Temperature and velocity dependence of mechanical properties of the alloy Ni3Al. III

The temperatures and velocity dependence of the yield point and flow stress of the inter-metallic compound Ni₃Al was studied. It was established that, within the range of temperatures where the anomalous temperature dependence of flow stress is observed, an anomalous velocity dependence of these stresses also occurs. The increase of the yield point and flow stress of the alloy Ni₃Al with the increase of temperature is under the full control of thermally activated mechanisms of hardening. Three stages in the increase of resistance to deformation under increasing temperature were disclosed. It is assumed that two of these are associated with the deposition of atomic defects on sliding dislocations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 11, pp. 107–113, November, 1971.     

Mechanical properties of single-crystal L12-superlattice alloys in tests on temperature and strain-rate variation

Abrupt stresses in L1₂-superlattice alloys caused by variation in temperature and strain rate are investigated. An interpretation of the experimentally observed facts is given. It is shown that the results obtained in the experiments on strain-rate and temperature variation might provide additional information on the mechanisms responsible for the anomalous temperature dependence of the yield and flow stresses. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 25–33, January, 2006.     

Stress relaxation and viscosity of a bulk Pd<Subscript>40</Subscript>Cu<Subscript>30</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> metallic glass under isochronous heating conditions

Isochronous relaxation of tensile stresses is measured in a bulk Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glass in the initial state and after certain thermal treatments. The results of measurements are used to find the energy spectrum of irreversible structural relaxation, from which the temperature dependence of shear viscosity is then calculated. This dependence is also found independently from measurements of creep in the same glass. The calculated viscosity is shown to agree well with the experimental data.     

Thermally activated mechanisms of plastic deformation of single crystals of the disordered alloy Ni3Fe

On the basis of experiments on the variation of temperatures in the process of deformation and relaxation of stresses, we make an analysis of the temperature dependence of the yield stresses of single crystals of the disordered alloy Ni₃Fe and of the nature of the processes determining it.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 109–113, July, 1978.     

Magnetic Properties of Iron-Based Amorphous Metal Wires

It has been studied how the conditions of machining and the elastic tensile stresses affect the magnetic properties of amorphous metal wires of composition Fe₇₅Si₁₀B₁₅ produced by drawing from a melt. The magnetic characteristics of wires subjected to both thermal treatment and treatment with a continuous electric current of different magnitude have been investigated. The residual induction of wires is their magnetic parameter most sensitive to the conditions of treatment. The dependences of the residual induction on temperature and on the magnitude of the treating electric current are qualitatively similar. The greatest changes in residual induction are observed in the range of treating electric currents from 0.5 to 0.8 A, which can be associated with the processes of structural relaxation and crystallization occurring in the wires. The run of the dependence of the residual induction on the magnitude of tensile stresses is nonmonotonic in character and is determined by the level of internal hardening stresses of the test wires.     

Effect of the stresses caused by substrate on the electrical conductivity of ferromagnetic manganite lanthanum–barium films

This is a complex study of the electrophysical and magnetic characteristics of epitaxial manganite La_0.7Ba_0.3MnO₃ (LBMO) films under conditions of crystal structure stresses caused by misfit in the lattice parameters of the LBMO crystal and a substrate. The substrate used had the lattice parameter smaller than that in the LBMO crystal. It is shown that the temperature dependence of the electrical resistance of the films at low temperatures is not dependent on the existence of stresses in the film and agrees well with the calculation with allowance made for the interaction of carriers with magnetic excitations in the presence of strongly correlated electronic states. The study of the ferromagnetic resonance line indicates an inhomogeneity of the ferromagnetic phase in the LBMO films and the increase in the ferromagnetic resonance line width with decreasing temperature.     

Thermal hardening of Ni3Ge alloy single crystals with an L12 superstructure at low temperatures

The mechanical properties and the dislocation structure of Ni₃Ge alloy single crystals have been experimentally studied at low temperatures. It is found that the flow stresses increase beginning with 4.2 K, and the observed rise in the stresses depends on the orientation of the strain axes of the crystals. The dislocation structure is investigated thoroughly. It is revealed that the mean density of dislocations and the interdislocation interaction parameter α anomalously increase as the temperature increases in the range 4.2–293 K. The mechanisms providing an explanation for the temperature anomaly of flow stresses and the α parameter are considered. The activation energy of thermal hardening is evaluated. It is assumed that the low activation energies of thermal hardening are due to the motion of dislocations at velocities close to the velocity of sound at these temperatures.     

Transformation behavior of Ni-Mn-Ga/Si(100) thin film composites with different film thicknesses

A series of Ni_51.4Mn_28.3Ga_20.3/Si(100) thin film composites with different film thicknesses varying from 0.1 to 5 μm have been prepared by magnetron sputtering and subsequently annealed. X-ray powder diffraction patterns of the films show the features associated with the lattice-modulated martensitic phase and/or cubic austenite at room temperature. 220-fiber texture was confirmed by the X-rays measurements made at 150 °C. While the Curie temperature is almost film thickness independent, the martensitic transformation temperature shows a strong descended dependence in the submicron range. The substrate curvature measurements demonstrate that the forward and reverse martensitic transformation in the films is accompanied by the reversible relaxation and accumulation of residual stress, originally created by the thermal treatment due to the difference in thermal expansion of the film and substrate. The values of residual stresses measured by both substrate curvature and X-rays diffraction methods at constant temperatures are found to be dependent on the film thickness. This behavior appears in correlation with the thickness dependence of the transformation temperature.     

