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.     

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.     

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.     

Metallic alloys suitable for YBCO melt-texturing at low temperature

Ag-Au-Pd alloys, having a gradient in the concentration of the elements (called diffusion alloys), were investigated in order to find homogeneous alloys suitable for melt-processing of YBCO superconductor. Interface aspects between the superconductor and diffusion alloys and composition profiles of the diffusion alloys after melt texturing are presented. The results obtained with the diffusion alloy led to the development of new ternary homogeneous Ag-Au-Pd alloys of composition (at %) Ag-(18–23)Au-(2–10)Pd. The interface between homogeneous alloys and the YBCO superconductor after melt-processing was characterized and the resistivity-temperature curve obtained with electrical contacts on the metallic part of the composite is shown.     

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.     

Considerations on the selection of alloys for use in pressure cells at low temperatures

Issues surrounding the choice of metals subject to large tensile stresses in a cryogenic environment are discussed. Problems with the use of materials under such conditions often arise from their loss of ductility and toughness. The focus of the present article is therefore mainly on these properties; their dependency on temperature, crystal structure, grain size and purity level; and their application to the quantitative prediction of fracture behaviour. It also discusses the selection of alloys (on the basis of their toughness, ductility and freedom from flaws) that can be used safely under ambient and cryogenic conditions. A recent proposal to employ a particular titanium alloy for the above purpose is considered, in light of the tendency of metals with a bcc crystal structure (including β-phase titanium alloys) to become brittle at low temperatures. Other materials, which are known to be (or are likely to be) satisfactory under such conditions are examined. For example, certain platinum alloys may possess very high cryogenic tensile strengths and ductilities, and low magnetic susceptibilities. Nanocrystalline metals have the potential to provide these characteristics, as well as low specific heats and high thermal conductivities. Such properties could make nanocrystalline materials very useful in (for example) work at microkelvin temperatures, or more generally where very accurate cryogenic temperature control is needed. Sources of cryogenic mechanical property information on a wide range of materials are also presented.     

New features of the low temperature ductile shear failure observed in bulk amorphous alloys

Fractographic studies of ductile shear failure under the uniaxial compression for rod–like samples of the Zr_41.2Ti_13.8Ni₁₀Cu_12.5Be_22.5 and Cu₅₀Zr₃₅Ti₈Hf₅Ni₂ bulk amorphous alloys at temperatures 300 and 77 K are presented. The mechanisms of shear deformation and failure appeared to have characteristics in common with other amorphous alloys prepared in the form of thin ribbons. However, there were a number of new fractographic features observed due to the bulk character of the samples and to the large supercooled liquid region of these alloys.