Changes in the potential and vibrational energies upon the formation of vacancies in oxygen-doped cores of crystallite-conjugation regions of polycrystalline 4d and 5d BCC transition metals

The changes in the potential (u _cmpl) and vibrational (?_cmpl) energies and the signs of changes in the interatomic spacings (Δa _cmpl) upon the formation of vacancies in oxygen-alloyed (OA) cores of crystallite-conjugation regions (CCR) in polycrystalline 4d and 5d transition metals Mo, Ta, and W have been determined. The potential energy upon the formation of vacancies (upon the formation of vacancy complexes with oxygen atoms—vacO complexes) in the OA cores of CCRs in polycrystalline Mo, Ta, and W increases (u _cmpl are positive), as upon the formation of vacancies in pure CCR cores of transition bcc d metals. The vibrational energy upon the formation of vacancies in OA CCR cores in polycrystalline Mo, Ta, and W decreases (?_vacO are negative). The negative sign of changes in the vibrational energy upon the formation of vacancies in the OA CCR cores in Mo, Ta, and W (?_vacO < 0) agrees with the independent determinations of the sign of changes in the vibrational energy in the OA CCR cores in polycrystalline tungsten (?_vacO)_W using Mössbauer measurements of the Debye temperature. The signs of changes in the interatomic spacings upon the formation of vacancies in the OA CCR cores in polycrystals of Mo, Ta and W are negative (Δa _VacO < 0), in contrast to positive (0 < Δa _BCC) changes in the interatomic distances in the nearest neighborhood of vacancies formed in pure CCR cores in Mo, Ta, and W.     

Changes in the potential energy upon the formation of vacancies in the volume and in the cores of crystallite-conjugation regions in cubic polycrystalline metals

Changes in the potential energy of atoms that constitute the nearest neighborhood of vacancies formed in the bulk of d transition and precious cubic metals have been determined. These changes agree with the available first-principles calculations of changes in the potential energy of atoms of the nearest neighborhood of vacancies. In the cores of crystallite-conjugation regions (CCRs) of bcc polycrystalline d transition metals, the formation of vacancies is accompanied by positive changes in the potential energy of atoms of their nearest neighborhood. The absolute magnitudes of these changes are several times less than the changes in the potential energy of atoms of the nearest neighborhood of vacancies in the bulk of these metals, in accordance with the relationship between the enthalpies of formation of vacancies in these regions of polycrystals. The changes in the potential energy of atoms of the nearest neighborhood of vacancies formed in the cores of CCRs of fcc polycrystalline metals are negative because of the split structure of vacancies in the CCR cores of such metals.     

Changes in the potential and vibrational energies upon the formation of vacancies in the oxygen-alloyed cores of crystallite-conjugation regions of polycrystalline Cr, Cu, and Ir

The changes in the potential and vibrational energies upon the formation of vacancies in the oxygen-alloyed (OA) cores of crystallite-conjugation regions (CCRs) in polycrystals of d transition metals Cr, Cu, and Ir have been determined. The potential energy increases upon the formation of vacancies (upon the formation of complexes of vacancies with oxygen atoms) in the OA cores of CCRs in the polycrystalline Cr, Cu, and Ir. However, the vibrational energy decreases upon the formation of vacancies in the OA cores of CCRs in these polycrystals, as in the OA cores of CCRs in polycrystals of the 4d and 5d transition bcc metals Mo, Ta, and W. The volume of the OA cores of CCRs in polycrystals of Cr, Cu, and Ir decreases upon the formation of vacancies. The changes in the interatomic interactions and dynamic properties in the regions of vacancy localization in the OA cores of CCRs coincide with analogous changes introduced into the lattices of metals by split interstitials.     

