Suchergebnisse

Microhardness tests of samples of technically pure lead are carried out without and with exposure to external magnetic fields with inductions of 0.3 T, 0.4 T, and 0.5 T. The dependences of the C2-grade lead surface microhardnesses on the exposure times in a magnetic field are obtained, thus reflecting the influence of the magnetic field on the plastic characteristics of lead. The exposure time at which the maximum effect on microhardness occurs is revealed. Additional microhardness tests are carried out for cases with magnetic fields with inductions of up to 0.3, 0.4, and 0.5 T and exposure times of 0.25, 0.5, and 1 hour. Following the obtained microhardness data, the plasticity parameters of lead samples in their initial state and after exposure are calculated. The dependences of the plasticity parameters on the exposure times are shown. The behavior of the plasticity parameter of lead samples during their exposure to external magnetic field with induction up to 0.5 T is revealed. The percent changes in the values of microhardness depending on the magnetic field induction are shown.

In this paper, the properties of rail steel subjected to fracture under uniaxial tension deformation are studied. Mechanical properties of rail steel, defective substructure of lamellar pearlite, and fracture surface of rail steel are investigated using methods of modern physical materials science.
Tensile strength is found to vary from 1247 to 1335 MPa, and the fracture strain of the samples changes from 0.22 to 0.26. The deformation of the rail steel is accompanied by the process of breaking the ferrite plates into fragments by low angle boundaries along with the significant increase in the scalar dislocation density to 7.9-1010 cm-2 (the scalar dislocation density of the original rail steel is 3.2 -1010 cm-2). Also, there is a formation of internal stress fields appearing in the form of bend extinction contours. The sources of stress fields are identified. It is revealed that cementite plates are fractured by cutting and dissolution with subsequent transfer of carbon to ferrite plates by moving dislocations and formation of round-shaped nanodimensional (8.3 nm) round shaped particles of tertiary cementite. It is shown that the dissolution of cementite plates is accompanied by fragmentation (into coherent scattering regions with an average size of 9.3 nm).

An external high-energy impact on the surface of a solid, for example, by ultrashort ultra-high-power laser pulses, can lead to phase transitions, as a result of which the surface structure undergoes cardinal changes. This paper presents the results of molecular dynamics modeling of changes in the surface layer of the computational cell under a short-term high-energy impact. The model constructed and described in the paper, in which the temperature of the computational cell is distributed in accordance with the solution of the linear problem of heat conduction, made it possible to reveal a violation of the continuity of the surface layer, which consists in the localization of excess free volume in the form of a group of spherical pores. The sizes of these imperfections, as well as the duration of their existence, have differences when modeling different energy densities of laser radiation. Further research made it possible to reveal the conditions under which the pores remain stable throughout the entire simulation time, as well as to reveal the relationship between the crystallographic orientation of the "solid — liquid" interface and the sizes of the formed pores.

The paper concerns improving the microhardness and wear resistance of steel 45 by the combined treatment of electroexplosive borocoppering with the subsequent electron-beam treatment. It is found that surface roughness at the area of the electroexplosive treatment increases along with the absorbed power density and the mass of boron powder. The electron-beam treatment leads to a decrease of roughness and appearance of craters instead of radial melt flow traces. The depth structure of the electroexplosive alloying area with a thickness of 25 µm includes a coating layer, near-surface, intermediate, and boundary layers. The surface microhardness and the depth of the hardening zone after the electroexlosive alloying increase along with the absorbed power density and boron concentration and reach the values of 1400 HV The electron-beam treatment causes merging of the coating and the surface layers and increases the hardening zone depth up to 80 µm. A cellular or dendritic crystallization structure is formed near the surface, and a grain structure is formed in the depth. The inhomogeneous distribution of alloying elements over the volume of the alloying area and its adjustment during the electron-beam treatment are established. The inter-dendritic distances and grain diameters increase as the absorbed power density becomes higher with the increase of the electron-beam treatment exposure time. Also, the size of martensite needles increases in the depth. The combined treatment produces the sub microcrystalline strengthening phases-borides FeB, Fe2B, FeB2, carboboride Fe23 (C, B)6 , and carbide B4C. The microhardness level is reduced to 800 HV, and the wear resistance increases up to five times when compared to the wear resistance of the base.

In this paper, the structure, phase composition, dislocation substructure, and carbon redistribution in differentially quenched 100-meter rails after the ultra long-term operation (passed gross tonnage since 2013 — 1770 mln. tons) are studied using modern materials science and engineering approaches. The regularities of main parameters variations, such as changes in a relative variety of different morphological structures, scalar and excessive dislocation density, volume fractions of cementite, and redistribution of carbon atoms, are identified at the rail head cross section along the central axis and fillet rounding radius at the distances of 0, 2, 10 mm from the surface. It is suggested that the decrease of volume fraction of carbide phase in head surface layer can be related to the decarburization and appearance of carbon atoms at the steel structure defects. It is noted that transformations along the central axis proceed slower in relation to changes of the rounding radius of the fillet.

The quantitative estimation of strengthening mechanisms of rails' surface layer is carried out on the basis of regularities and formation mechanisms of structure-phase states revealed by the methods of modern physical materials science. It is performed at different depths of the rail head along the central axis and fillet of differentially quenched 100-meter rails after the extremely long-term operation (gross passed tonnage of 1411 mln tons). A long-term operation of rails is accompanied by the formation of structural constituent gradient consisting of a regular change in the relative content of lamellar pearlite, fractured pearlite, the structure of ferrite-carbide mixture, scalar, and excess dislocation density along the cross-section of the rail head. As the distance to the rail fillet surface decreases, the relative content of metal volume with lamellar pearlite decreases. However, the relative content of metal volume with the presence of the fractured pearlite structure and ferrite-carbide mixture increases. The contributions caused by the matrix lattice friction, intraphase boundaries, dislocation substructure, presence of carbide particles, internal stress fields, solid-solution strengthening, pearlite component of steel structure are estimated. It is shown that the main mechanism of strengthening in the surface layer is due to the interaction of moving dislocations with low-angle boundaries of nanometer dimensional fragments and subgrains. The main dislocation strengthening mechanism in a near-surface layer at a depth of 2-10 mm is due to the interaction of moving dislocations with immobile ones.

High-entropy alloys are a new class of materials consisting of at least five elements in an equiatomic or close to equiatomic ratio, which provides them with unique properties. Non-equiatomic high-entropy Fe-Co-Cr-Ni-Mn and Fe-Co-Cr-Ni-Al alloy coatings were applied to the 5083 alloy substrate using wire arc additive manufacturing and the cold metal transfer process. The structure, elemental composition, mechanical and tribological properties of coating / substrate systems were analyzed using modern methods of materials physics. The deposition of FeCoCrNiMn and FeCoNiCrAl HEA coatings on the surface of 5083 alloy was accompanied by the formation of gradients of elemental composition and mechanical properties. A transition layer with a thickness up to 450 μm was formed at the coating / substrate interface located at the coating-substrate boundary. The elemental composition gradient of the transition layer was studied, and a high level of chemical homogeneity of the coating was revealed. Alloying of the coating with substrate elements was observed. The alloying of the substrate with coating elements is accompanied by nonmonotonic changes of element composition in the 500 μm depth layer.