Yu. M. Chernov

Influence of the deformation type and medium on the mechanodynamic penetration of nitrogen molecules into surface layers of armco iron

The extraction of nitrogen molecules from deformed samples of armco iron with different initial structures (annealed and subjected to equal-channel angular pressing) and different deformation prehistories (deformation in liquid nitrogen at 77 K, rolling in air at room temperature, and their combination) has been studied. It has been shown that the preliminary deformation in liquid nitrogen increases its concentration in the surface layer of the material and shifts the principal peak of its release toward low temperatures during heating. The results are associated with the existence of different types of nitrogen traps in annealed and nanostructured armco iron and with their changes during subsequent deformation.

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.

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.

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.

Influence of the defect and structural state of FCC and BCC metals on the intensity of mechanodynamic penetration of helium atoms

The specific features of the mechanodynamic penetration of helium under plastic deformation into fcc (Cu) and bcc (Fe, Nb) metals with different initial defect structures (single-crystal, nanocrystalline, and porous samples) are investigated. The intensity of mechanodynamic penetration into these metals is shown to depend on the type of bonding (metallic or covalent), which determines the degree of localization of the plastic flow of these metals, as well as on the type of defect structure and on the character of plastic flow (dislocation deformation, twinning, grain-boundary sliding). Curves of helium extraction from samples at different strains are obtained. It is found that the helium release exhibits a wide variety of peaks depending on the degree and character of plastic deformation of the metals under investigation. This suggests that the metals contain different types of helium traps, which determine the content of helium and the specific features of its release in the temperature range studied.

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 ∼1094He 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.

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.

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.

Mechanodynamic diffusion of helium atoms into porous copper

Mechanodynamic penetration of helium atoms into porous copper compressively strained at 4.2 K is studied. Porous copper is obtained by vaporizing zinc out of brass in vacuum at a temperature of 800°C for 8 h. The number of helium atoms which penetrated into the sample increased monotonically with strain to reach 2.9 × 1016 atoms/cm2 at ɛ = 42%. This amount of helium is two and even more orders of magnitude larger than that obtained from the data available thus far on mechanodynamic penetration of atoms of an external medium into crystalline and amorphous materials under strain. The relations obtained suggest that specific types of helium traps determine the kinetics of mechanodynamic diffusion of helium into solids.

Effect of dynamic diffusion of air, nitrogen, and helium gaseous media on the microhardness of ionic crystals with juvenile surfaces

The load dependences of the microhardness of surface layers of NaCl and LiF ionic single crystals with juvenile surfaces and surfaces exposed to air for a long time measured in the air, nitrogen, and helium gaseous media have been investigated. It has been found that there is a change in the sign of the derivative of the microhardness as a function of the load for LiF crystals indented in helium and after their aging in air, as well as a weaker effect of the nitrogen and air gaseous media on the studied dependences as compared to NaCl crystals. It has also been found that, after the aging of the surface of NaCl crystals in air, there is a change in the sign of the derivative of the microhardness in the nitrogen and air gaseous media, as well as a pronounced change in the microhardness as a function of the time of aging the samples in air as compared to the weaker effect of the gaseous medium for LiF crystals. The obtained data have been analyzed in terms of the phenomenon of dislocation-dynamic diffusion of particles from the external medium into crystalline materials during their plastic deformation along the nucleating and moving dislocations. It has been shown that this phenomenon affects the microhardness through changes in the intensity of dislocation multiplication upon the formation of indentation rosettes in different gaseous media. The performed investigation of the microhardness of the juvenile surface of NaCl and LiF crystals in different gaseous media has revealed for the first time a different character of dislocation-dynamic diffusion of these media in a “pure” form.