Effect of elastic stresses on the thermoelectric properties of Pb1−x GexTe epitaxial layers

The temperature dependence of the thermopower of Pb_1?x Ge_xTe (x = 0.01?0.05) epitaxial films grown on BaF₂(111) substrates was measured. The temperature of the ferroelectric phase transition was found to disagree with that obtained for bulk single crystals of the same composition. This disagreement was explained as resulting from the effect of elastic stresses arising in the crystal lattice of the Pb_1?x Ge_xTe compound due to the difference between the thermal expansion coefficients of the film and the substrate during cooling of the layers from the growth temperature to the temperature of measurement.     

Internal stresses of deformation origin in bulk metallic glasses of the Pd-Cu-Ni-P system

The effective internal shear stresses arising during unloading of bulk Pd-Cu-Ni-P metallic glasses deformed under conditions of homogeneous flow below the glass transition temperature T _g have been determined using measurements of the torque relaxation. It has been shown that the level of effective internal stresses gradually decreases with increasing temperature and approaches zero at T ?? T _g . Possible mechanisms of the formation of internal stresses with variations in the experimental conditions have been discussed.     

Self-similar solutions of equations of the nonlinear Schrödinger type

Experimental data are presented for the temperature dependence of the conductivity of Cu: SiO₂ metal-insulator composite films containing 3-nm Cu granules. At low temperatures in the concentration range 17–33 vol % Cu, all of the conductivity curves have a temperature dependence of the form σ ∝ exp{ (T ₀/T)^1/2}, while at higher temperatures a transition is observed to an activational dependence. A numerical simulation of the conduction in a composite material shows that an explanation of the observed temperature dependence must include the Coulomb interaction and the presence of a rather large random potential. The simulation also yields the size dependence and temperature dependence of the mesoscopic scatter of the conductivities of composite conductors. It is shown that a self-selecting percolation channel of current flow is formed in the region of strong mesoscopic scatter.     

Gradient-stress on polarization in Ba1−x Sr x TiO3 films

A group of position-thickness-dependent stresses are used to modified Landau-Devonshire theory to investigate the second-order phase transition in Ba_1?x Sr _x TiO₃ films. The result shows that the short-range interaction between the unit cells of the film and the substrate induces the phase transition dispersion and the rise of the transition temperature in the films. The dependence of the effective dielectric constant on the temperature and the average spontaneous polarization on the film thickness are computed, which qualitatively agree with the experiments.     

Temperature and velocity dependence of the mechanical properties of intermetallic compound Ni3Al. II

This paper examines the retardation of superdislocations in LI₂ superstructure, due to the capture by superpartial dislocations (SPD) of atomic defects on the slip plane, and also resulting from diffusion of atomic defects in PD. When the jogs formed by the settling of defects on dislocations remain nondissociated, the sliding of superdislocations is accompanied by the generation of displaced rows of atoms the maximum linear energy of which in the L1₂ superstructure amounts to v/b (v is the ordering energy, b the interatomic distance). The maximum magnitude of retardation of superdislocations, dependent upon generation of displaced rows, is twice as high on cubic planes as on octahedral planes. The estimations presented indicate that the diffusion settling of atomic defects on the SPD of sliding superdislocations can be a cause of the anomalous temperature dependence of flow stresses under high temperatures. Some effects associated with the possible dissociability of jogs on SPD are examined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 10, pp. 63–67, October, 1971.The authors express their thanks to É. V. Kozlov for helpful discussion.     

Tb1-xGdxFe2的磁矩和矫顽力

测量了赝二元立方Laves相化合物TbGdFe₂(0≤x≤1)的磁化曲线、居里温度和矫顽力随温度的变化。发现化合物由TbFe₂向GdFe₂过渡时,样品的饱和磁化强度几乎直线下降。在所有成分下,Fe原子的磁矩都是常数(μ_Fe=1.60±0.04μ_B),但Tb原子的磁矩,由于受晶场影响,却小于自由离子的值(gJ=9.Oμ_B)。矫顽力由两部分组成:一为畴壁受Peierls势垒和稀土 关键词：     

Influence of the growth mechanism and thermoelastic stresses on the lattice dynamics of heteroepitaxial films of barium strontium titanate

Heteroepitaxial thin films of Ba_0.7Sr_0.3TiO₃ (BST-0.3) solid solutions were grown on single-crystal (001) MgO substrates by high-frequency cathode sputtering of a stoichiometric ceramic target. The parameters of the tetragonal unit cell of a film were determined by x-ray diffraction methods, and the temperature dependence of the parameter c was studied depending on the synthesis conditions in the temperature range 293–520 K. An E(TO) soft mode was observed in the Raman spectra, the frequency of which correlates with two-dimensional stresses arising in films. It is shown that the two-dimensional stresses in a film are controlled not only by the film-substrate lattice constant mismatch and the difference of their thermal expansion coefficients but also are significantly dependent on the heteroepitaxial growth mechanism. It is shown that the phase transition to the tetragonal paraelectric phase during film heating occurs irrespective of the growth mechanism.     

