Physics of Metals and Metallography (v.114, #5)

A model of the magnetization distribution in thin ferromagnetic films that are in the so-called supercritical state has been considered. The model makes it possible to take into account the real distribution of the magnetization more precisely in ferromagnetic films than open and closed models. The theoretical results have been compared to the experimental data.
Keywords: stripe domain structure; single-crystal ferromagnetic film; positive constant of magnetocrystalline anisotropy; perpendicular anisotropy; supercritical state; model of magnetization distribution

The effect of the isothermal-holding temperature and cooling rate upon in-air heat treatment on the magnetic properties and magnetization distribution in ribbons of amorphous soft magnetic iron-based alloys with positive saturation magnetostriction has been investigated. The results of the investigation showed that the dependence of the maximum magnetic permeability on the isothermal-holding temperature correlates with the corresponding changes in the magnetization distribution in the ribbon and is determined by diffusion processes that occur upon in-air heat treatment at a specific isothermal-holding temperature. An increase in the cooling rate leads to an ambiguous effect on the level of magnetic properties. The increase favors an improvement in magnetic properties when, after in-air heat treatment, either a predominantly amorphous state of the surface or a state with the formed amorphous-crystalline surface layer with a nearly optimal thickness is obtained.
Keywords: magnetic permeability; magnetization; amorphous soft magnetic alloys; heat treatment; cooling rate; isothermal-holding temperature

Degree of perfection of the 〈111〉 texture and the hysteresis of magnetoresistance in MnIr-based top spin valves by M. A. Milyaev; L. I. Naumova; V. V. Proglyado; T. P. Krinitsina; I. S. Bannikova; A. M. Burkhanov; V. V. Ustinov (383-389).
Dc magnetron sputtering was used to prepare spin valves based on an Mn75Ir25 antiferromagnet with a composite free layer Ni80Fe20/Co90Fe10 and different thicknesses of the magnetic and nonmagnetic layers. Investigations of the microstructure and texture have been performed using X-ray diffraction and transmission electron microscopy. Field dependences of the magnetoresistance were measured for samples with a different degree of perfection of the axial texture 〈111〉: from sharp (with an orientation spread angle of 3.3°) to strongly smeared (with an orientation spread angle of 19.3°). It has been found that the width of the low-field loop of the magnetoresistance hysteresis increases monotonically with a decrease in the texture strength from sample to sample. The results are interpreted with allowance for the dependence of the hysteresis-loop width on the mutual orientation of the external magnetic field and easy axes of individual crystallites of the textured film.
Keywords: spin valve; multilayer nanostructures; giant magnetoresistance effect; magnetron sputtering; hysteresis; Mn75Ir25 ; 〈111〉 texture

Elastic and magnetic properties of single-crystal La0.4Pr0.3Ca0.3MnO0.3 by R. I. Zainullina; N. G. Bebenin; V. V. Ustinov; Ya. M. Mukovskii (390-394).
Results are reported on the experimental investigation of the temperature dependences of the magnetization, velocity of longitudinal sound, and internal friction of single-crystal La0.4Pr0.3Ca0.3MnO0.3. In the paramagnetic region, extended temperature hysteresis of the sound velocity has been revealed, which is due to a nonequilibrium state of the crystal structure of the compound.
Keywords: manganite; magnetization; ultrasound

