effect of temperature on the amplitude dependence and the effect of high-amplitude loading at 7 K on the temperature and amplitude dependences of the internal friction of indium have been analyzed. It has been demonstrated that the thermocycling leads to microplastic deformation of indium due to the anisotropy of thermal expansion and the appearance of a “recrystallization” maximum in the spectrum of the amplitude-dependent internal friction. The conclusion has been drawn that the bulk diffusion of vacancies and impurities begins at temperatures of approximately 90 K and that, at lower temperatures, the diffusion occurs in the vicinity of dislocations. It has been revealed that the high-temperature internal friction background becomes noticeable after the dissolution of Cottrell atmospheres.     

Martensitic structure of a Cu-Al-Ni alloy after a single cyclic change in the temperature and the action of reactive stresses

The martensitic transformations in a Cu-13.4 wt % Al-4.0 wt % Ni alloy subjected to a single cyclic change in the temperature in the range 293–680 K under conditions of constrained shape-memory deformation are studied by differential scanning calorimetry. These martensitic transformations are found to be closely related to the temperature dependences of the reactive stresses generated in constrained alloy samples during a single heating-cooling cycle. The substantial change in the behavior of these dependences during heating to a temperature above 600 K is caused by the strong effect of the decomposition of the β-phase solid solution on the parameters of the martensitic transformations in this alloy.     

预应变对TiNi丝/Ni基复合材料的内耗行为的影响

本文对预应变TiNi形状记忆合金N／Ni基复合材料的内耗行为进行了研究。TiNi丝／Ni基复合材料是将TiNi丝作为阴极，Ni基作为阳极通过化学电镀法制成的。结果表明：随着TiNi丝预应变的增加，复合材料的相变内耗峰逐渐增宽。由于TiNi丝与Ni基之间的热膨胀系数不匹配以及回复力的产生，使复合材料在高温段的内耗有一个急剧增加过程。与TiNi合金相比，TiNi／Ni基复合材料的整体内耗整体随着温度的升高而增加的。     

Amplitude-dependent internal friction and similarity of temperature dependences of microflow and macroflow stresses of a crystal

A dislocation-kinetic mechanism is discussed of amplitude-dependent internal friction and of the similar temperature dependences of the microflow and yield stresses of a crystal. The similarity of these dependences is shown to be due to the similarity of strain-(dislocation-) strengthening curves of the crystal in the microplastic-and macroplastic-strain ranges, respectively.     

Temperature-dependent plasmon excitations in the surface layer of ordered Cu-22.5 at % Mn alloy

Temperature and energy dependences of the characteristic electron energy losses on plasmon excitations are studied in the surface layer of ordered polycrystalline Cu-22.5 at % Mn alloy. Features in the near-surface distribution of Mn are found from an analysis of plasmon excitations and the data of Auger spectroscopy. The observed temperature dependences of the electron energy loss spectra have features in the range of 650–750 K that include the temperature of atomic disordering (T _k = 675 K) in the bulk of the alloy.     

Elasticity and inelasticity of silicon nitride/boron nitride fibrous monoliths

A study is reported on the effect of temperature and elastic vibration amplitude on Young’s modulus E and internal friction in Si₃N₄ and BN ceramic samples and Si₃N₄/BN monoliths obtained by hot pressing of BN-coated Si₃N₄ fibers. The fibers were arranged along, across, or both along and across the specimen axis. The E measurements were carried out under thermal cycling within the 20–600°C range. It was found that high-modulus silicon-nitride specimens possess a high thermal stability; the E(T) dependences obtained under heating and cooling coincide well with one another. The low-modulus BN ceramic exhibits a considerable hysteresis, thus indicating evolution of the defect structure under the action of thermoelastic (internal) stresses. Monoliths demonstrate a qualitatively similar behavior (with hysteresis). This behavior of the elastic modulus is possible under microplastic deformation initiated by internal stresses. The presence of microplastic shear in all the materials studied is supported by the character of the amplitude dependences of internal friction and the Young’s modulus. The experimental data obtained are discussed in terms of a model in which the temperature dependences of the elastic modulus and their features are accounted for by both microplastic deformation and nonlinear lattice-atom vibrations, which depend on internal stresses.     

Singularities of magnetic and elastic characteristics of La2/3Ba1/3MnO3: Analysis of martensitic kinetics

A coordinated temperature behavior of magnetic susceptibility and internal friction has been observed in the La_2/3Ba_1/3MnO₃ manganite in the temperature region of the crystal phase separation 5–340 K. Stepwise temperature behavior of the susceptibility of the single crystal sample and corresponding singular behavior of the internal friction in the polycrystalline manganite have been found. These small-scale features of the temperature dependences of the susceptibility and the internal friction are considered to be a reflection of martensitic kinetics of the structural phase transformation R3¯c↔Imma in the 200 K temperature region.     

Phenomenological theory for the melting of a thin lubricant film between two atomically smooth solid surfaces

A thermodynamic model is developed for the melting of an ultrathin lubricant film squeezed between two atomically smooth solid surfaces. To describe the state of lubricant, an excess volume parameter is introduced; it appears due to the chaos in the structure of a solid body induced by melting. This parameter increases with the total internal energy upon melting. Thermodynamic melting and shear melting are described. The dependences of the friction force on the lubricant temperature and the shear rate of friction surfaces are analyzed. The calculated results are compared to the experimental data.     

Elastic properties of a La<Subscript>0.5</Subscript>Pr<Subscript>0.2</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single crystal

The temperature dependences of the velocity of longitudinal sound and internal friction in the ferromagnetic La_0.5Pr_0.2Ca_0.3MnO₃ single crystal with magnetic first-order phase transition were studied. It was found that the sound velocity decreases by ≈20% in transition from the ferromagnetic to paramagnetic state. In the paramagnetic region, the extended temperature hysteresis of the sound velocity and the internal friction was observed. It was shown that La_0.5Pr_0.2Ca_0.3MnO₃ has two paramagnetic phases with different sound velocities.     

