Physics of Metals and Metallography (v.116, #6)

Study of magnetic properties and structural and phase transformations in the Co-19 at % Al-6 at % W alloy by N. V. Kazantseva; N. N. Stepanova; M. B. Rigmant; D. I. Davydov; D. A. Shishkin; S. L. Demakov; M. A. Ryzhkov; E. P. Romanov (531-537).
The Co-19 at % Al-6 at % W alloy prepared by two methods of melting in an inert atmosphere, namely, by arc melting followed by casting into a water cold copper mold and induction melting followed by casting in a ceramic (Al2O3) mold, has been studied. It was found that the phase composition of the alloy and its magnetic properties depend on the cooling rate of ingot after melting. Samples are ferromagnetic up to 800°C; the specific magnetization equal to σ= 10 emu/g is retained up to ∼700°C. The Curie temperatures of all phases found in the alloy have been determined. It was found that the formation of the Co7W6 phase in the alloy increases the coercive force of the alloy, whereas the saturation magnetization demonstrates a 1.5-fold decrease.
Keywords: magnetic properties; cobalt alloys; phase transitions

Methods of optimizing the magnetic domain structure and functional characteristics of electrical-sheet anisotropic steel based on the Fe-Si alloy and the laminated magnetic circuits of transformers have been developed. The use of an efficient complex based on the local laser treatment and application of magnetically active electrically insulating coatings provided a significant reduction in the magnetic losses in the sheets of the steel (by 18–22%) and in the magnetic circuits of transformers (by 9–14%).
Keywords: laminated magnetic circuit; electrical-sheet steel; magnetic domains; magnetic losses; laser treatment; local pressure

Using different prediction methods, such as the General Solution Model of Kohler and Muggianu, the excess energy and activities of molybdenum for the sections of the phase diagram for the penternary Ni-Cr-Co-Al-Mo system with mole ratios xNi/xMo = 1, xCr/xMo = 1, xCo/xMo = 1, and xAl/xMo = r = 0.5 and 1, were thermodynamically investigated at a temperature of 2000 K, whereas the excess energy and activities of Bi for the section corresponding to the ternary Bi-Ga-Sb system with mole ratio xGa/xSb = 1/9 were thermodynamically investigated at a temperature of 1073 K. In the case of r = 0.5 and 1 in the alloys Ni-Cr-Co-Al-Mo, a positive deviation in the activity coefficient was revealed, as molybdenum content increased. Moreover, in the calculations performed in Chou’s GSM model, the obtained values for excess Gibbs energies are negative in the whole concentration range of bismuth at 1073 K and exhibit the minimum of about −2.2 kJ/mol at the mole ratio xGa/xSb = 1/9 in the alloy Bi-Ga-Sb.
Keywords: geometrical model; Redlich-Kister coefficients; excess Gibbs energy; activity coefficient; thermodynamic model

Phases and defects upon the solidification of nitrogen-alloyed stainless steels by A. G. Svyazhin; L. M. Kaputkina; V. E. Bazhenov; Z. Skuza; E. Siwka; V. E. Kindop (552-561).
One of the technological problems in alloying molten steels with nitrogen is the precipitation of nitrogen into the gas phase upon the solidification of steels and the formation of nitrogen bubbles and porosity in steel ingots with the result that potentials of nitrogen as an alloying element are used incompletely. The formation of bubbles and pores in nitrogen-alloyed stainless steels occurs heterogeneously and is limited by the desorption rate of nitrogen from the molten-metal surface into bubbles. For this reason, it is convenient to determine the critical nitrogen concentration in steels as the concentration at which in the process of equilibrium solidification the content of nitrogen in the residual liquid phase at all temperatures of semisolid state does not exceed its solubility in the residual liquid at a given pressure. Hereafter, this concentration should be refined experimentally for concrete conditions of solidification. To calculate the nitrogen concentration in the arising phases, a Thermo-Calc software can be employed.
Keywords: stainless steel; solidification; nitrogen alloying; gas bubbles