Structure and phase composition of Nb-C deposited films by V. N. Volodin; Yu. Zh. Teleushev; E. A. Zhakanbaev (395-399).
Using the methods of ion-plasma sputtering and the coprecipitation of ultradispersed Nb and C particles, samples of Nb-C coatings have been obtained in the range of carbon concentrations of 1.9–56.6 at %. The structural analysis supports the existence of metastable solid solutions below 1.9 at % C, the direct synthesis of Nb2C in the range of 1.9–8.5 at % C, the presence of an amorphous phase between 8.5 and 41.7 at % C, and the direct synthesis of NbC in the range of 41.7–56.6 at % C. Thermal treatment below 900°C leads to a considerable widening of concentration intervals of Nb2C and NbC existence and the intervals of coexistence of these phases with other phases. The field of NbC carbide extends from 41.7 to 56.6 at %. At a concentration above 50 at % C, the carbon phase in the coating is not revealed, which is probably caused by difficulties detecting it via X-ray diffraction method. Upon cryogenic tests, a change in the polarity of conductivity from metallic to semiconductor type upon an increase in the carbon concentration has been demonstrated. The possibility of applying the technique of material formation using small particles for other systems has been mentioned.
Keywords: niobium; carbon; solid solution; carbides; phase; phase transition; lattice parameters

A nearly equiatomic MnNi alloy was fabricated from the elemental powders by means of mechanical alloying in a planetary ball milling apparatus. X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and measurements of magnetization were conducted to identify the structural states and properties of the prepared alloys. After ball milling for 20 h, a disordered face-centered cubic (f.c.c.) solid solution was formed which increased in lattice parameter by further milling up to 50 h. An exothermic reaction took place at around 300–400°C during continuous heating of the disordered f.c.c. solid solution. This reaction is attributed to a structural ordering leading to the formation of a face-centered tetragonal (f.c.t.) phase with L10 type ordering. Examination of the magnetic properties indicated that the structural ordering increases remnant magnetization and decreases coerecivity.
Keywords: mechanical alloying; solid solution; disorder; MnNi

Structure of diffusion layers formed at liquid aluminum alloy-steel contact boundary by I. G. Brodova; O. A. Chikova; I. G. Shirinkina; T. I. Yablonskikh; V. V. Astaf’ev (406-410).
The microstructure and composition of diffusion layers that arise upon the spread of the liquid aluminum alloys AL5, AL9, AL852, and A7075 over the St3 steel has been investigated using optical and electron microscopy. The thermophysical conditions of the interaction of the melt with the substrate under which at the boundary of the substrate the plastic transition layer of iron-based solid solution improving quality of the coating is formed have been determined.
Keywords: melt; diffusion layers; steel; structure; phase composition

Structure of chromium-zirconium bronze subjected to dynamic channel-angular pressing and aging by V. I. Zel’dovich; I. V. Khomskaya; N. Yu. Frolova; A. E. Kheifets; E. V. Shorokhov; P. A. Nasonov (411-418).
Structure changes in chromium-zirconium bronze upon high-speed deformation and subsequent annealing have been studied using the methods of metallography and electron microscopy and microhardness measurements. Deformation was performed by the method of dynamic channel-angular pressing in one and three passes. The deformation creates a submicrocrystalline structure and increases the microhardness by 2.4 times. Aging additionally increases the microhardness by 10%. During annealing at temperatures of 400–700°C, aging and recrystallization take place. At early stages of aging, nanosized particles pin dislocations, thus hampering the formation of recrystallization centers. At subsequent stages, chromium particles impede the migration of large-angle boundaries, thereby blocking the development of recrystallization. At the beginning of aging process in the deformed structure, chromium particles are coherent with the copper matrix and have an fcc structure. Upon the coarsening of the particles in the recrystallized structure, they acquire a bcc structure that is typical of chromium.
Keywords: chromium-zirconium bronze; dynamic pressing; high-strain-rate deformation; aging; recrystallization; thermal stability

