Volume 60, Nº 7-8 (2018)

The effect of the mode of isothermal quenching on the special features of formation of bainitic structures in high-strength cast irons with globular graphite is studied. The regular features of the influence of isothermal quenching on the mechanical properties of the alloyed irons are determined. It is shown that the process of formation of bainite in the irons is affected considerably by the austenitization temperature. Control of this temperature makes it possible to control the mechanical properties of the metal.

The effect of silicon on the transformations of austenite under cooling, the content of silicon and carbon in the austenite, the conditions of precipitation of structurally free carbon in the form of globular graphite, and the formation of microstructure in chromium-nickel iron is studied for attaining a high level of mechanical properties in cast condition.

The composition of wear-resistant Al/AlC/a-C:H coatings deposited on a magnesium alloy by combination of radio frequency plasma assisted chemical vapor deposition and magnetron sputtering is studied. It is shown that the mechanical properties of the coatings vary depending on the composition of the amorphous C:H layers after changing the negative bias polarization of the process from 150 to 600 V; the carbon structure may be varied from a disordered one to a graphite one as well as the sp²/sp³ bonds ratio.

Variation of the sizes and chemical compositions of martensite plates in the structure of powder-metallurgy shape memory alloy Fe – 14 wt.% Mn – 6 wt.% Si – 9 wt.% Cr – 5 wt.% Ni is studied. Apart of the specimens is fabricated from mechanically alloyed powder. Tensile tests of the alloy are performed after 4% deformation and after deformation and heat treatment at 700 and 1100°C. The effect of the treatment on the structure of the alloy is determined.

Titanium aluminide is synthesized in a chemical reaction of elemental powders. The synthesis is conducted at a high temperature in vacuum for two variants of pressure variation. To obtain a quality product at a minimum loss of the material to spillage, it is important to apply pressure at the right moment. The reaction synthesis is also accompanied by “swelling” of the billet. The temperature-pressure cycle is optimized. The results of the x-ray diffractometry show the presence of an aluminum-rich phase in the form of a shell surrounding the particles of pure titanium.

Sm – Co – Fe – Cu – Zr sintered high-temperature permanent magnets (HTPM) produced by the “POZ-Progress” Company are studied. The microstructure of the magnets is determined by scanning electron microscopy and x-ray diffraction analysis. The Curie temperature and the magnetic properties of the materials are described. The hysteresis loops of the magnets are measured in strong pulsed fields at room and elevated temperatures. The temperature dependence of the magnetic susceptibility is plotted using the method of compensated transformer in a variable magnetic field.

Processes of reversal magnetization in Sm(Co, Cu, Fe, Zr) _7.4-type alloys are simulated on the basis of the assumption that this process is determined by difficulty of detachment of domain walls from the places of their fixation. The behavior of the material is studied under magnetization after different methods of demagnetization, i.e., cyclic and thermal ones, and by a negative field. The obtained distributions of the fixing fields are used to plot minor loops for actual magnets. The computed and experimental data are compared.

The crystal structure, the temperature and field dependences of the magnetization M and of the magnetic contribution into the entropy ΔS_m, the temperature dependences of the high-field susceptibility χ and the heat capacity C_p of specimens of polycrystal R(Co_{1 – x} Fe_x)₂ compounds, where R is Gd, Dy, Ho or Er, are studied. The width of the ΔS_m peak at half-height of its maximum (ΔT_FWHM) is estimated. The dependences of ΔT_FWHM and of the temperature behavior of ΔS_m of the specimens on the magnetic field, on the iron content (x ) and on the atomic number of the element R are determined. The causes of the broadening of the ΔS_m (T ) peak upon substitution of cobalt with iron in the R(Co_{1 – x} Fe_x)₂ compounds are considered.

The method of slow cooling from melting temperature is used to obtain coarse-grain ingots of (Fe_xCo_{1 – x})₂B with columnar structure. It is shown that at room temperature the configuration of the domain structure of the specimens of (Fe_xCo_{1 – x})₂B corresponds to magnetocrystalline anisotropy (MCA) of different types. Alloys Fe₂B, (Fe_0.4Co_0.6)₂B, (Fe_0.2Co_0.8)₂B and Co₂B exhibit MCA of the type of a “plane of easy axes.” Alloy (Fe_0.92Co_0.08)₂B has a state with spin-reorientation transition of type “easy axis” – “plane of easy axes”.

The possibilities of application of known models of domain structure to multiphase alloys are analyzed with the aim to assess the anisotropy constants and to search for magnetically uniaxial highly anisotropic phases. It is shown in what cases and how the standard models of domain structure may be applied to such alloys. Recommendations on the necessary amendments of the model to be applied to multiphase materials are given.

The methods of optical, electron, and atomic force microscopy are used to study the morphology of bainite formed in high-strength structural steel 25G2S2N2MA (HY-TUF) under isothermal quenching. It is shown that the temperature of the transformation affects the substructure of the bainite, which is represented by ordered layers of plates with a width obeying a lognormal distribution law. It is shown that the bainite plates consist of nanosize subplates, the size of which is determined by the temperature of the bainitic transformation.

The aging kinetics, the mechanical properties, and the structure of cast alloy IMV7-1 of the Mg – Y – Gd – Zr system alloyed additionally with dysprosium are studied. It is shown that yttrium and gadolinium may be replaced partially with dysprosium without changes in the aging kinetics and lowering the hardening and the level of the mechanical properties of alloy IMV7-1.

Results of a complex thermochemical treatment of test specimens from different heats of a novel high-strength heat-resistant precipitation-hardening steel microalloyed with REM are presented. The optimum chemical composition of the steel is determined with the aim to provide precipitation hardening after vacuum carburizing. In the process of finishing nitriding in glow discharge the surface hardness of the new steel increases substantially at inconsiderable decrease in the hardness of the core. The developed modes of complex thermochemical treatment exclude the appearance of such flaws as grain boundary net and excess retained austenite.

Schemes for creating rationally directed stresses of contact friction in operations of compaction of preforms are suggested for lowering the residual porosity of powder parts and raising the efficiency of their subsequent heat treatment.

The properties of railroad rails produced from bainitic steels are compared to those from traditional pearlitic steels. Formation and propagation of contact fatigue cracks in the rails is considered. It is shown that bainitic steels exhibit a better combination of strength and ductility characteristics than pearlitic steels.

Causes of reversible changes in the coercivity of alloys for permanent magnets as a result of cyclic heat treatment are analyzed. The magnetic properties and the metallographic and magnetic domain structures of alloy Sm(Co_0.65Fe_0.26Cu_0.07Zr_0.02)₇ are studied as a function of the temperature of interruption of the process cooling. The own results and data of foreign authors are used for developing a scheme of the processes occurring in the alloy in order to explain the mechanism of the “damage – restoration” phenomenon in alloys of the Sm – Co – Cu – Fe – Zr system of type Sm(Co_0.65Fe_0.26Cu_0.07Zr_0.02)₇.