The effect of alloying with chromium and manganese nitrides is studied on a fine crystal structure of powder iron produced by hot forging. The features of the fine structure and the phase composition are found to strongly depend on the kind of alloying nitrides. It has been shown that the introduction of both nitrides in the initial composition of powder mixture causes an increase in the lattice parameter of a matrix, its defectiveness, and the dislocation density, which results in an increase in the hardness of steel alloyed with nitrides. The defectiveness of the matrix crystal lattice, the dislocation density, and the hardness of hot-forged steels are slightly higher when manganese nitride is used as a nitrogen-bearing additive.
Keywords: nitride; structure; alloying; powder steel; solid solution; lattice defectiveness; dislocation density; hot forging

Analysis of the heat capacity of nanoclusters of FCC metals on the example of Al, Ni, Cu, Pd, and Au by Yu. Ya. Gafner; S. L. Gafner; I. S. Zamulin; L. V. Redel; V. S. Baidyshev (568-575).
The heat capacity of ideal nickel, copper, gold, aluminum, and palladium fcc clusters with diameter of up to 6 nm has been studied in the temperature range of 150–800 K in terms of the molecular-dynamics theory using a tight-binding potential. The heat capacity of individual metallic nanoclusters has been found to exceed that characteristic of the bulk state, but by no more than 16–20%, even in the case of very small clusters. To explain the discrepancy between the simulated data and the experimental results on the compacted metals, aluminum and palladium samples with 80% theoretical density have also been investigated. Based on the simulation results and analysis of the experimental data, it has been established that the increased heat capacity of the compacted nanomaterials does not depend on the enhanced heat capacity of the individual clusters but rather, can be due to either the disordered state of the nanomaterial or a significant content of impurities (mainly, hydrogen).
Keywords: fcc metals; computer simulation; molecular dynamics; tight binding; heat capacity

Peculiarities of the formation of multicomponent AlN-TiB2-TiSi2 composite ceramics coatings during heat treatment by A. D. Pogrebnyak; Yu. A. Kravchenko; A. A. Dem’yanenko; O. V. Sobol’; V. M. Beresnev; A. V. Pshik (576-585).
Results of studies of the morphology, elemental and phase compositions of coatings prepared by pulsed magnetron sputtering of the AlN-TiB2-TiSi2 composite ceramic target have been reported. The used technology allows us to prepare a protective amorphous-like layer with ordering areas with sizes up to 1 nm. The heat treatment of samples at 900 and 1300°C leads to the depletion of the coating surface of boride phases and the formation of α-Al2O3- and β]-TiO2-based protective films on the surface. The growth of crystallites of the nanostructured coating to 11–25 nm is observed. The annealing at 1300°C allows us to obtain the thermally stable crystalline state of substance, the nanohardness of which is 11 GPa.
Keywords: nanocomposite coating; pulsed magnetron sputtering; high temperature annealing; phase and elemental composition; amorphous-like structure; nanohardness

The molecular-dynamic method has been used to study the interaction of lattice vacancies with symmetrical grain boundaries (GBs) in aluminum. The fraction of trapped vacancies has been found to depend linearly on the distance to the GB plane. The average velocity of the vacancy migration toward the boundary decreases exponentially with an increase in the distance between the GB plane and vacancy. The radius of the region of trapping of a vacancy by the boundary is limited to two to three lattice parameters and grows with an increase in temperature. Four types of boundaries, which are characterized by different capability for the trapping of vacancies, have been determined.
Keywords: grain boundary; vacancy; computer simulation; method of molecular dynamics

