V. D. Natsik

The structure, slip systems, and microhardness of C60 crystals

The structure and microplasticity of high-purity fullerite C60 have been investigated comprehensively. The crystalline structure, lattice parameters, and phase transitions have been studied by x-ray diffractometry in the temperature range 30–293 K. It is found that the temperature corresponding to the orientational order–disorder phase transition is Tc=260 K. A considerable number of regions with stacking faults discovered in the samples leads to blurring of the fcc→sc phase transition in the temperature interval Tc±3 K. The a(T) dependences of the lattice parameter display peculiarities at the following characteristic temperatures: Tc at which the lattice parameter jump Δa/a=3.3×10−3 is observed, and the temperatures T0≃155 K, and Tg≃95 K which are associated with the beginning and end of molecular orientation freezing. It is shown that the formation of orientational glass is accompanied by a considerable increase in the width of x-ray reflections. The slip geometry and the temperature dependence of micr...

Verification of the theory of Brownian motion of a particle through a potential barrier in a viscous medium during experimental study of dislocation acoustic relaxation in normal and superconducting niobium

The Kramers theory (1940) describing a thermally activated escape of a particle from a potential well in a viscous medium is used for explaining the acoustic absorption peak discovered by Kramer and Bauer (1967) in niobium in the liquid helium temperature range. It is shown that the properties of the peak observed in experiments correspond to the model of resonant interaction of acoustic vibrations with dislocation kink chains. Kinks, viz., pseudoparticle excitations on dislocation lines, perform diffusive drift in the second-order Peierls relief under the action of acoustic vibrations, experiencing simultaneously viscous drag exerted by conduction electrons. The possibility of a sharp controllable change in electron viscosity during a superconducting transition can be used for verifying the conclusions of the Kramers theory in experimental investigations of the dislocation contribution to attenuation of sound. It is found that at low temperatures, the conditions for the observation of the anomaly predict...

Theory of orientational relaxation in the low-temperature phase of fullerite C60

A microscopic extension of the phenomenological model of double-well orientational states, viz., pentagonal and hexagonal configurations of molecules, which is widely used for describing the low-temperature phase of fullerite C60, is proposed. A simple kinetic equation and a set of thermodynamic relations connecting the crystal lattice deformation, the concentration of orientational excitations of molecules, and temperature are derived. Basic physical properties of the low-temperature phase, including orientational glass transition, heat capacity, thermal expansion, rheological properties and damping of elastic vibrations are described on a unified basis. The conclusions of the theory are compared with the experimental data, and empirical estimates are obtained for the parameters of double-well states and the lattice–orientational interaction. It is shown that the large values of thermal expansion and acoustic damping above the orientation glass-transition temperature of fullerite are due to high-intensit...

Effect of plastic deformation on the shape and parameters of the low-temperature peak of internal friction in niobium

Temperature dependences of the decrement of longitudinal vibrations are studied at moderately low temperatures in polycrystalline niobium with the residual resistivity ratio RRR=60 at frequencies 78 and 363 kHz. A peak of internal friction is detected in the vicinity of 200 K. The height, width, and temperature of the peak change significantly upon a variation of vibrational frequency and as a result of changes in the defect structure of the sample under thermocycling, plastic deformation, or prolonged low-temperature recovery. It is shown that the absorption peak is due to the interaction of elastic vibrations of the sample with a system of identical thermally activated relaxators with an activation energy of 0.15 eV and an attack frequency of 1×1010 s−1 in a nearly perfect crystal. A theory is proposed for describing the variation of the shape and parameters of the internal friction peak due to statistical dispersion of the values of activation energy of the relaxators.

Low-temperature plasticity of Pb–Bi alloys: the role of thermal activation and inertial effects

Detailed studies of temperature dependences of critical shear stress and strain-rate sensitivity of deforming stress of Pb–Bi single crystals with 0.1–6.0 at. % Bi are carried out in the temperature range 0.5–295 K. The deforming stress decrease during a superconducting transition of the sample is studied and the concentration dependence of the effect is measured. The ideas of a gradual transition (upon cooling) from thermally activated motion of dislocations through impurity barriers to the thermoinertial mechanism in the temperature range 10–25 K and further to the quantum-inertial motion at temperatures ≲1 K are developed. A detailed thermoactivation analysis of experimental data is carried out, and empirical estimates of internal stresses, dislocation–impurity interaction parameters, electron and phonon components of dynamic drag coefficient for dislocations are obtained.