Crystallite-conjugation regions and adjacent lattice regions in polycrystalline iridium: III. Enthalpies of formation of vacancies and the energies of interaction of partners in vacancy complexes with oxygen formed in the cores of crystallite-conjugation regions in polycrystalline iridium

The interatomic spacings in the cores of crystallite-conjugation regions (CCRs) and adjacent lattice regions (ALRs) of polycrystals either decrease or increase upon alloying of nominally pure metals with oxygen and vacancy complexes with oxygen (mVacO, where m is the number of vacancies in the complex) that are formed during annealing. These changes in the interatomic spacings lead to an increase or decrease in the isomer shifts δ of the components of the Mössbauer spectra of atomic probes ⁵⁷Fe that are localized in the CCR cores and ALRs of polycrystals [1–6]. The enthalpies Q _mcmpl1, of formation of vacancy-oxygen complexes mVacO in the CCR cores have been measured for Ir and Cr polycrystals, and the enthalpies Q _Vac,1 of formation of vacancies in CCR cores have been determined for Ta, W, Ir, and Cr polycrystals. The enthalpies E _mcmpl of the interaction between the partners of the complexes increase with increasing number m of vacancies in the complexes.     

Crystallite-conjugation regions and adjacent lattice regions in polycrystalline iridium: II. Composition and properties of cores of crystallite-conjugation regions in polycrystalline iridium during annealing in an ultrahigh vacuum

Composition and properties of cores of crystallite-conjugation regions (CCRs) formed during annealing of polycrystalline iridium (poly-Ir) in an ultrahigh vacuum (UHV) have been studied using the method of intercrystalline diffusion in combination with Mössbauer spectroscopy (ID+MS) that has been developed in our previous works. Upon annealing in a UHV, poly-Ir is doped with oxygen from the atmosphere of the vacuum chamber. Complexes containing two vacancies per oxygen atom are formed in the CCR cores of poly-Ir because of a rearrangement of the atomic structure of the CCR cores upon their doping with oxygen. Using the ID+MS method, we for the first time revealed a “compensated” state of CCR cores in poly-Ir samples annealed in a UHV and of CCR cores in poly-Cr annealed in technical vacuum. The cause of the appearance of “compensated” states in CCR cores is the mutual compensation of relaxation volumes with opposite signs characteristic of different point defects. The relaxation volume of an oxygen atom in the CCR core of poly-Ir is by an order of magnitude greater than that in poly-Cr.     

On the structure formation in polycrystalline metals and alloys. I: Thermocycling

It has been found that the thermocycling of the Fe-3 wt % Si commercial alloy with a bcc lattice considerably changes its microstructure even in the absence of a temperature gradient over the cross section of the sample. This phenomenon is ascribed to the difference in the properties of grains and intergranular layers. These changes in the microstructure have been found to exert considerable influence on the structure and texture formation upon subsequent annealing.     

On the structure formation in polycrystalline metals and alloys: II. Recrystallization

Effects of the difference in the thermal expansion coefficients of grains and intergranular interlayers on the process of recrystallization in metals and alloys are considered.     

Pressure effect upon the formation of the physical contact during welding metals in a solid state with the use of current pulses

By the example of the welding of heterogeneous metals of a Steel 20 + M1 Copper pair in the solid state with the use of current pulses, it is shown that the changes in the pressure within the range from 0.35 to 0.5 of the yield point of the less strong metal are more expedient. In this case, the level of the local microdeformations is found to make up from 40 to 50%, which provides a rather tight contact between the welding surfaces as applied in the studied method of welding. Thus, this allows us in the course of the further treatment to insulate them from the surrounding medium by the current and creates the possibility to obtain a welded junction without degassing or using a protective medium, which is confirmed by experimental testing.     

Changes in the electronic structure and energy of nanoclusters of 3d metals and TiFe and TiNi compounds upon BCC-HCP transformations

The electronic structure and the energy of the ground state of nanoclusters of a number of 3d transition metals and their compounds have been studied in terms of the method of scattered waves. It is shown that a substantial difference in the electronic structure of cluster fragments and bulky materials leads to significantly different dependences of the differences of the energies of crystalline modifications on the average electron concentration.     

