The effect of antiphase domains on the energy spectra of special boundaries in alloys with the <Emphasis Type="Italic">L</Emphasis>1<Subscript>2</Subscript> superstructure

The special grain boundaries in ordering alloys with the L1₂ superstructure have been investigated by optical metallography and transmission diffraction electron microscopy. The relative energy of the boundaries Σ3 and Σ9 in Ni₃Fe alloy with a short-range order is determined. The energy of these boundaries in an alloy with long-range order is estimated. The energy of twin grain boundaries increases at the phase transitions A1 → L1₂ due to the formation of antiphase grain boundaries in those boundaries. The spectra of special boundaries over Σ and their distributions, depending on the relative energy, change as well. The average relative energy of special grain boundaries in alloys with the L1₂ superstructure increases with increasing the energy of antiphase boundaries.     

Kinetics of Polydomain Ordering at Second-Order Phase Transitions (by the Example of the AuCu<Subscript>3</Subscript> Alloy)

Kinetics of polydomain spinodal ordering is studied in alloys of AuCu₃ type. We introduce four non-conserved long-range order parameters whose sum, however, is conserved and, using the statistical approach, follow the temporal evolution of their random spatial distribution after a rapid temperature quench. A system of nonlinear differential equations for correlators of second and third order is derived. Asymptotical analysis of this system allows to investigate the scaling regime, which develops on the late stages of evolution and to extract additional information concerning the rate of decrease of the specific volume of disordered regions and the rate of decrease of the average thickness of antiphase boundaries. Comparison of these results to experimental data is given. The quench below the spinodal and the onset of long-range order may be separated by the incubation time, whose origin is different from that in first-order phase transitions. Numerical integration of equations for correlators shows also, that it is possible to prepare a sample in such a way that its further evolution will go with formation of transient kinetically slowed polydomain structures different from the final L1₂ structure.     

Simulation of the process of deformation-induced destruction of long-range order in alloys with an L12 superstructure

Plastic-deformation-induced destruction of long-range order in alloys with an L1₂ superstructure is considered. A mathematical model is suggested which takes into account the following mechanisms that lead to the destruction of long-range order: generation of superdislocations, generation of single dislocations, multiplication of antiphase boundaries (APBs) upon the conservative motion of dislocations, multiplication of APBs upon dislocation climb, formation of APB tubes on superdislocations, generation of point defects, and thermal ordering. A mathematical model of deformation strengthening and long-range order destruction with allowance for the change in the type of shear-forming dislocations from superdislocations to single dislocations is formulated.     

Specific features of the strain-induced order-disorder phase transition and its mechanisms

In the present paper, the methods of x-ray diffractometry are used to investigate the influence of plastic deformation on the long-range atomic order in alloys with L1₂, L1₂(M), and L1₂(MM) superlattices. The influence of ordering energy, initial values of the long-range order parameter, and average sizes of antiphase domains on the specific features of the dependence of long-range order parameter on the strain degree is considered. It is demonstrated that the wetted strain-induced order-disorder phase transition occurs with rapid accumulation of antiphase boundaries that play an important role in the destruction of long-range atomic order.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 21–32, September, 2004.     

Effect of atomic ordering on the role of grain boundaries in the plastic deformation of Ni<Subscript>3</Subscript>Fe alloy

The structure of dislocations and the defect structure of grain boundaries and their parameters in Ni₃Fe alloy with short-range order (SRO) and long–range order (LRO) at different stages of plastic deformation are studied by means of transmission diffraction electron microscopy using thin foils and replicas. It is found that atomic ordering reduces the Σ3 twins plasticizing effect, increases the density of grain boundary defects, slows their annihilation during deformation, and intensifies the microstrains at the triple junctions of grain boundaries.     

