On solid solution hardening in crystals with randomly distributed solute atoms

The temperature dependence of the critical resolved shear stress (CRSS) of solid solution single crystals is explained on the basis of random distribution of solute atoms in the alloy. The calculated temperature dependence of CRSS in a crystal with both isolated solute atoms and their pairs is very similar to the experimentally observed one, i.e., the pronounced temperature independent region, so called plateau, appears.     

Cyclic hardening of copper single crystals

Cyclic hardening of copper single crystals was investigated at room temperature at stress amplitudes corresponding to stages II and III of the monotonic work-hardening. At constant shear stress amplitudes corresponding to stage II of monotonic work-hardening, the cyclic hardening as a function of the number of stress cycles is independent of stress amplitude and of strain rate. Further, it seems to be able to decrease the mean free slip area of mobile dislocations so that their motion becomes almost completely reversible. Thus, neither structural changes nor final fatigue failure can be expected at these stress amplitudes. At shear stress amplitudes corresponding to stage III of monotonic work-hardening, cyclic hardening becomes the smaller the higher the stress amplitude used. The mean free slip area of mobile dislocations stays generally large enough to allow the structural changes required for cyclic softening and final fatigue failure. In the lower portion of this stress amplitude region, the strain amplitude associated with the constant stress cycling is continuously controlled by cyclic hardening, while in the higher portion, it is controlled first by cyclic hardening and then by work-hardening during the half cycle in question. Creep can also be found at these stress amplitudes corresponding to a high level in stage III.     

Magnetoresonant hardening of silicon single crystals

JETP Letters - A microwave magnetic field crossed with a static field was found to exert a resonance effect on the dislocation mobility in single crystals of p-type silicon. The frequency of...     

High-pressure torsion applied to nickel single crystals

Nickel single crystals with different crystallographic orientations were deformed by high-pressure torsion. Special attention is devoted to examining the evolution of the micro-texture and microstructure. The initial crystal orientation was found to have a significant effect on the mechanical hardening and evolution of micro-texture at low and medium equivalent strains, whereas at very high strains no effect of the initial orientation was observed and the behaviour was very similar to a polycrystal. The evolution of micro-texture is in good qualitative agreement with the full constrained Taylor model. At very high equivalent strains the initial crystal orientation has no influence on micro-texture. At such strains, the hardening, the refinement of the structure and the texture reaches a saturation. The final micro-texture is explained by the change from one preferred crystallographic orientation to another.     

Deuteron NMR relaxometry applied to confined liquid crystals

We discuss the capability of deuteron nuclear magnetic resonance (NMR) spectroscopy and relaxometry to reveal molecular ordering and dynamics in confined liquid crystals. The attention is focused on the high-temperature phase above the nematic-isotropic transition, which is — in the absence of the long-range orientational order — very suitable for the study of surface interactions. Deuteron NMR spectra and relaxation rates are presented for two representatives of confined liquidcrystal systems: 8CB in cylindrical cavities of Anopore membranes and 5CB with an embedded polymer network. A substantial increase in the transverse spin relaxation rate, stimulated by the surface-induced order in enclosures, has been observed. In cylindrical cavities, it exhibits a strong temperature dependence on approaching the phase transition, whereas in the polymer network dispersion it is temperature-independent. The increase of T ₂ ^?1 provides information on the effect of spatial constraints on molecular mobility and on the surface orientational order parameter. Using deuteron relaxometry, one can measure the degree of orientational order in the isotropic phase not only in cylindrical but also in spherical cavities and enclosures of irregular shape, where the standard approach based on quadrupolar splitting of the NMR spectrum fails.     

The hardening of NaCl single crystals by X-rays

The influence of X-ray irradiation and the following bleaching on microhardness and dislocation mobility was investigated. It was found that dislocations were pinned in irradiated samples and the pinning was not destroyed by illumination. The microhardness, increased by irradiation, increased still further by bleaching.     

On the role of solute atom — dislocation elastic interaction in solid solution hardening of Cu single crystals

Elastic interaction of solute atoms with dislocations has been reconsidered in Labusch's theory of solid solution hardening. New interaction parametersε are suggested and tested on dΤ/dc ^2/3 dependence onε ^4/3 for Cu base solid solutions.     

Electric stimulation of magnetoplasticity and magnetic hardening in crystals

The effect of electric field E on the magnetoplastic effect (MPE) has been investigated in NaCl crystals with different impurities, which provide either the plasticization of the samples in the magnetic field (positive MPE) or their magnetic hardening (negative MPE). The mobility of individual dislocations under the joint action of the magnetic and electric fields and the mechanical load on the crystals has been studied. The sharp electric stimulation of the MPE of both signs has been revealed, i.e., an increase or a decrease in the mean free path of dislocations that is roughly proportional to exp(±E/E ₀) at E ? E ₀ ～ 1–10 kV/m. In particular, in the negative-MPE NaCl(Pb) crystals, the accompanying electric field enhances the magnetic suppression of plasticity. The results are attributed to the electrically induced transformation of the additional part of the pinning impurity ions Me⁺⁺ to the magnetically active state of Me⁺ on the dislocations. The subsequent magnetic transformation of the structure of these pinning centers should lead to a sharper variation of the dislocation pinning force (either an increase or a decrease, depending on the MPE sign).     

Magnetically stimulated hardening of NaCl(Pb) crystals

Dislocation motion in NaCl(Pb) crystals under a pulsed mechanical load with and without a magnetic field is investigated. It is found that the dislocation mobility decreases when these crystals are deformed in a magnetic field. It is inferred that the observed magnetically stimulated hardening of NaCl(Pb) is due to a characteristic feature of spin-dependent electronic transitions in the dislocation-lead impurity system which increase the barrier for dislocation motion. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 226–228 (10 August 1999)     

Quasiparticle scattering induced by charge doping of iron-pnictide superconductors probed by collective vortex pinning

Charge doping of iron-pnictide superconductors leads to collective pinning of flux vortices, whereas isovalent doping does not. Moreover, flux pinning in the charge-doped compounds is consistently described by the mean-free path fluctuations introduced by the dopant atoms, allowing for the extraction of the elastic quasiparticle scattering rate. The absence of scattering by dopant atoms in isovalently doped BaFe2(As(1-x)P(x))(2) is consistent with the observation of a linear temperature dependence of the low-temperature penetration depth in this material.     

Analysis of strain hardening of crystals under large plastic strains

A strain-hardening mechanism is considered theoretically for crystals with large shear strains (γ=1–10). According to this mechanism, the emergence of the fourth and fifth stages in the strain-hardening curve for a crystal is associated with geometrically essential dislocations (GEDs) appearing as a result of accommodation processes accompanying large plastic deformations. Geometrically essential dislocations are concentrated in the boundaries of strongly disoriented fragments of the crystal. A comparison of the experimental and theoretical results leads to the conclusion that the density evolution of GEDs with deformation at the fourth and fifth stages is described by the same kinetic equation as for the evolution of the density of statistically random dislocations at the second and third stages, but with lower values of the kinetic coefficients.     

On the particle hardening in Cu-Cd alloy single crystals

The transmission electron microscopy of Cu-Cd single crystals deformed previously was performed to explain the hardening effect measured. The increment of the critical resolved shear stress is explained by the size misfit effect of the observed particles.