利用内耗研究淬火空位对Cu-11.9Al-2.5Mn(wt%)形状记忆合金逆马氏体相变温度的影响

利用内耗手段研究了淬火空位的演变行为及其对Cu-11.9Al-2.5Mn(wt%)形状记忆合金逆马氏体相变温度的影响.通过对不同冷却方式的样品研究表明,较高的冷却速度可以导致较高的逆马氏体相变峰峰温.而对于水淬样品,相变峰峰温随淬火温度呈非单调变化行为,这可能与有序相及无序相中不同的空位形成能有关.经历热循环以后,逆马氏体相变峰峰温显著降低. 关键词： 形状记忆合金 热处理 马氏体相变 阻尼     

Specific features of the magnetic aftereffect in high-purity diamagnetic beryllium

The behavior of the effective shear modulus and low-frequency internal friction in magnesium-thermal beryllium condensate before and after the treatment of samples in a weak constant magnetic field has been studied. The data obtained have been used for calculating the temperature and time dependences of the ratio between the dislocation motion velocities during and after the magnetic action. It has been shown that the exposure of samples to a magnetic field and their aging after the magnetic action radically change the character of the temperature dependences of the dislocation motion velocities. Empirical expressions describing these dependences have been obtained. It has been demonstrated that the magnetic aftereffect in diamagnetic beryllium has a complex character, which indicates the presence of several magnetic memory mechanisms.     

Elastic and kinetic properties of single-crystal La<Subscript>0.75</Subscript>Ba<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript>

An experimental study of the temperature behavior of longitudinal sound velocity, internal friction, electrical resistivity, and thermopower of single-crystal La_0.75Ba_0.25MnO₃ is reported. A structural transition accompanied by a large jump (18%) in the sound velocity was found to occur at T _S ≈170 K. Within the interval 156–350 K, the temperature dependences of the sound velocity and internal friction reveal a temperature hysteresis. An internal-friction peak due to relaxation processes was detected. The metallic and semiconducting regions are separated by a transition domain about 80 K wide lying below the Curie temperature T _C =300 K.     

Amplitude dependence of the internal friction and Young’s modulus defect of polycrystalline indium

The dependences of the internal friction and the Young’s modulus defect of polycrystalline indium on the oscillatory strain amplitude have been studied over a wide range of temperatures (7–320 K) and oscillatory strain amplitudes (10⁻⁷−3.5 × 10⁻⁴) at oscillatory loading frequencies of about 100 kHz. It has been revealed that the amplitude dependences of the internal friction and the Young’s modulus defect include stages associated with the interaction of dislocations with point defects and the interdislocation interaction. The temperature range characterized by the formation of point-defect atmospheres (the Cottrell atmospheres) near dislocations in indium has been determined.     

Synthesis and physical properties of the ferroelectric magnet SrBi3Nb2FeO12

The dielectric, elastic, and inelastic properties of a ceramic ferroelectric SrBi₃Nb₂FeO₁₂ are studied over the temperature range 300–900 K. The observed anomalies in the temperature dependences of the permitivity, dielectric loss, shear modulus, and internal friction indicate the occurrence of a structural phase transition in the compound at ～700 K. It is suggested that the transition is a proper ferroelectric and improper ferroelastic second-order phase transition.     

Computer simulation of amplitude-dependent internal friction

The internal friction in crystalline materials has been studied in the model of viscous motion of dislocations through a system of randomly arranged defects. The results of the calculation of the amplitude dependence of the internal friction for different frequencies and defects of different powers are presented. Three characteristic portions have been revealed in the amplitude dependences of the internal friction, which correspond to different modes of dislocation motion.     

On the nature of the magnetoplastic effect in a beryllium condensate

The effect of a weak constant magnetic field on the temperature and amplitude dependences of the effective shear modulus, low-frequency internal friction, and the ratio of velocities of the dislocation motion in a magnetic field and without it in a beryllium condensate has been investigated. It has been shown that the simultaneous action of a constant magnetic field, mechanical stresses, and a variable temperature results in sign reversal of the magnetoplastic effect in beryllium. It has been found that the sign reversal of the magnetoplastic effect is accompanied by the sign reversal of the ratio of velocities of the dislocation motion, as well as by oscillations in the temperature dependence of this ratio. Possible mechanisms of manifestation of the magnetoplastic effect in beryllium have been discussed in terms of the results of the comprehensive analysis of the amplitude dependences of the studied characteristics.     

Internal friction in alloys due to plasticity of the diffusion phase transformation

An analytic expression is obtained for the time dependence Q ^?1(t) of internal friction associated with plasticity of a phase transformation. Time dependences Q ^?1(t) of internal friction of the Pb-62Sn and Pb-1.9Sn alloys (wt.%) alloys were studied in the regime of continuous excitation of resonant flexural vibrations. The measurements of the Q ^?1(t) dependences for 1 h at room temperature and a fixed strain amplitude ε₀ ≈ 7 and 19 min) for the Pb-62Sn alloy. For the Pb-1.9Sn alloy under the same conditions, an exponential decrease followed by an internal friction peak (at t _m ≈ 7 min) is observed. It is shown numerically that the above singularities of internal friction are formed by processes of intermittent phase decomposition of Pb-Sn alloys in the cyclic stress field produced by an external load. Experimental data on Q ^?1(t) are used for reconstructing the kinetic curves describing the decomposition (conversion) ratio as a function of time and for calculating the corresponding values of parameters K and n of the Avrami kinetic equation for the Pb-62Sn alloy.