Alexander Bakai

On the polyamorphism of fullerite-based orientational glasses.

The dilatometric investigation in the temperature range of 2–28 K shows that a first-orderpolyamorphous transition occurs in the orientational glasses based on C60 doped with H2, D2 andXe. A polyam ...

The pre-factor D0 of the diffusion coefficient in amorphous alloys and grain boundaries

Abstract Self-diffusion in several amorphous alloys and in silver grain boundaries of different orientation are analyzed and compared with those of crystals with the aim of a better understanding of the diffusion mechanism operating in disordered structures. Several experimental observations are discussed, including the concentration depth profiles, the Arrhenius plots of the diffusion coefficients and the effective activation volume and isotope effect. As an important difference between the diffusion behavior of the amorphous alloys and general grain boundaries on the one side and of crystals on the other side a distinctly different correlation between the pre-factor, D 0 , of the diffusion coefficients and the effective activation enthalpy is identified. This correlation is regarded as key information to determine the validity of different diffusion models proposed for disordered structures. It is shown that a detailed consideration of the structural disorder and the related distribution of energy states of the migrating atoms is necessary in order to understand the diffusion in disordered structures.

On thermodynamics of supercooled liquids and glasses

Thermodynamics of glass-forming liquids and glasses is considered taking into account the contribution of heterophase fluctuations to the free energy. The role of heterophase fluctuations in liquids and glasses is played by nuclei of noncrystalline solids and nuclei of the liquid phase respectively. Crystalline solid state is not considered. Free energy is described on the bases of the droplet model and the model of mixed state. It is shown that three regions of stability of different structural states of the system exist on the (P,T) plane. Two of them are stability regions for droplet structural states, while the mixed state is stable in the third region in which heterophase fluctuations have the form of isolated drops. In this state, two infinite percolating clusters (liquid and noncrystalline solid) coexist. The width of the temperature interval [Tt1,Tt2] in which the mixed state is stable increases with configuration entropy (complexity) of the noncrystalline solid fraction. In the approach used, wea...

On the nature of the Kaiser effect in metallic glasses

The Kaiser effect was observed in the measurement of acoustic emission (AE) during the course of uniaxial compression of the bulk samples of metallic glasses Zr41Ti14Cu12.5Ni10Be22.5 and Zr52.5Ti5Cu17.9Ni14.6Be22.5. The field-ion microscopy study of bulk Zr41Ti14Cu12.5Ni10Be22.5 shows that this glass has a polycluster structure. This fact allows one to reveal the nature of AE appearing during the course of deformation of metallic glasses and to interpret the observed Kaiser effect. The dislocations generated at the intercluster boundaries and moving through the glass bulk are the sources of AE.

On low-temperature polyamorphous transformations

A theory of polyamorphous transformations in glasses is constructed in the framework of a model of heterophase fluctuations with allowance for the fact that a glass inherits the short- and intermediate-range order from the liquid. A multicomponent order parameter describing the concentration of fluctuons with different types of short-range order is introduced, along with the concepts of isoconfigurational and non-isoconfigurational transitions in the glass. Taking the nonergodicity, nonequilibrium, and multiplicity of structural states of a glass into account leads to a kinetic criterion of observability of polyamorphism of a glass. As an example, a theory is constructed for the low-temperature first-order phase transition in an orientational glass based on doped fullerite. The relaxation processes of this system are described, including the subsystem of tunneling states. The possibility of a hierarchy of polyamorphous transformations in a glass is discussed.

Construction of radiation-modified phase diagrams under cascade-producing irradiation: application to Zr–Nb alloy

Abstract A general theoretical formalism developed for the description of phase stability alteration in substitutional binary alloys under irradiation is applied to Zr–Nb alloys. We examine the stability of β-Nb precipitates in Zr–Nb alloy subjected to the cascade-producing irradiation. The results of phase stability studies are presented in the form of radiation-modified phase diagram. Evolution of large precipitates (as compared to the size of cascade region) differs from that of small precipitates. In the radiation-modified phase diagram there exists a low temperature boundary for stability of large precipitates, the location of which depends on interface type and displacement rate. Above this boundary large precipitates coarsen with radiation-enhanced rate. Below it the alloy is maintained in a quasi-steady-state of supersaturated solid solution with a population of fine-grained precipitates. The competition between processes of cascade destruction; nucleation and growth of coherent precipitates; and coherency loss can lead to the formation of the distribution of fine-grained precipitates with slowly varying parameters. In particular, such a distribution may form in Zr–Nb alloys under thermal reactor conditions.

Recombination mechanism of point defect loss to coherent precipitates in alloys under irradiation

Abstract A new mechanism of defect loss by enhanced recombination inside coherent precipitates in alloys under irradiation is described. The mechanism is examined quantitatively to find the microstructural parameters responsible for resistance to dimensional instability. The proposed model explains why radiation properties of Zr–Nb alloys depend on density of fine-grained precipitates of βNb-phase.

On the effect of radiation-induced segregation on void shape and growth rate

Abstract Microstructural changes have been studied in Cr18Ni10Ti steel irradiated in BOR-60 up to 40 dpa at 580–600 °C. It was found that in the vicinity of the voids associated with G-phase particles the composition of steel is practically the same as the average matrix one, whereas the isolated voids are surrounded by Ni and Si-enriched zones. The shape and size of a void depend on local composition near it — the mean size of octahedral voids (associated with G-phase particles) is greater than that of isolated cubic voids. A theoretical model is developed to explain the observed difference between growth rates of free and precipitate-attached voids.

On chemical bonding and helium distribution in hcp beryllium

The electron densities of states and spatial distribution of electron density in the system hcp beryllium-helium were investigated by means of ab-initio methods of simulation. It was found that contrary to predictions of the “jelly” model, the energetically more favorable configuration is that where a helium atom is located at the most restricted position, on a triangular face of two adjacent tetrahedrons, and where the charge density of electrons is maximal. It is established that this occurs due to hybridization of electron states of helium and nearest beryllium atom. The helium binding energy is about 5.6u2009eV. The spatial distribution of the charge density is investigated in details. Calculation of solution energy of helium in hcp beryllium was performed. The helium location at lattice sites in different interstitial positions and in divacancy complexes were considered. It is found that helium implemented into hcp beryllium favors formation of divacancies.

On the kinetics of spontaneous amorphization of a metastable crystalline phase

The phase transformations of a metastable crystalline phase obtained under high pressure occurring upon heating are considered: amorphization and subsequent crystallization. A model for the kinetics of these processes taking into account the capability for competitive formation of crystalline and amorphous phases at the boundaries of the grains of the initial phase is constructed. Expressions for the volume fractions as well as for the nucleation rates and growth velocities of the phases formed are obtained. Differential scanning calorimetry curves are described. A numerical analysis of the equations of the kinetics and a comparison with the results from experiment are carried out with reference to Cd43Sb57 alloy.