A two dimensional model for ferromagnetic martensites

We consider a recently introduced 2-D square-to-rectangle martensite model that explains several unusual features of martensites to study ferromagnetic martensites. The strain order parameter is coupled to the magnetic order parameter through a 4-state clock model. Studies are carried out for several combinations of the ordering of the Curie temperatures of the austenite and martensite phases and, the martensite transformation temperature. We find that the orientation of the magnetic order which generally points along the short axis of the rectangular variant, changes as one crosses the twin or the martensite-austenite interface. The model shows the possibility of a subtle interplay between the growth of strain and magnetic order parameters as the temperature is decreased. In some cases, this leads to qualitatively different magnetization curves from those predicted by earlier mean field models. Further, we find that strain morphology can be substantially altered by the magnetic order. We have also studied the dynamic hysteresis behavior. The corresponding dissipation during the forward and reverse cycles has features similar to the Barkhausen's noise.Comment: 9 pages, 11 figure

Influence of ionic balance on the process of microcrystal AgBr (111) maturing

The research of the effect of photosensitivity level increase found in the process of maturing microcrystals AgBr with octahedron facets (111) without addition of sulphur containing additives (natural maturing) is carried out. It is stated that formation of sensitivity centres occurs during modification of AgBr (111) microcrystal form owing to difference of chemical potentials of microcrystal sides. Thus, the direct proofs of silver nature of sensitivity centres at maturing of AgBr (111) microcrystals in the conditions involved are obtained. The reasons of absence of natural maturing for AgBr microcrystals of a cubic facet (100) and flat AgBr (Т-МC) microcrystals are discussed

Homogenization in magnetic-shape-memory polymer composites

Magnetic-shape-memory materials (e.g. specific NiMnGa alloys) react with a large change of shape to the presence of an external magnetic field. As an alternative for the difficult to manifacture single crystal of these alloys we study composite materials in which small magnetic-shape-memory particles are embedded in a polymer matrix. The macroscopic properties of the composite depend strongly on the geometry of the microstructure and on the characteristics of the particles and the polymer. We present a variational model based on micromagnetism and elasticity, and derive via homogenization an effective macroscopic model under the assumption that the microstructure is periodic. We then study numerically the resulting cell problem, and discuss the effect of the microstructure on the macroscopic material behavior. Our results may be used to optimize the shape of the particles and the microstructure.Comment: 17 pages, 4 figure

First-principles study of lattice instabilities in the ferromagnetic martensite Ni2_2MnGa

The phonon dispersion relations and elastic constants for ferromagnetic Ni$_2$MnGa in the cubic and tetragonally distorted Heusler structures are computed using density-functional and density-functional perturbation theory within the spin-polarized generalized-gradient approximation. For $0.9<c/a<1.06$, the TA$_2$ tranverse acoustic branch along $[110]$ and symmetry-related directions displays a dynamical instability at a wavevector that depends on $c/a$. Through examination of the Fermi-surface nesting and electron-phonon coupling, this is identified as a Kohn anomaly. In the parent cubic phase the computed tetragonal shear elastic constant, C$^\prime$=(C$_{11}-$C$_{12}$)/2, is close to zero, indicating a marginal elastic instability towards a uniform tetragonal distortion. We conclude that the cubic Heusler structure is unstable against a family of energy-lowering distortions produced by the coupling between a uniform tetragonal distortion and the corresponding $[110]$ modulation. The computed relation between the $c/a$ ratio and the modulation wavevector is in excellent agreement with structural data on the premartensitic ($c/a$ = 1) and martensitic ($c/a$ = 0.94) phases of Ni$_2$MnGa.Comment: submitted to Phys. Rev.

XRD Characterization of the Structure of Graphites and Carbon Materials Obtained by the Low-Temperature Graphitization of Coal Tar Pitch

The structure of some commercial graphites and carbon materials (CMs) obtained by the low-temperature catalytic graphitization of coal tar pitch with iron salt, needle coke, foamed graphite as the catalysts has been studied. The study was performed using the X-ray diffraction technique with reflections from base plane and their decomposition into two components corresponding to the structural phases of graphite which have different XRD characteristics. Various CMs were compared with respect to the structural phase ratio, distance between polyarene layers in these phases, and sizes of the coherent scattering regions. The (004) reflection provided a better fit of some properties of graphites to the calculated XRD characteristics as compared to calculation from the (002) reflection. In the case of carbonization of coal tar pitch with investigated catalyst additions, prepared carbon materials have a higher degree of graphitization and a crystallite size greater than in the other case of carbonization of the individual pitch. The highest catalytic activity is shown by foamed graphite. It was found that the use of foamed graphite as the catalyst at 800-900 ºC produced carbon materials possessing a crystalline structure with interplanar spacing close to that in commercial graphites, while in the absence of catalyst the coal tar pitch material has an amorphous structure

Review of properties of magnetic shape memory (MSM) alloys and MSM actuator designs

Magnetic shape memory alloys are a new group of "smart" materials that exhibit large strain of 6-12% when subjected to magnetic fields. This indicates their enormous potential to be used in different electromagnetic (EM) devices such as actuators, sensors, energy harvesters and dampers. Shape change in MSM materials is controlled by magnetic field and doesn't involve phase transformation, allowing it to overcome a number of disadvantages of conventional shape memory alloys (SMAs). MSM devices are capable of producing large force and stroke output in considerably small dimensions. At the same time they can have fast response and potentially very long lifetime. This paper discusses different modern designs and approaches to MSM actuator design with their advantages and disadvantages. An overview on characteristics of MSM alloys is also presented in order to highlight how different properties of the material influence the total output of a device

CHANGES IN A TWO-PHASE TITANIUM ALLOY UNDER HOT PLASTIC DEFORMATION

The results of experiments with hot plastic deformation of a two-phase titanium alloy in the temperature range of 600–1100 °C with compression rates from 10–3 to 10 s–1 are presented. A process map has been constructed to determine the efficiency and mechanisms of dissipation. The data obtained are verified by the methods of metallography and X‑ray diffraction analysis.Представлены результаты экспериментов с горячей пластической деформацией двухфазного сплава титана в диапазоне температур 600–1100 °C со скоростями сжатия от 10–3 до 10 с–1. Построена карта процесса для определения эффективности и механизмов рассеяния (диссипации) вводимой механической энергии в условиях горячего пластического сжатия с учетом структурно-фазового превращения. Полученные данные верифицированы методами металлографии и рентгеноструктурного анализа.Экспериментальные исследования проведены на оборудовании Центра коллективного пользования «Состав, структура и свойства конструкционных и функциональных материалов» НИЦ «Курчатовский институт» — ЦНИИ КМ «Прометей» при финансовой поддержке государства в лице Минобрнауки в рамках соглашения № 14.595.21 0004

Pediatricians 64

Article is based on performance of the Rector of the Kazan State Medical University at the VIII Conference «Pediatrics and Pediatric Surgery in the Volga Federal District» September 20, 2011, devoted to the 70th anniversary of the famous Russian pediatricians — academician A.A. Baranov, professors V.Yu. Albitsky, A.V. Kuznetsova, S.V. Maltsev, O.I. Pikuza, who graduated Department of Pediatrics KSMA in 1964. Presents data on scientific creativity and contribution of anniversaries in pediatric science, to the protection of the health of the child population of Tatarstan and Russia