Dynamic diffusion of helium into various types of solids during their deformation and dispersion

Quantitative relations governing the penetration of helium atoms into various types of solids in the course of their plastic deformation in liquid ³He (T = 0.6–1.8 K) and ⁴He (T = 4.2 K) and dispersion in gaseous helium at 300 K were obtained and analyzed. Experiments were carried out on metals with different lattice types, ionic single crystals, amorphous alloys, and barite and titanium dioxide powders dispersed in helium. Curves illustrating helium extraction from deformed specimens under dynamic annealing were obtained. The temperature range of helium extraction was found to correlate with the melting temperature and the initial and deformed structures of a material, which determine the number and character of helium traps present in the material. The dependence of helium penetration intensity on the type of defects forming under plastic deformation for various materials, as well as the formation of chemical bonds of helium atoms to the defected structure of these materials, is discussed.     

Analysis of dynamic dislocation-assisted penetration of helium into tin and cadmium deformed by stretching in liquid helium

Curves describing the liberation of helium from Sn single crystals deformed by stretching in liquid ³He and ⁴He and from Cd polycrystals in ⁴He are analyzed. It is found that the stress-strain diagrams for Sn are in qualitative agreement with the concentration of helium in the samples. The peaks in the amount of He liberated from Cd and Sn at temperatures both below and above the melting point are found to be of different types. The reasons for this difference are investigated, and the assumption concerning the existence of a chemical bond between helium atoms and structural defects of the metals under investigation is formulated.     

Mechanodynamic diffusion of nitrogen molecules into armco-iron under its deformation in liquid nitrogen medium

Data on mechanodynamic penetration of nitrogen molecules are obtained under deformation of armco-iron samples. It is shown that molecular nitrogen diffuses into the surface layer of samples under their deformation in a liquid nitrogen medium, and the nitrogen concentration compares well with the helium concentration in samples deformed in a liquid helium medium and in some cases exceeds the latter.     

Use of the helium medium for preparation of nano-sized powder materials by the example of industrial cement

Based on the phenomenon of the mechanodynamic diffusion of particles of the external medium in solids, a new in principle method has been proposed for the first time for producing nano-sized powder materials using industrial cement milled in a helium medium as an example. The temperature dependences of the extraction rate and the amount of helium in powders upon their heating in a temperature range of 20–1200°C have been obtained using mass spectrometry. It has been shown that milling of the cement powder of the M-400 brand using an MK-1 laboratory mill in helium leads to a considerable shift of its extraction curve towards lower temperatures compared with the air medium. The particle sizes of the powder milled in helium lie in a range of 5–10 nm, which is smaller than the powder size (～500 nm) after milling in the air medium by a factor of 100. The compression strength of cement samples obtained from the powders milled in helium increased by a factor of 2 compared with the strength of the samples from the initial material. The activation energies of helium extraction from the cement powders milled in helium and in air have been analyzed. The obtained results indicate a high efficiency of the method for producing nano-sized powder materials in the helium medium. The method can be used in the industrial scale based on the existing mill equipment with its minimal modernization.     

Specific features of helium penetration into single-crystal and nanocrystalline copper under deformation in liquid helium

This paper reports on experimental data on the penetration of helium atoms into single-crystal and nanocrystalline copper samples subjected to tensile and compressive strains at T=4.2 K, respectively. The dependences of the helium concentration N in the samples on the strain ? and the curves of helium extraction in the temperature range 300–1000 K at different strains ? are determined. It is found that the dependences N(?) and σ(?) correlate qualitatively with each other for single-crystal copper and do not correlate for nanocrystalline copper. This is associated with the different mechanisms of deformation in these samples. The deformation proceeds through the dislocation mechanism in single-crystal copper and through the jumpwise (twinning, rotational) mechanisms in nanocrystalline copper during local heating in regions of plastic shears. These factors are also responsible for the considerable difference between the curves of helium extraction from samples of both types. The curves of helium extraction exhibit two maxima for single-crystal copper and five maxima for nanocrystalline copper samples. The results obtained are discussed in terms of both the dynamic dislocation pipe diffusion and grain-boundary mechanisms of particle penetration from the surrounding medium into copper through different-type moving defects under applied stresses and due to the gradient of the chemical potential at the metal-surrounding medium interface.     