Effect of impurity barriers on low-temperature anomalies in plasticity parameters of β-Sn

Plastic deformation determined by the slip of (100)〈010〉 screw dislocations in single crystals of β-Sn solid solutions with different concentrations of Cd, In, and Zn impurities is studied in the temperature range 1.6–150 K. Low-temperature anomalies of the yield stress τ0 and activation volume γ (kinks in the temperature dependence of τ0(T) and peaks on the stress dependence of the activation volume γ(τ)) are observed. The parameters of anomalies are found to be determined to a considerable extent by the concentration of impurity atoms in the alloy and by the height of the barriers formed by impurity atoms for dislocations. The observed effects are compared with the predictions of the theory describing the influence of impurities on the motion of dislocations through Peierls barriers of complex shape. The results of analysis of experimental data are in agreement with the assumption concerning the two-hump shape of the lattice potential relief in the (100)〈010〉 slip system.

Low-temperature plasticity of Zn-doped β–Sn crystals

Work hardening curves are obtained for Sn–Zn single crystals with Zn concentration 0.01 and 0.53 at.% in the temperature interval 1.6–300 K. It is found that the Zn impurity significantly affects the shape of the curves as well as the work hardening parameters like the strain-hardening factor, duration of slip stage, temperature dependences of critical shear stress, and strain-rate sensitivity. A comparative analysis is carried out for the effect of Zn on low-temperature creep of Sn, and the effect of the same concentration of Cd impurity (which was studied earlier).

The influence of Peierls relief on low-temperature plasticity of CdTe single crystals

Abstract The following was investigated on CdTe single crystals: the kinetics of spontaneous elongation of dislocation arms arising near the indentor impression on the (111) face at room temperature; the kinetics of stress relaxation in the early stages of deformation in the vicinity of the yield point at 200, 225, 250, 273 and 300 K; the temperature dependence of the yield point in the temperature range 200–300 K. The experimental data for the temperature dependence of macroscopic plasticity parameters are well described by the model of dislocation movement in the Peierls relief by the kink pair mechanism for the case of low effective stresses τ∗ when the equation for the average dislocation velocity is ν∼ exp [ −H kp (τ∗) κT ] with the activation enthalpy H kp = 2H k −2α(τ∗) 1 2 . According to our estimates, the parameters of the theory have the following values: enthalpy of kink pair formation 2H k = 0.6 eV , α = 10 −23 N 1 2 m 2 , and Peierls stress τp = 21 MPa. To explain the temperature dependence of the yield point we suppose that it is determined not only by the natural contribution τ∗(T) but also by the temperature dependence of internal stresses τi(T). The empirical values of τi(T) obtained in the temperature region studied can be assigned to development of the superjogs structure on dislocations. The average distance between the superjogs impenetrable to kinks is estimated to be L⩽10 μm. The process of dislocation arm elongation can be described by the empirical formula l(t,P)∼P 1 3 t 1 3 (P is the indentation load, t is time. This dependence follows from the assumption that the velocity of a single dislocation in an array can be described by the equation ν∼(τ∗) m exp ( −H 0 κT ) at m = 1.

Low-temperature structure transformation in In–Cd solid solutions

In the temperature range from 6 to 300 K, the resistivity ρ, microhardness HV, dynamic Young’s modulus E and logarithmic decrement δ (vibration frequency ∼75 kHz) have been studied in In–4.3 at.% Cd polycrystalline alloy. The measurements were carried out during cooling and heating of samples at a rate of 1 K min−1. A hysteresis with boundaries Tmin≅150 K and Tmax≅290 K is observed in the temperature dependences ρ(T), E(T) and δ(T). The maximum relative values of the hysteresis loops are Δρ/ρ≅0.1, ΔE/E≅0.25 and Δδ/δ≅0.25. Within the hysteresis region, the characteristics measured are unstable and their values change slowly with time at fixed temperatures. The hysteresis has not been registered in HV(T), but the derivative d/dT Hv(T) has an essential feature in the temperature interval Tmin<T<Tmax. The anomalies mentioned are indicative of the existence of some low-temperature structural transformation of hysteretic type controlled by complicated kinetics with large relaxation times. The structure and the physical and mechanical properties of pure In remain stable in the temperature range studied and reveal no anomalies mentioned. It is also shown that the alloy In–4.3 at.% Cd has sufficiently high plastic compliance and internal friction in a wide range of low temperatures that qualify it as a high damping material.

Crowdions in atomic cryocrystals and metals with fcc and bcc lattices

Numerical values are obtained for the parameters of the crystal-field potentials and crowdions (self-energy, effective mass, characteristic length) for Ar and Kr fcc cryocrystals, Cu and Al fcc metals, and α- and δ-Fe bcc metals. The calculations are performed assuming that the interatomic interaction in the crystals can be described by empirical Lenard–Jones and Morse pair potentials. A new algorithm is developed and used for calculating the crystal-field potentials. The algorithm is based on a representation of the crystal lattice as a collection of parallel atomic rows. An analytic expression in the form of a trigonometric series is obtained for the potential describing the interaction between an atom in a close-packed row and the crystal matrix. An explicit analytic expression is also obtained for the energy parameter characterizing the interatomic interaction inside a distinguished row. It is shown that the main condition for weak coupling between close-packed rows and the crystal matrix, admitting t...