S. A. Firstov

Kinetics of sulfur surface segregation in Fe-6at%Si

Abstract The kinetics of Si and S segregation to the (100) surface in Fe-6at%Si single crystals has been measured by Auger electron spectroscopy for the temperature range from 770 to 900 K. A rather unexpected behaviour for sulfur was found which was not controlled by bulk diffusion. Sulfur segregation kinetics can be explained by taking into account both sulfide formation as well as pipe diffusion. At higher sulfur surface coverage the reaction rate drops. The latter may be a consequence of a surface phase transition. Moreover, the model is extended to such cases where the history of the sample should also be considered. Thus, the experiments of the segregation sequence can be described.

Effect of dislocation structure on fracture toughness of strained BCC-metals

Abstract Mechanical properties of materials depend on their structure. Examined are the effects of dislocation structure on fracture toughness and mechanisms of fracture of BCC-metals. Fracture toughness was determined by depending specimens with cracks introduced into the plane perpendicular to the plane of rolling. Fracture toughness increases with decreasing yield stress (for e=15–25%). This is due to instability of slightly misoriented cell structure under repeated loading. The peak of fracture toughness at the temperature 77 K was not observed. The increase of fracture toughness for high strained metals (e>60% for Mo, and e>85% for Cr) corresponds to cell size reduction and the change of fracture mechanisms.

Effect of Electron Density on Phase Composition of High-Entropy Equiatomic Alloys

A series of high-entropy equiatomic alloys have been analyzed to determine the main factors that influence the formation of various solid solutions and chemical compounds. The key factor leading to the formation of phases in high-entropy equiatomic alloys is mean electron density (e/a). The necessary condition for the high-entropy σ-phase to emerge is the presence of elements forming it in two-component alloys in various ratios, the electron density of the alloy is to be between 6.7 and 7.3 e/a. The Laves phase shows up in the high-entropy equiatomic alloys at a mean electron density of 6–7 e/a in the presence of atoms differing by more than 12% in size and having mixing enthalpy lower than −30 kJ/mol. It is revealed that the lattice parameter in bcc high-entropy equiatomic alloys influences their elastic modulus and hardness.

Ultimate Strengthening, Theoretical and Limit Tool Hardness

The influence of passing from a microcrystalline to a nanocrystalline structure on the mechanical properties of chromium deposited by magnetron sputtering is studied. The possibility of additional strengthening nanomaterials due to enrichment of grain boundaries by “useful” additives elements is established. A wide spectrum of materials in different structural states was investigated by the method of micromechanical tests. The notions of the “theoretical” hardness (largest hardness for the material) and “limit tool” hardness, connected with tool limitations in indentation, are introduced.

Toughness characterization of iron-based powder material

Abstract Fracture mode dependence on the material structure is applied to characterize the fracture toughness of iron-based powder material. The porosity parameter is varied to exhibit the change in the observed fracture mechanism and hence the fracture toughness of powder materials. Useful information is obtained to enhance the production technology in terms of porosity content and inter-particle contacts. The proposed simulation procedure provides data that are in good agreement with experiments and makes it possible to predict the mechanical behavior of powder materials with different structures. Complex dependence of fracture toughness on the porosity of powder materials is reflected by competing microfracture mechanisms and inherent peculiar behavior of the compacting powder.

Properties of coatings of the Al–Cr–Fe–Co–Ni–Cu–V high entropy alloy produced by the magnetron sputtering

It has been found that coatings from an Al–Fe–Co–Ni–Cu–Cr–V high entropy equiatomic alloy produced by the magnetron sputtering have nanocrystalline microstructures, are textured, and present a solid two-phase solution, which crystallizes in the bcc (a = 2.91 Å) and fcc (a = 3.65 Å) phases. The ion bombardment of a growing coating caused by the bias voltage (0–(–200) V), which has been applied to the substrate, decreases the growth rate of a condensate and affects its composition and structure. It has been shown that the composition of coatings deposited without an ion bombardment coincides with the target composition, whereas an increase of the ion bombardment intensity leads to the depletion of the coating composition in Al, Cu, and Ni and increase the microhardness. The anisotropy of the coating produced has been revealed.

Superhard Vacuum Coatings Based on High-Entropy Alloys

High-entropy TiZrVNbTa, AlCrFeCoNiCuV, and TiZrHfNbTaCr alloy coatings with a thickness of 2.5–6 μm and with various phase compositions were deposited by dc magnetron sputtering. The chemical and phase composition of the TiZrVNbTa and TiZrHfNbTaCr coatings do not change substantially during deposition. Only when the substrate bias is higher than –180 V, the deposited AlCrFeCoNiCuV alloy coatings are depleted of Al and Cu. All the coatings are nanostructured, and their microhardness varies between 10 and 19 GPa, and reduced elastic modulus changes between 106 and 192 GPa depending on the phase composition.

Features of phase and structure formation in high-entropy alloys of the AlCrFeCoNiCux system (x = 0, 0.5, 1.0, 2.0, 3.0)

Alloys of the AlCrFeCoNiCux system (x = 0, 0.5, 1.0, 2.0, 3.0) were smelted by argon-arc smelting in pure argon. The phase composition and structure of fabricated alloys are investigated and their mechanical properties are determined. The results showed that an increase in the amount of copper in alloys leads to a change in the phase composition from single phase (bcc) to three phase (bcc + fcc1 + fcc2), which is accompanied by the structural change from coarse-grain polygonal structure to complex dendritic structure (primary dendrites (DR) + secondary dendrites (SDR) + interdendrite phase (ID)). The region of electron concentrations of alloys, in which bcc and fcc phases are present simultaneously, is determined. The limiting electron concentration of stability of the bcc lattice is found experimentally. Microhardness is measured and Young moduli of alloys over the entire range of varying the copper concentration are determined.

Gradient Structure Formation by Severe Plastic Deformation

Severe plastic deformation (SPD) techniques are the best for producing of massive nanostructured materials. The methods of equal channel angular pressure (ECAP) and twist extrusion (TE) are realized by simple shear uniform deformation without change of cross-section sizes of sample. In the case of roll forming (RF) the shear strain is localized in the near-surface layer of metal. Intensity of shear strain in the near-surface layer depends on variation of parameters of deformation and conditions of friction in a contact. Steel 65G (0.65C, 0.3Si, 0.6Mn, 0.3Cr, and 0.3Ni) was deformed by roll forming. Transmission electron microscopy (TEM) of “cross-section” samples was used for studying of gradient structure of deformed material. TEM investigation shown that cell substructure in a near-surface layer have been formed. The depth of deformed layer is approximately 40 micrometers. Average cell size in cross-section direction is about 100 - 200 nm. Thin nanostructure layer with cell size about 20-30 nm was detected. In our opinion such substructure formed due to effect of “good” impurities.

Structural Changes in Iron upon Large Plastic Deformations and Their Influence on the Complex of Its Mechanical Properties

The influence of the degree of preliminary strain, obtained by equal-channel angular pressing, on the character of work hardening of highly deformed armco-iron is investigated. The effect of the deformation on the evolution of dislocation and cellular structures is analyzed. The structural sensitivity of the mechanical properties is studied.