The deformation behavior of copper under conditions of high-strain-rate deformation has been investigated based on the model of elastoplastic medium with allowance for the kinetics of plastic deformation. Data have been obtained on the evolution of the dislocation subsystem, namely, on the average dislocation density, density of mobile dislocations, velocity of dislocation slip, concentration of deformation-induced vacancies, and density of twins. The coefficient of the annihilation of screw dislocations has been estimated depending on pressure and temperature. It has been shown that severe shear stresses that arise upon high-strain-rate deformation can lead to a significant increase in the concentration of vacancies. The time of the dislocation annihilation upon their nonconservative motion has been estimated. It has been shown that this time is much greater than the time of the deformation process in the samples, which makes it possible to exclude the annihilation of dislocations upon their nonconservative motion from the active mechanisms of deformation.
Keywords: elastoplastic medium; dislocations; vacancies; twins

In this article, we give a preferential regime of selective laser melting for the production of parts from a cobalt superalloy using a PTK-PS domestic machine, which can find application upon the production of components from various superalloys in the aviation and atomic industry and in the automobile industry. We have investigated the phase composition and determined the physicomechanical properties of the samples prepared under the preferential regime of selective laser melting. It has been established that the structure of the alloy obtained by selective laser melting consists of two supersaturated solid solutions based on the low-temperature hexagonal and high-temperature cubic cobalt modifications, which leads to an increase in the strength characteristics of the samples in comparison with the cast samples.
Keywords: selective laser melting; cobalt-based alloys

Structural changes and the main features of the fracture of the base metal and the coarse-grained region of the heat-affected zone of the welded joints of high-strength steels have been studied by simulating the thermal cycle of welding and post-welding heat treatment. The effects of the simultaneous action of heating for high-temperature tempering and of deformation allowing the estimation of the impact of residual welding stresses have been studied. The probable reasons of the formation of cracks in welds upon the postwelding tempering have been determined.
Keywords: high-strength steel; heat-affected zone; welded joint; microstructure; failure

The influence of the types and regimes of heat treatment, as well as of the temperature and magnitude of the shape-memory-inducing deformation on the structural changes, martensitic transformations, parameters of the crystal lattice and substructure, and the mechanical and thermomechanical characteristics have been studied in the new shape-memory alloy of composition 43Ti-46Ni-9Nb-2Zr (at %). The conditions of the appearance and realization of the shape-memory effect have been determined. The relationship between the structural features and the values of the thermomechanical characteristics of the alloy has been revealed. The regimes of the heat treatment and of the deformation that induces the shape-memory effect, which provide in this alloy the obtaining of high thermomechanical characteristics, have been determined.
Keywords: Ti-Ni-Nb-Zr alloy; shape-memory effect; heat treatment; magnitude and temperature of deformation inducing the shape-memory effect; structure; martensitic transformations; phase composition; mechanical characteristics; thermomechanical characteristics

Comparative analysis of corrosion cracking of austenitic steels with different contents of nitrogen in chloride- and hydrogen-containing media by S. Yu. Mushnikova; V. V. Sagaradze; Yu. I. Filippov; N. V. Kataeva; V. A. Zavalishin; V. A. Malyshevskii; G. Yu. Kalinin; S. K. Kostin (626-635).
The structural state and the resistance to stress-corrosion cracking (SCC) at constant loads have been studied using samples with a grown crack by the method of the cantilever bending on quenched austenitic stainless steels of the 20Cr-6Ni-11Mn-2Mo-N-V-Nb (Kh20N6G11M2AFB) type, with different contents of nitrogen (0.17, 0.34, 0.43, and 0.50 wt % N). The tests were conducted in a 3.5% aqueous solution of NaCl (without providing polarization) and in a similar solution under cathodic polarization, which causes the formation of hydrogen. It has been shown that, in a chloride solution without polarization, the steels do not undergo SCC for 2000 h. In the case of significant cathodic polarization via employment of a magnesium protector, there was revealed a brittle character of fracture upon SCC in all steels. It has been shown that steel with a nitrogen content of 0.43 wt % possesses the maximum absolute values of rupture stresses under the conditions of cathodic polarization.
Keywords: nitrogen-containing austenitic steels; nitrides; corrosion cracking; hydrogen brittleness; electron microscopy