Effect of austenite-decomposition temperature on bainite morphology and properties of low-carbon steel after thermomechanical treatment by V. M. Schastlivtsev; T. I. Tabatchikova; I. L. Yakovleva; S. Yu. Klyueva; A. A. Kruglova; E. I. Khlusova; V. V. Orlov (419-429).
Peculiarities of the bainite structure formed in low-carbon steel 07G2NDMBT during isothermal austenite decomposition, namely, the sizes of crystallites, their mutual orientation, and the presence of cementite precipitates, are considered. The temperature range of the formation of bainite with the subgrain structure was determined. The size of the austenite grain and degree of hot deformation were found to affect the transformation of bainite that occurs upon subsequent cooling and the submicrocrystalline bainite structure. We studied the structure and mechanical properties of a rolled sheet 16 mm thick, which was subjected to thermomechanical treatment (TMT) under plant conditions in accordance with optimum regimes. It was shown that the high structure dispersion of the steel subjected to TMT is due to not only the formation of bainite with the subgrain structure, but also the partial transfer of crystal-structure defects from hot-rolled austenite to the final bainite structure.
Keywords: low-carbon steel; isothermal transformation; structure; bainite; thermomechanical treatment; mechanical properties

Structure levels of pearlite deformation in carbon steel of eutectoid composition by N. A. Tereshchenko; I. L. Yakovleva; T. A. Zubkova; M. V. Chukin; N. V. Koptseva (430-439).
The structure of eutectoid steel 80 has been studied at various stages of producing high-strength large-diameter reinforcing bars using transmission and scanning electron microscopy. Peculiar features of structural state have been revealed by the scale factor. It has been demonstrated that the stage of intensive strengthening does not exist at the initial steps of drawing a large-diameter wire. Using EBSD it has been established that, in the course of drawing, within the limits of the former austenite grain, a conglomerate is formed that consists of pearlitic colonies with the single orientation of the ferrite constituent along the {100} cleavage plane, which causes the poor technological plasticity of the studied steel.
Keywords: pearlitic colony; interlamellar spacing; cementite lamella; microcrack

Dislocation density in copper and tantalum subjected to shock compression depending on loading parameters and original microstructure by A. M. Podurets; M. I. Tkachenko; O. N. Ignatova; A. I. Lebedev; V. V. Igonin; V. A. Raevskii (440-447).
The dislocation density ρ in copper and tantalum specimens with various grain size that remained after high-strain-rate loading by shock and quasi-isentropic waves with amplitudes of 20–100 GPa has been studied using X-ray diffraction analysis. The deformation rate was 106–109 s−1. It has been confirmed that high-strain-rate loading generates a higher dislocation density in copper than does quasi-static deformation, as well as that the shock-wave loading generates a higher dislocation density than quasi-isentropic loading. In copper, a maximum of ρ has been found in the pressure range of P = 30–40 GPa, which corresponds to a degree of deformation of 0.25–0.3, followed by a drop. This drop in ρ is explained by the partial annealing of defects during adiabatic heating resulting from compression. An increase in ρ in copper with decreasing specimen temperature has been noted. In tantalum, an increase in the shock wave pressure leads to a monotonic increase in the dislocation density. No effect of heating on the annealing of defects in tantalum has been found, even under the maximum pressure. As the average grain size increases, ρ increases in both copper and tantalum.
Keywords: copper; tantalum; shock wave; dislocation density

Texture and structure contribution to low-temperature plasticity enhancement of Mg-Al-Zn-Mn Alloy MA2-1hp after ECAP and annealing by V. N. Serebryany; G. S. D’yakonov; V. I. Kopylov; G. A. Salishchev; S. V. Dobatkin (448-456).
Equal channel angular pressing (ECAP) in magnesium alloys due to severe plastic shear deformations provides both grain refinement and the slope of the initial basal texture at 40°–50° to the pressing direction. These changes in microstructure and texture contribute to the improvement of low-temperature plasticity of the alloys. Quantitative texture X-ray diffraction analysis and diffraction of backscattered electrons are used to study the main textural and structural factors responsible for enhanced low-temperature plasticity based on the example of magnesium alloy MA2-1hp of the Mg-Al-Zn-Mn system. The possible mechanisms of deformation that lead to this positive effect are discussed.
Keywords: texture; microstructure; plasticity; magnesium alloys; equal channel angular pressing; annealing