The plastic deformation of potassium single crystals at low temperatures

The yield stress of potassium single crystals was measured in the temperature range of 1.5 to 30 K for a wide range of orientations. The effects of nobs itutional solutes (sodium and rubidium) on the yield strength of potassium single crystals were also investigated. The Schmid law of critical resolved shear stress for slip is invalkid in the temperature-dependent region of yield stress, and the orientation dependence of yield stress is in agreement with the Peierls mechanism for screw dislocation motion. The validity of the Schmid law may be questionable even in the athermal region, as suggested by the observations at 30K. Alloy softening is observed in the temperature-dependent region of yield stress by adding substitutional solutes to potassium.     

Lattice thermal conductivity of bismuth-antimony alloy single crystals at low temperatures

Thermal conductivity measurements in a number of bismuth-antimony alloy single crystals, with antimony concentrations from 1.72 at.% to 20 at.% are reported. Analysis of the results shows that in this alloy system, below about 25 K, the heat carriers are predominantly phonons. Callaway's theoretical model for lattice thermal conductivity has been utilized for the analysis of the results, using various phonon-scattering processes, and this model has been found to give a good fit to the experimental data, particularly below 25 K, where lattice conduction predominates. The Casimir model for finding the characteristic length for boundary scattering fails to explain the experimental results. This indicates the existence of microscale fluctuations in the composition of the alloy.     

Observations of crack healing in sodium chloride single crystals at low temperatures

Journal of Materials Science -     

Temperature and concentration dependence of the critical resolved shear stress of cadmium-zinc alloy single crystals

The increase of the critical resolved shear stress of cadmium single crystals by additions of zinc has been investigated in the temperature range 77 to 295 K. The temperature dependence of the critical resolved shear stress can be divided into two temperature regions. At all temperatures the critical resolved shear stress was found to increase withc ^2/3 wherec is the atomic concentration of zinc as solute. The concentration dependence of the plateau stress is explained according to the theory of Labusch [5].     

Theory for saturation stress difference in torsion versus other types of deformation at low temperatures

Torsion tests carried out to large plastic strains typically exhibit a leveling out (saturation) of the flow stress. On the other hand, saturation is usually not observed for other modes of deformation such as wire drawing or rolling. In iron and other body-centered cubic materials linear hardening persists to strains of $\text{≈ 10}$. For face-centered cubic materials the behavior is mixed, with saturation not being achieved for most impure metals. We propose an explanation for the difference in flow stress at large strains between torsion and other modes of deformation. The explanation is based on Holt's dislocation cell wall formation theory and on the fact that apparently dislocations of only one Burgers vector are required to be active in torsion at large strains, whereas several are required for other deformation modes. Consequently, at the same dislocation density and applied stress level dislocations of opposite character but of the same kind are closer together in torsion. The dislocations have a stronger force of attraction between them and a faster rate of mutual annihilation through pipe diffusion controlled climb. The saturation stress is larger the larger is the number of different Burgers vectors of the dislocations involved in the deformation. Shear band instabilities are explained as a consequence of material locally reducing the number of active slip systems.     

Ultrasonic fatigue of nickel-chromium alloys at low temperatures

The influence of low temperature ultrasonic deformation on the electrical resistivity and magnetic properties of nickel-chromium dilute solid solutions is examined. It is shown that changes in the electrical resistivity and magnetic properties of prequenched samples after ultrasonic fatigue can be explained by order-disorder processes during fatigue. It is concluded that ordering at low temperatures during the ultrasonic deformation is associated with high strain rate and high vacancy concentration produced by deformation.     

Hypervelocity crater formation in aluminum alloys at low temperatures

The penetration and perforation of three kinds of aluminum alloys at room temperatures and low temperatures in the velocity range from about 0.5 to about 3.7 km/s were investigated experimentally. Main interests were focused on the depth and diameter of craters and their relations to the impact velocity. As a result, very distinctive features of the temperature effect on the shape and size of craters were found. Also, the effect of impact-induced phase transition of projectiles on the crater formation was examined about carbon steel, aluminum alloy and NaCl projectiles.     

Elastic properties of two titanium alloys at low temperatures

Sound velocities and elastic constants were determined semi-continuously for two annealed polycrystalline titanium alloys between 4 and 300 K. Results are given for: longitudinal sound velocity, transverse sound velocity. Young's modulus, shear modulus, bulk modulus, Poisson's ratio, and elastic Debye temperature. A pulse-superposition technique was used.     

The lattice thermal conductivity of alloys at low temperatures

The scattering of phonons by solute atoms in concentrated, binary, substitutional alloys is re-examined, and the corresponding thermal resistivity at low temperatures is calculated for several alloys. Satisfactory agreement with experimentally determined values is obtained. It is shown that the solute atoms are relatively inefficient scattering centres in concentrated alloys compared with their behaviour in dilute alloys.     

A method for the preparation of He3 single crystals at ultralow temperatures

A method for the preparation of He₃ single crystals at ultralow temperatures is proposed. The method is based on the shift of the melting curve when a magnetic field is applied. With the use of the proposed technique, it is thought possible to grow single crystals by the Bridgman or the zone-melting method. A trial of this method is being planned and the results will hopefully be published soon.