Role of transition metals in biological systems: Mn-protein center of water oxidation and its function in photosynthetic oxygen formation

The formation of oxygen during photosynthesis results from the photooxidation of intracellular water molecules and is one of the most important and least decipherable processes in animate nature. The O₂ formation has been ascertained to take place in the oxygen-evolving complex (OEC) of chloroplast membranes at the expense of effective transformation of absorbed light energy into chemical energy of oxidative equivalents localized on Mn cations of the water-oxidizing center of the OEC. Based on the study of the molecular composition, photochemical properties, and structural organization of the isolated oxygen-evolving complex, it has been ascertained that the OEC represents the dimer of the pigment-lipoprotein complexes of photosystem 2 (PLPC PS-2) associated according to the dissymmetry rule into the integral structure on the basis of hydrophobic bonds. The studies on the regularities of the functioning of the OEC has permitted us to develop a mechanism of photosynthetic water oxidation and oxygen formation that is based on the concept of the formation of the two-anode reactor in the water-oxidizing center, which is determined by the symmetry in the location of structural D1-proteins and unidentant ligands (Tyr_z) that are coordinated with Mn cations that perform water molecule oxidation, as well as in the structure of the hydrophobic boiler. Taking into consideration these data, we have conducted the quantum-chemical modeling of the reaction process, which confirms the developed mechanism. It has been shown that this mechanism is also possible in the monomeric complex of PS-2, the structure of which is determined by homologous D1/D2 proteins.     

Simulation of characteristics determining pressure effects on the concentration and diffusivity of vacancies in BCC metals: A new approach

This work is devoted to the development of a new model (based on the molecular-statics method) for studying the diffusion properties of point defects in metals. In the model, a new algorithm is realized, which makes it possible in a self-consistent manner to calculate the atomic structure in the vicinity of a defect and the constants which determine atomic displacements in the elastic medium surrounding the computational cell. We also took into account the fact that the energy of the system depends on pressure. This dependence is different in the case of an ideal system and a system with a defect, which gives an additional contribution to the volumes of formation and migration. Furthermore, we took into consideration that the time required for an atom to jump into a vacancy is about that required for an atom to execute only a few vibrations in a lattice site. In this period, only atoms that are located in the immediate proximity to the center of dilation have time to respond for the disturbances arising in the system; therefore, when calculating the volume for the vacancy migration we carried out only a partial relaxation of the system. Within the framework of this model, we calculated the energies and volumes of vacancy formation and migration in different bcc metals.     

On the formation of serrated grain boundaries and fan type structures in an advanced polycrystalline nickel-base superalloy

The formation of serrated grain boundaries and fan-type structures in an advanced polycrystalline nickel-base superalloy has been extensively studied and linked to the thermal history. The development of grain boundary serrations has been quantitatively linked to the cooling rate from the solution heat treatment temperature. Slower cooling rates were found to result in more convoluted grain boundaries formed as a result of impingement of growing secondary γ′. Subtle differences were also noted in the γ′ distributions in the vicinities of the grain boundaries, in particular they exhibit differences in size, morphology and chemistry as a function of location within the grain, all of which are reported in the paper. Fan-type structures were found to form in the alloy, irrespective of the cooling rate employed, although their size and volume fraction increased with a decrease in cooling rate. The presence of these structures is shown to impact upon the total length of grain boundary and the degree of serration. The fan-type structures exhibit a typical dendritic morphology and were found to nucleate on minor phases present within the alloy.     

Influence of the relaxation processes on the structure formation in pure metals and alloys under high-pressure torsion

A study has been performed on three classes of materials in which different structure-forming processes dominate, namely, cold hardening in iron and structural steels, dynamic recrystallization in copper, and pressure-induced transformation in austenite steel. These processes are shown to affect a staged character of the structure formation upon high-pressure torsion. In the case of cold hardening, the character is controlled by true strain, the growth of which results in an increase in hardness and a refinement of the structure elements. The termination of these processes can be caused by another mechanism of relaxation, e.g. phase transformation induced by high pressure. Upon dynamic recrystallization, the staged character of deformation is determined by the temperature and rate of deformation.     

Electron-microscopic study of the formation and distribution of microcracks in austenitic steels upon low-cycle fatigue tests

Problems of nano- and micromechanics of fracture of thin foils of an austenitic steel have been studied using transmission electron microscopy (TEM) after low-cycle fatigue (LCF) tests. In view of the large urgency of these problems, a new technique of their study is proposed. The places of the formation of microcracks, their distribution, and their interaction both with each other and with the structural elements of the steel and inclusions in it have been studied. The crystallographic orientations of slip bands and microcracks have been determined. It is shown that the coalescence or stoppage of microcracks depends on the angle of their joining in the limits of a given microregion of a grain. The influence of the anisotropy of the grain and inhomogeneity of the distribution of stresses on the growth and the trajectory of the propagation of microcrack has been examined.     

