The Kinetics of Destruction of Piezoceramics under the Electric-Field Action

The electrical durability (time since the application of a constant voltage up to the breakdown) as a function of the electric field strength E and the breakdown field intensity E _br as a function of the rate of rise of the electric field strength E are measured on ferroelectric-piezoceramic disks. It is concluded that the destruction of ferroelectric-piezoceramics has a kinetic character.     

Superplasticity of a microcrystalline aluminum-lithium alloy under torsion

The deformation of an aluminum-lithium alloy under torsion in the temperature range 523–673 K at angular velocities of 0.322 and 0.0322 rad/s is studied. The shear strain γ prior to failure is found to increase with decreasing strain rate, and its temperature dependence has a maximum at 553 K (γ≈30). The initial loading-induced jump in the dependence of the torque on the angle of rotation (stress-strain curve) is followed by a softening stage, which changes into a hardening stage or a stage with a constant torque at all temperatures except those near 673 K. The stress distribution over a cross section is analyzed, and the dependence of the shear-strain rate \(\dot \gamma \) on the stress τ and temperature T is found to be \(\dot \gamma \sim \tau ^n \exp ( - {U \mathord{\left/ {\vphantom {U {kT}}} \right. \kern-\nulldelimiterspace} {kT}})\). The results are compared with those obtained earlier from tensile tests of this alloy.     

Generation and relaxation of reactive stresses in a Cu-Al-Ni shape memory alloy upon cyclic temperature variation in the range 293–800 K

The generation and relaxation of reactive stresses in Cu-Al-Ni shape-memory alloy single crystals studied during a single cycle of temperature variation in the range 293–800 K under conditions of the β-phase decomposition (above 600 K) are found to depend on the degree of β-austenite decomposition at the stage of decreasing temperature. The higher this degree, the lower the stresses relaxed and generated upon decreasing temperature and the higher the critical temperatures of the reverse martensitic transitions. Moreover, loading the alloy by reactive stresses during a heating half-cycle causes not only a reversible martensitic shape memory deformation but also an additional austenitic shape memory deformation to occur when the temperature is decreased.     

Elasticity and inelasticity of ceramic samples of graphitelike boron nitride

This paper reports a study of the effect of temperature and strain amplitude on Young’s modulus and ultrasonic damping in ceramic samples of graphitelike boron nitride prepared in the RF and USA by different technologies and using various additions to improve the mechanical properties of the ceramic. It is shown that, in addition to the influence on the effective elasticity modulus, additions improve the sample microplasticity, which apparently increases the strength of the material.     

Kinetics of failure of polycrystalline ferroelectric ceramics in mechanical and electric fields

The kinetics of mechanical failure and electrical breakdown in polycrystalline ferroelectric ceramics was studied under the simultaneous action of an electric field and a mechanical load. A kinetic approach is shown to be preferable as compared to the concepts that treat the failure and breakdown as critical phenomena. The mechanical failure and electrical breakdown are shown to be interrelated. It is found that a weak action of one of the fields retards failure caused by the other field and that the simultaneous action of these strong fields accelerates both the mechanical failure and electrical breakdown. Methods for determining the activation characteristics of both processes only from the failure kinetics in one of these processes are developed.     

Deformation instability and fracture at liquid-helium temperature

The instability of plastic deformation during the extension and compression of aluminum and during the extension of titanium and stainless steel is studied, and the temperature-rate region for the existence of deformation instability is determined. The form of the jumps in the flow stresses and their relationship to the macroscopic localization of strain and to fracture of the sample are also investigated. The role of the localization of microstrain and of fracture in the macroscopic manifestations of strain inhomogeneity is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 647–651 (April 1997)     

Durability of polycrystalline ferroelectric ceramics in a constant electric field

The kinetics of mechanical fracture and electric breakdown of polycrystalline ferroelectric ceramics under simultaneous action of an electric field and mechanical load have been investigated. The procedures for determining the activation characteristics of both processes from only the fracture kinetics in one of them have been developed.     

Low-temperature deformation of nanocrystalline niobium

The strain characteristics of nanocrystalline niobium are measured in the temperature range 4.2–300 K. It is shown that the development of a strong local deformation with clearly delineated macroscopic slip bands occurs at 4.2 K and 10 K. The thermal effects at a stress jump observed upon transition of the sample (or a niobium strip placed close to the sample) from the superconducting state to the normal state are estimated. It is demonstrated that the temperature dependence of the yield point σ_s(T) can be divided into three portions: two portions (T<10 K and T>70 K) with a slight change in σ_s and the third portion with a strong dependence σ_s(T). The strain characteristics of polycrystals with nano-and larger-sized grains are compared with those of single crystals.     

Effect of low-temperature compression on the parameters of the phase transition at 250–260 K in C60 single crystals

A thermodynamic transition in C₆₀ crystals, subjected beforehand to uniaxial compression at various temperatures and to thermal action, was investigated by differential scanning calorimetry. It was established that compression of the crystal at temperatures below the phase transition has a much smaller effect on the transition itself than a similar or even much weaker action at a temperature above the phase transition. A “quenching” effect was also found. This effect is probably due to the orientational order of individual fullerene molecules. A correlation was established between the magnitude of the effect and the preliminary deformation of C₆₀ crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 550–553 (March 1999)