Dispersion of crystalline powder materials in gaseous media of different chemical compositions

The process of grinding of rutile and barite crystalline powders in a laboratory ball mill in different types of gaseous media (air, nitrogen, helium) has been investigated. Comparative evaluations of the intensity of the dispersion of these minerals have been performed and the particle sizes of powders obtained in different modes of their dispersion have been measured. A sharp increase in the intensity of this process in the helium medium as compared to the air and nitrogen media has been revealed and ultrafine-grained particles of barite powders have been obtained. The results of the performed investigations have demonstrated that the helium medium can be recommended for producing nanoparticles of powder materials in modern types of ball and bead mills with a drastic decrease in the time and energy consumptions required for their preparation.     

Mechanodynamic penetration of helium atoms into aluminum and its alloys under strain in liquid helium

The specific features of helium penetration into aluminum and its alloys, V95 and D16T, at a temperature of 4.2 K under uniaxial tension, compression, and reversal of the sign of the load are investigated. The role played by serrated strain in the intensity of the effect under consideration and the influence of impurities on the number of helium atoms penetrating into strained samples are elucidated. It is shown that the condition of additivity of the effect observed under successive reversal of the sign of the load depends on the specific features of the tensile and compressive strains.     

Investigation of the concentration gradient and extraction of helium atoms from copper deformed in a liquid-helium medium

A complex investigation of the penetration, accumulation, and extraction of helium atoms in porous copper samples deformed in a liquid-helium medium has been performed. The experiments have been carried out using three mass spectrometric techniques: (1) ionization of helium atoms by an electron impact in an MSCh-6 mass spectrometer, (2) secondary ion mass spectroscopy, and (3) an original high-resolution method with a sensitivity threshold of ∼10^{9 4}He atoms. The results obtained have made it possible to determine important characteristics of mechanodynamic diffusion of helium atoms, such as their penetration depth, the true concentration of helium trapped under deformation, and its gradient with an increase in the distance from the surface, as well as to estimate the binding energy of helium in traps.     

Heating of slip lines and bands as a quasi-athermic mechanism of plastic deformation of crystals at low temperatures

We discuss theoretically a mechanism of violation of the Arrhenius law for the rate of plastic deformation, on the one hand, and of the appearance of plateau-like segments in the temperature dependence of the thermal-activation parameters, on the other, during deformation of crystals at low (<10 K) temperatures, which is associated with heating of the crystal by slip lines and bands. Via a self-consistent solution of the heat-conduction equation with allowance for variation of its coefficients and the rate of plastic deformation with temperature it is found that both a stable and an unstable regime (in the thermal sense) of propagation of slip lines and expansion of slip bands are possible depending on the ratio between the heating level and the level of strain hardening of the strain localization sites. The first regime is associated with the appearance of quasi-athermic plateaus in the temperature dependences of the thermally-activation parameters, and the second one leads to an instability (stepped) in the plastic deformation that is characteristic at low temperatures. Fiz. Tverd. Tela (St. Petersburg) 40, 1479–1485 (August 1998)     

Mechanodynamic penetration of helium atoms into nanocrystalline iron

A relation between the characteristics of plastic deformation and the specific features of mechanodynamic penetration of helium into nanocrystalline iron compressively strained at 4.2 K is investigated. Iron samples with a grain size of about 200 nm are prepared by the multiple equal-channel angular pressing technique. The samples deformed in giant (6–7%) sample-averaged serrations, which amounts to several thousand percent strain in a shear band. The amount of helium in samples strained to various degrees is measured, and curves of helium extraction from these samples are obtained in the temperature range 300–1400 K. At a strain of ～50%, the amount of helium built up in a sample is found to be substantially higher (more than hundredfold) than that in samples subjected to lower strains. It is found that an increase in the strain rate gives rise to a strain within a serration (the strain localization is enhanced) and that the amount of accumulated helium decreases, most probably, because of the shorter deformation time. The helium extraction curves obtained with increasing temperature exhibit several peaks. The temperature positions of some of them are about the same for samples strained to different extents, while the other peaks are characteristic of samples subjected to a specific strain only. The results obtained suggest the existence of helium traps of different types, which depend on the original structure and the magnitude of the strain and differ both in the amount of helium they contain and in the temperatures at which helium is released from these traps.