Shift of the transition temperature and the change in the free energy upon directional solidification of solutions at a given rate

Conditions of equilibrium at the interface between the solid and liquid phases in the course of directional solidification of a two-component melt and the rate of the decrease in the free energy of such a system in the case where diffusion in the solid phase can be neglected have been investigated. Using the previously suggested new boundary conditions for the diffusion equation, analytical expressions have been obtained for the shifts of the values of the concentrations in the liquid and solid phases at their interface, as well as for the solidification temperature as compared to the initial equilibrium values corresponding to a zero growth rate. It is shown that the magnitudes of these shifts are directly proportional to the solidification rate and are inversely proportional to the coefficients that describe the transport of atoms of various sorts through the interface. Using the above-mentioned boundary conditions, we have found an expression for the rate of the decrease in the free energy of the system under consideration and have shown that it can be divided into two parts, one of which is determined by the processes at the interface between the phases and the other is controlled by diffusion processes that occur inside the volume of the phases. The results are compared with those obtained in terms of a different approach, which is based on the determination of the balance between the changes in the free energy in the solid and liquid phases far from the solidification front. It is shown that both approaches yield similar results, so that in some cases they can be used as mutually complementary.     

MD study of the glass transition in binary liquid metals: Ni6Cu4 and Ag6Cu4

The structure, thermodynamic and kinetic evolution of the binary Ni₆Cu₄ and Ag₆Cu₄ liquids during a rapid cooling process were studied in detail by using the molecular dynamics (MD) simulation method. The Gibbs free-energy difference between the supercooled liquid and the crystal (ΔG_lc) for the pure metals Ni and Ag and for alloys Ni₆Cu₄ and Ag₆Cu₄ is calculated. A time–temperature-nucleation diagram for Ni₆Cu₄ is constructed based on the temperature dependence of the characteristic relaxation times. The results indicate that the driving force for the crystallization ΔG_lc plays an important role in the glass transition process. The increase of the metallic glass forming ability (GFA) is in consistent with the decrease of ΔG_lc.     

Changes in the state of sulfides and impact toughness of the quenched 40KhGM steel upon heat treatment in the austenitic region

The impact toughness of the quenched 40KhGM steel has been investigated as a function of the heat treatment conditions in the austenitic region. The maximum impact toughness was observed after heating to 1300°C, intermediate annealing at 1050°C, and quenching with low-temperature tempering. According to the results of the fractographic and electron-microscopic examinations, the high impact toughness is due to the specific type of the manganese sulfide precipitation from the γ solid solution. The results of a theoretical estimation of the average sulfur solubility in the γ solid solution are presented.     

The transition from the formation of mixed scales to the selective oxidation of the most-reactive component in the corrosion of single and two-phase binary alloys

The conditions for the transition from the formation of mixed scales to the exclusive oxidation of the component B, forming the most stable oxide, are examined for both single-phase and two-phase binary A-B alloys by taking into account the displacement of the alloy-scale interface due to the growth of the protective oxide. This procedure eliminates the inconsistencies arising from Wagner's classical treatment for single-phase alloys when the interdiffusion coefficient in the alloy is small with respect to the parabolic rate constant for outer-scale growth; but the same procedure leads to a significantly-improved treatment also for two-phase alloys. For the latter systems, the transition is shown to depend also on the solubility of B in the A-rich phase.Moreover, the exclusive growth of the most-stable oxide is more difficult than for single-phase alloys because it requires higher average concentrations of B in the alloy and may even become impossible if the parabolic rate constant of oxidation is large with respect to the interdiffusion coefficient in the alloy.     

The influence of oxide films on a steel surface and their role in adhesion contact formation upon the deposition of condensation coatings

The mechanism of contact formation, determined by the temperature mode of condensation and technological parameters of the process, is explained. Theoretical calculations and their experimental verification show that, in the case of ordinary thermovacuum deposition, the preliminary heating of steel in a vacuum in an atmosphere of residual gases determines the formation of an oxide film corresponding to the γ-Fe₂O₃ structure.