Reaction of different types of attractive dislocations in alloys with long-range order

The strength of junctions is calculated arising from the reaction of attractive superdislocations of the primary glide system with a tree of the forest, and from the reaction of a single glide dislocation with a superdislocation of the secondary glide system. The calculated strength and stability of the junctions are compared with the strength and stability of junctions formed between attractive trees in fcc metals, and superdislocations in an LI₂ superstructure.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 108–113, May, 1976.     

Analysis of the conditions favoring the onset of long-range order in disordered relaxors of different types

The possibility of realizing a stable relaxor behavior in PbSc_1/2Nb_1/2O₃ (PSN) compounds and the conditions favoring the formation of a phase with long-range order in PbZn_1/3Nb_2/3O₃ (PZN) are studied using optical methods. It is found that the replacement of only 6% of Pb ions with Ba ions (PBSN-6) freezes the relaxor state in PSN compounds, while adding even 7% of PbTiO₃ to the PZN relaxor (PZN-7PT) leads to the formation of a phase with long-range order and a phase transition occurring at T _maxε through the percolation mechanism. Common features in the formation and destruction of the long-range order phase in relaxors of different types are revealed. The differences associated with the presence of spontaneously polarized ferroelectric regions in the cubic nonpolar matrix of PSN-type compounds are determined.     

Spin-glass-like behaviour and long-range order in dilute YTb alloys

Using neutron diffraction, we have examined the magnetic ordering of two YTb alloys whose magnetic properties are characteristic of spin-glass materials. For YTb_{5 at %} we have determined that a spiral state appears below the susceptibility cusp at 26.6 K, despite differences between field coolled and zero field cooled magnetization. No evidence for long-range order was found for YTb_{3 at %} below its cusp at 15.5 K. We conclude that irreversibility alone can not be used to characterize the spin-glass state.     

Saddle-point energies and Monte Carlo simulation of the long-range order relaxation in CoPt

We present atomic-scale computer simulations in equiatomic L1₀-CoPt where Molecular Dynamics and Monte Carlo techniques have both been applied to study the vacancy-atom exchange and kinetics relaxation. The atomic potential is determined using a Tight-Binding formalism within the Second-Moment Approximation. It is used to evaluate the different saddle-point energies involved in a vacancy-atom exchange between nearest-neighbour sites. The potential and the saddle-point energies have been used to simulate the relaxation of the long-range order in CoPt using a Monte Carlo technique. A vacancy migration energy of 0.73±0.15 eV and an order-disorder transition temperature of 935 K have been found.     

Vacancies in binary alloys: II. Equilibrium concentration of vacancies in alloys with long-range order

General formulae are presented for the equilibrium concentration of vacancies in binary alloys having two types of lattice sites. The simple model used takes in account the pair interaction between nearest neighbouring atoms. The formulae derived in this paper yield in special cases identical results as the precedent theories.     

Kinetics of spinodal decomposition in Fe-Si alloys

The spinodal decomposition in the two-phase region of the phase diagram (B₂ + DO₃) of Fe-Si alloys is studied on the basis of the generalized diffusion equation. The configurational free energy is calculated in the Bragg-Williams-Gorsky approximation. The effect of the coherency strain energy is estimated. In the main part of the paper the Langer's method is applied in the investigation of the coarsening rate in Fe-Si system with a good agreement with experimental data in a broad range of annealing times. The limitations of such approach are discussed with respect to the Khachaturyan's theory of the periodic distributions of decomposition products.The author is grateful to Dr. F.Kroupa for his guidance and encouragement during the course of this work, to Dr. S.Libovický and Dr. A.Gemperle for discussions of their latest experimental results.     

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.     

The lattice deformation and the energy of antiphase boundaries in Fe-Si alloys

The lattice deformation across the antiphase boundary and the energy of both types (a/2111 anda(100) of antiphase boundaries lying in {110} plane are calculated using a series of three interatomic potentials fitted to experimental data. It is shown that the relaxation of atomic planes in the vicinity of antiphase boundary is important for thea/2111 antiphase boundary and is negligible for a 100 antiphase boundary in the DO₃ structure.The author is grateful to Dr. F.Kroupa and to Dr. A.Gemperle for valuable discussions.     

Generalized stacking faults in model lattice of ordered Fe-Si alloys

The energies of the generalized stacking faults lying on {110} and {112} planes are calculated for Fe-Si alloys with the DO₃ long-range order for concentrations 10–25 at. % Si. In the used model, the interaction between atoms is described by the series of three central interatomic potentials fitted to experimental data. The lattice relaxations perpendicular to the stacking fault plane are considered. The displacement vectors corresponding to stable antiphase boundaries are found and some consequences for the structure of superdislocations in the DO₃ superlattice are mentioned.The author is indebted to Dr. F.Kroupa and Dr. L.Lejek for valuable discussion.     

The short-range order and ordering energies in gold-copper polycrystalline alloys that are rich in gold

The concentration dependence of the short-range order parameters in gold-rich polycrystalline disordered solid gold-copper solutions was investigated using the X-ray diffuse-scattering method. On the X-ray diffuse-scattering patterns of disordered alloys with 75, 83, and 90% gold, diffuse maximums that are typical of short-range order were absent in the region of the possible superstructure reflex (100), but in the region of the possible reflex (110), an intensity rise on the scattering patterns was revealed that noticeably depended on the gold concentration. The application of the method, taking the effects of static displacements on the considered coordination spheres into account, allowed the determination of short-range order parameters for nine coordination spheres. The spectra of short-range order parameters, which was calculated using the least-squares method, had the long-range sign-alternating representation that is inherent in the Cu₃Au superstructure. The absolute values of short-range order parameters were detected to decrease with an increase of the gold content in the alloys. The concentration dependence and small values of ordering energy of investigated polycrystalline alloys were revealed on the first coordination sphere using the Clapp-Moss method.     

Existence of long-range order in the Migdal recursion equations

A modification of XY — model is introduced for which Migdal recursion equation are exact. High- and low-temperature fixed points of these equations are investigated. As a result the existence of long-range order at low temperature and its absence at high temperature are proved rigorously for the model under consideration in the case when dimensiond>2.     

The formation of an ultrafine-grained structure in long-range order metals and alloys subjected to phase transformations and irradiation

The generation of an ultrafine-grained structure in long-range order metals and alloys by phase transformations and ion irradiation is studied. It is shown that the structure of pure iridium can be significantly refined under certain regimes of ion irradiation, and a microduplex structure possessing high mechanical properties can be produced in binary and ternary alloys, combining the processes of atomic ordering, aging, and phase recrystallization.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 81–89, August, 2004.