Gradient-stress on polarization in Ba1-xSrxTiO3 films

A group of position-thickness-dependent stresses are used to modified Landau-Devonshire theory to investigate the second-order phase transition in Ba_1−x Sr _x TiO₃ films. The result shows that the short-range interaction between the unit cells of the film and the substrate induces the phase transition dispersion and the rise of the transition temperature in the films. The dependence of the effective dielectric constant on the temperature and the average spontaneous polarization on the film thickness are computed, which qualitatively agree with the experiments.      

Magnetic-field-induced orientational phase transition in a nonuniformly stressed iron borate single crystal

Spatially nonuniform magnetic anisotropy was induced in the basal plane of an iron borate (FeBO₃) single crystal by applying low-symmetry stresses. The effect of nonuniform magnetic anisotropy on the magnetic state of this weak ferromagnet was studied by magnetooptic methods. It is revealed that, when a nonuniformly stressed FeBO₃ crystal is magnetized in the basal plane along a certain direction (depending on the symmetry of the applied stress), a transition from a uniform to a spatially modulated magnetic state occurs, which is not observed in the crystal in the absence of stresses. The modulated magnetic phase of the crystal can be represented as a static spin wave linearly polarized in the basal plane, with the azimuth of the weak-ferromagnetism vector oscillating about the direction of the mean magnetization. The temperature and field dependences of the spatial period of the modulated magnetic structure and the amplitude of oscillations of the ferromagnetism vector are studied, and the temperature dependence of the field range over which the modulated phase exists in the nonuniformly stressed FeBO₃ crystal is found. The results are discussed in terms of the phenomenological theory of magnetic phase transformations. It is shown that the theoretical model used consistently describes all the experimental results of the study of the noncollinear magnetic phase of the nonuniformly stressed FeBO₃ crystal.     

Nonlinear dielectric response of epitaxial Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript> TiO<Subscript> 3</Subscript> thin films

Based on Landau-Devonshire (LD)-type phenomenological thermodynamic theory, the electric field dependence of the dielectric properties of tetragonal single-domain barium strontium titanate(Ba_1-xSr_xTiO₃) films on cubic substrates is theoretically investigated by taking into account the high order terms of the polarization. At room temperature, the nonlinear dielectric responses of epitaxial Ba_0.6Sr_0.4TiO₃ films are provided by adjusting the film thickness and growth temperature. The strong nonlinearity of relative dielectric constant and pyroelectric coefficient are attained around critical film thickness on MgO (69 nm) and LaAlO₃ (132 nm) substrates or critical growth temperature on MgO (337 °C) substrate with respect to epitaxy-induced lattice misfit and thermal stresses during deposition. This can be explained that small compressive stresses are effective to support high nonlinearity of dielectric constant and pyroelectric coefficient for Ba_0.6Sr_0.4TiO₃ films irrespective of whether they are on compressive substrate or tensile substrate. It is also predicted that a large tunability may be achieved by altering processing conditions, such as the film thickness and growth temperature for different substrates. Our theoretical results are in good agreement with the experimental data reported in literature.     

Residual stress in Ni-Mn-Ga thin films deposited on different substrates

Four series of Ni_51.4Mn_28.3Ga_20.3/substrate thin film composites, where the substrate is either Si(100), MgO(100), alumina or Mo foil, and two series of Ni_53.5Mn_23.8Ga_22.7/substrate composites where the substrate is either alumina or Mo foil, with different film thicknesses varying from 0.1 to 5 μm have been studied in the cubic phase by XRD stress measurements. The values of residual stresses are found to be dependent on both substrate and film thickness. In the submicron range, a correlation between thickness dependencies of residual stress and transformation temperatures is experimentally obtained. The temperature dependence of the d-spacing d₂₂₀ is studied for the films deposited on Si(100).     

Dislocation retardation due to the destruction of short-range order in the L12 superstructure

The magnitude and temperature dependence of the dislocation retardation due to the destruction of short-range order are calculated in the quasichemical approximation. The atomic interaction in two coordination spheres and dislocation slip in cubic and octahedral planes are taken into account. The retardation stress for the first superdislocation is essentially the same in these planes for T < T _k (where T _k is the phase-transition temperature), while the retardation stress for subsequent superdislocations is much lower, so large planar accumulations of superdislocations may arise. The retardation stress is maximal at T = T _k , and for T > T _k the first dislocations should move in pairs. Comparison of the experimental cleavage stresses with calculated values shows that, by itself, superdislocation retardation due to correlation destruction cannot explain the behavior of the yield point.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 78–85, April, 1971.