Short-range magnetic order,irreversibility and giant magnetoresistance near the triple points in the (x,T) magnetic phase diagram of ZrMn<Subscript>6</Subscript>Sn<Subscript>6-x</Subscript>Ga<Subscript>x</Subscript>

We have studied pseudo-layered ZrMn₆Sn_6-xGa_x intermetallics (0.55 ≤ x ≤ 0.81) using magnetic, magnetoresistivity and powder neutron diffraction measurements. All the alloys studied have magnetic ordering temperatures in the 450-490 K temperature range. They present complex temperature-dependent partially disordered magnetic structures whose ferromagnetic component develops upon increasing the Ga content. ZrMn₆Sn_6-xGa_x alloys with x ≤ 0.69 are essentially collinear antiferromagnets at high-temperature and adopt antifan-like arrangements at low temperature. For x ≥ 0.75, the alloys order ferromagnetically and evolve to a fan-like structure upon cooling. The intermediate compositions (x = 0.71 and 0.73) present a canted fan-like order at high temperature and another kind of antifan-like arrangement at low temperature. The degree of short-range order tends to increase upon approaching the intermediate compositions. The (x, T) phase diagram contains two triple points (x ~ 0.70; T ~ 460 K and x ~ 0.74; T ~ 455 K), where the paramagnetic, an incommensurate and a commensurate phases meet, which possess some of the features of Lifshitz point. Irreversibilities manifest in the low-temperature magnetization curves at the antifan-fan or fan-ferromagnetic boundaries as well as inside the fan region. Giant magnetoresistance is observed, even above room temperature.     

Magnetoresistive effect in RCu<Subscript>3</Subscript>Mn<Subscript>4</Subscript>O<Subscript>12</Subscript> perovskite-like oxides

The electrical properties of and the magnetoresistive effect in RCu₃Mn₄O₁₂ (R=rare-earth ion or Th) are studied. In all compounds of this series, the magnetoresistive effect amounts to 20% at liquid nitrogen temperature in the presence of a field of 0.9 T. An increase in the magnetoresistance with decreasing temperature and a high sensitivity to weak magnetic fields at low temperatures point to the intergranular nature of the effect. The magnetoresistance shows a peak in the vicinity of the Curie temperature T_C. Based on the dependences of the magnetoresistance on an external magnetic field, it is assumed that the magnetoresistance peak near T_C is related to the charge carrier scattering by magnetic inhomogeneities as in substituted orthomanganites. We believe that the magnetoresistance value near the magnetic ordering temperature depends on the synthesis conditions and the effect of the intergranular spacer on the transport properties of these compounds.     

Effect of severe plastic deformation and ultrarapid quenching on the properties of magnetic shape memory alloys near the Ni<Subscript>2</Subscript>MnGa composition

The effect of ultrarapid quenching from melt and subsequent severe plastic deformation by torsion in Bridgman anvils on the temperatures of martensitic (T _M) and magnetic (T _C ) phase transitions in magnetic shape memory alloys has been investigated. The features of the crystal structure and magnetic, electric, and optical properties of Ni₂MnGa-based alloys with different degrees of structural ordering are discussed. The effect of doping with copper and cobalt on the properties of nonstoichiometric Ni₅₀Mn_28.5Ga_21.5 alloy is considered.     

Specific features of the properties of half-metallic ferromagnetic Heusler alloys Fe<Subscript>2</Subscript>MnAl,Fe<Subscript>2</Subscript>MnSi,and Co<Subscript>2</Subscript>MnAl

The structural, magnetic, and electrical properties of half-metallic Heusler alloys Fe₂MnAl, Fe₂MnSi, and Co₂MnAl have been investigated in the temperature range of 4–900 K. According to the X-ray diffraction analysis, these alloys have the B2 and L2₁ structures with different degrees of atomic order. The magnetic state of the alloys is considered as a two-sublattice ferrimagnet. The electrical resistivity and thermoelectric power have been discussed in the framework of the two-current conduction model taking into account the existence of an energy gap in the electronic spectrum of the alloys near the Fermi level for the subband with spin-down (minority) electrons.     

Special features of the deformation-induced order-disorder phase transition and its modeling

Methods of x-ray diffractometry are used to investigate the influence of plastic deformation on the state of the long-range order in alloys with the L1₂, L1₂(M), and L1₂(MM) superstructure. It is demonstrated that the heterogeneous order-disorder phase transition accompanied by intensive accumulation of antiphase boundaries occurs under plastic deformation. The antiphase boundaries play an important role in the destruction of long-range order. Based on the available experimental results, a model of destruction of long-range order is constructed that takes into account the dislocation mechanisms of accumulation of antiphase boundaries under deformation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 9–24, January, 2006.     

Etching of slip produced antiphase domain boundaries in iron-silicon alloys

A method for revealing the slip produced antiphase domain boundaries in nearly stoichiometric ordered Fe₃Si alloys by chemical etching in a solution of HF + H₂O₂ + H₂O is reported. The method was applied to determine the character of superlattice dislocations in strained crystals. Perfect superlattice dislocations were found at the beginning of deformation. Imperfect superlattice dislocations producing antiphase domain boundaries were found at higher deformations.     

Peculiarities of antiferromagnetic ordering in orthorhombic LiMnO<Subscript>2</Subscript>

Data on the antiferromagnetic ordering in orthorhombic lithium manganite LiMnO₂ are obtained from magnetic-susceptibility, calorimetry, and nuclear magnetic resonance studies. The minimal hysteresis and the absence of jumps in the temperature dependences of the sublattice magnetization M(T) and the magnetic susceptibility near T _N indicate that the ordering occurs through a continuous second-order phase transition. Within the critical temperature range, the M(T?T _N) variation is satisfactorily described by a power-law dependence with a critical exponent β = 0.25(4), which is substantially smaller than that predicted for 3D magnetic systems with isotropic Heisenberg exchange. The band structure of orthorhombic LiMnO₂ is calculated using the LMTO-ASA method. Taking into account the spin states of manganese ions, an adequate pattern is obtained for the density-of-states distribution with an energy gap near the Fermi level (～0.7 eV), which is in agreement with the measured electrical parameters of lithium manganite. The calculations demonstrate that the exchange interactions between Mn³⁺ ions leading to antiferromagnetic ordering are significantly anisotropic. It is found that small paramagnetic regions persist in the manganite below the Néel temperature, and it is concluded that the reason for this is partial structural disordering of LiMnO₂. As a result, a certain fraction of the manganese positions is occupied by lithium ions (Li_Mn) and vise versa (Mn_Li). These defects are not involved in the formation of the ordered magnetic structure and compose a paramagnetic fraction.     

Effect of atomic displacement on the parameters of the grain boundary ensemble in nickel-based alloys with <Emphasis Type="Italic">L</Emphasis>1<Subscript>2</Subscript> superstructure

The effect complex stacking fault energy and mean-square atomic displacement have on the parameters of the grain boundary ensemble in nickel-based alloys with L1₂ superstructure is established experimentally. The higher the complex stacking fault energy, the lower the average number of special grain boundaries per parent grain. The lower the mean-square atomic displacement, the smaller the proportion of the Σ3 twins in the special grain boundary spectrum.     

Atomic configuration of antiphase boundaries in alloys

Computer simulation methods in the approximation of pairwise interatomic Morse potentials are used to calculate the atomic configurations and the energy of formation of 1/2 <110> {111} antiphase boundaries in ordered Cu₃Au and Ni₃Fe alloys. Atomic displacements result in considerable smearing of the first five planes closest to an antiphase boundary and a slip of atomic planes. These effects substantially reduce the energy of formation of antiphase boundaries. The influence of the size factor on the atomic configuration on the antiphase boundaries is discussed.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 2, pp. 43–47, February, 1985.In conclusion, the authors express their thanks to A. I. Tsaregorodtsev for useful discussion of this work.     

Energy of planar defects in the Ni<Subscript>3</Subscript>Al phase: Theory and experiment

The Ni₃Al phase with an L1₂ surperstructure exhibits a clearly pronounced positive temperature dependence of the yield stress. The nature of this phenomenon is still not clearly understood. The temperature dependence of the yield stress under consideration is determined, in many respects, by the structure of glide superdislocations and planar defects of these superdislocations involved in the L1₂ superstructure. A critical analysis is made of the current state of the art in the solution of the problem regarding the energy of antiphase boundaries in planes of an octahedron and a cube, as well as the energy of stacking faults in the Ni₃Al phase.     

Mechanism of Strain-Induced Breaking the Long-Range Atomic Order in Alloys with the L12 Superlattice Caused by Accumulation of Dislocation Walls

The mechanism of strain-induced breaking the long-range atomic order in alloys with the L1₂ superlattice caused by rearrangement of dislocations in dislocation walls is examined. A mathematical model of strain-induced breaking the long-range atomic order caused by accumulation of dislocation walls is constructed. The dependence of the long-range order parameter on the strain degree is calculated for alloys with high- and low-energy antiphase boundaries.     

Pressure-induced metamagnetic transition in the Cd<Subscript>0.7</Subscript>Mn<Subscript>0.3</Subscript>GeAs<Subscript>2</Subscript> ferromagnetic semiconductor

The magnetic susceptibility χ/χ₀ and the longitudinal Δρ _zz /ρ₀ and transverse Δρ _xx /ρ₀ magnetoresistances have been measured as functions of the hydrostatic pressure P ≤ 7 GPa at room temperature in the high-temperature ferromagnetic semiconductor Cd_0.7Mn_0.3GeAs₂ with a chalcopyrite structure and the Curie temperature T _c = 355 K. A pressure-induced metamagnetic transition from the low-magnetization state to the high-magnetization state has been observed in Cd_0.7Mn_0.3GeAs₂ near the magnetic ordering temperature. This transition is accompanied by the hysteresis of the magnetic susceptibility and magnetoresistance.     

Metamagnetism near <Emphasis Type="Italic">T</Emphasis><Subscript>C</Subscript> in Mn-substituted chalcopyrite Cd<Subscript>0.90</Subscript>Mn<Subscript>0.10</Subscript>GeAs<Subscript>2</Subscript>

The magnetic and electric properties of a Cd_0.90Mn_0.10GeAs₂ solid solution with the chalcopyrite structure have been investigated in wide temperature and field ranges. It has been found that a metamagnetic transition from a low-magnetization state to a high-magnetization one is initiated in Cd_0.90Mn_0.10GeAs₂ near the magnetic ordering temperature. This transition is accompanied by the hysteresis of magnetic properties. An external magnetic field at temperatures above T _C also induces the metamagnetic transition. When the temperature increases above T _C, the magnetization jump decreases, whereas fields inducing the metamagnetic transition increase. The band character of magnetism and metamagnetism in the effective magnetic field is assumed on the basis of the behavior of magnetization in the metamagnetic transition and analysis of the band structure of the solid solution of cadmium-germanium diarsenide with manganese.     

Role of Ge incorporation in the physical and dielectric properties of Se<Subscript>75</Subscript>Te<Subscript>25</Subscript> and Se<Subscript>85</Subscript>Te<Subscript>15</Subscript> glassy alloys

The effect of Ge additive on the physical and dielectric properties of Se₇₅Te₂₅ and Se₈₅Te₁₅ glassy alloys has been investigated. It is inferred that on adding Ge, the physical properties i.e., average coordination number, average number of constraints and average heat of atomization increase but lone pair electrons, fraction of floppy modes, electronegativity, degree of crosslinking and deviation of stoichiometry (R) decrease. The effect of Ge doping on the dielectric properties of the bulk Se₇₅Te₂₅ and Se₈₅Te₁₅ glassy alloys has also been studied in the temperature range 300–350 K for different frequencies (1 kHz–5 MHz). It is found that, with doping, the dielectric constant ε^′ and dielectric loss ε^″ increase with increase in temperature and decrease with increase in frequency. The role of the third element Ge, as an impurity in the two pure binary Se₇₅Te₂₅ and Se₈₅Te₁₅ glassy alloys has been discussed in terms of the nature of covalent bonding and electronegativity difference between the elements used in making the aforesaid glassy systems.