Grain-boundary diffusion of oxygen in polycrystalline ferrites

It is shown that layer-by-layer measurements of the electron-transfer activation energy in polycrystalline ferrites allow the profiles of depth distribution of oxygen introduced from the atmosphere into a ferrite at the stage of sintering or thermal treatment to be studied using a simple experimental technique. The analytical equations proposed here make it possible to determine the volume and grain-boundary diffusion coefficients of oxygen using a minimum number of adjustable parameters. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 64–69, May, 1999.     

Temperature dependence of the hardening factor and the microstrain stage for polycrystalline substitutional fcc solid solutions

Russian Physics Journal -     

Contribution from dislocation to deformation resistance in polycrystals of copper-based solid solutions

The evolution of dislocation structure in solid solutions of Cu-Al and Cu-Mn systems with different grain sizes and at different test temperatures is studied by means of transmission electron diffraction microscopy. The scalar density of dislocations is measured and its relationship to the flow stress of alloys is determined. Changes in the contribution from dislocation hardening to deformation resistance upon variations in the contributions associated with changes in grain size, solid-solution hardening, and test temperature are analyzed.     

Resistance to dislocation motion in copper-based solid solutions with a statistically disordered atomic distribution

The stress _F due to friction forces in copper-based solid solutions was determined. Under the conditions of the procedure used to measure _F, on the basis of the half-wave hysteresis with polycrystalline samples, the value of d: _F = _F0 + K_Fd^–(1/2) where _F0 is the resistance to dislocation motion in an alloy having an infinite grain size, and K_F is a constant. It is shown that _F0 is governed by the interaction of moving dislocations with impurity atoms in the case of a statistically disordered atomic distribution. A study was made of the effects of various factors on _F and of the nature of the changes in _F0 caused by alloying.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No.3, pp. 18–23, March, 1971.     

Grain-boundary physics in polycrystalline CuInSe2 revisited: experiment and theory

Current studies have attributed the remarkable performance of polycrystalline CuInSe2 (CIS) to anomalous grain-boundary (GB) physics in CIS. The recent theory predicts that GBs in CIS are hole barriers, which prevent GB electrons from recombining. We examine the atomic structure and chemical composition of (112) GBs in Cu(In,Ga)Se2 (CIGS) using high-resolution Z-contrast imaging and nanoprobe x-ray energy-dispersive spectroscopy. We show that the theoretically predicted Cu-vacancy rows are not observed in (112) GBs in CIGS. Our first-principles modeling further reveals that the (112) GBs in CIS do not act as hole barriers. Our results suggest that the superior performance of polycrystalline CIS should not be explained solely by the GB behaviors.     

A phase-field model for elastically anisotropic polycrystalline binary solid solutions

A phase-field model for modeling the diffusional processes in an elastically anisotropic polycrystalline binary solid solution is described. The elastic interactions due to coherency elastic strain are incorporated by solving the mechanical equilibrium equation using an iterative-perturbation scheme taking into account elastic modulus inhomogeneity stemming from different grain orientations. We studied the precipitate interactions among precipitates across a grain boundary and grain boundary segregation–precipitate interactions. It was shown that the local pressure field from one coherent precipitate influences the shape of precipitates in other grains. The local pressure distribution due to primary coherent precipitates near the grain boundary leads to inhomogeneous solute distribution along the grain boundary, resulting in non-uniform distribution of secondary nuclei at the grain boundary.     

Ionic conductivity of polycrystalline samples of KBrKI solid solutions

Ionic conductivity of KBr_xI_1?x(0?x?1) mixed crystals, σ_x, has been measured as a function of temperature in the range of 370°C to close to their melting points. The variation in conductivity, σ_x, with composition in the intrinsic region was found to be non-linear, having a maximum value at x=0.3. The maximum conductivity of KBr_xI_1?x mixed crystals was never far outside the range of conductivity of the component crystals. Several expressions of the relative conductivity, σ_x/σ₁ (σ₁ refers to KBr) have been suggested.     

Temperature dependence of the macroscopic elastic limit for polycrystalline Fe-Si solid solutions

The influences of alloy composition have been examined for the temperature dependence of the macroscopic elastic limit σ″ over the range 77–293?K for polycrystalline Fe-Si solid solutions. Various factors contribute to the temperature dependence of σ″, and it is found that the variation on alloying is determined by the interaction between dislocations and dissolved atoms (substitutional and interstitial), as well as by the resistance to dislocation motion due to the short-range order; furthermore, the effect on the temperature dependence of the limit on alloying iron with silicon is determined by the short-range forces between the dislocations in the silicon atoms. Interstitial impurities tend to segregate at grain boundaries, which weakens the temperature dependence of the limit.     

Role of packing-defect energy in the localization of plastic strain during shock-wave loading of copper-based solid solutions

The formation of deformation structure accompanying a change in the packing-defect energy (PDE) and the conditions of deformation of copper-based solid solutions under shock-wave loading were investigated by the method of optical microscopy. It is shown that in the case of solid solutions with high PDE plastic strain is localized. This localization increases with the amplitude and duration of the shock pulse, and this in turn gives rise to the generation and growth of microcracks. V. D. Kuznetsov Siberian Physicotechnical Institute at Tomsk State University. Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 2, pp. 30–34, February, 1993.     

Correlation between work hardening and stress relaxation in polycrystalline nickel

Stress-relaxation rates at a constant strain in A-grade nickel polycrystals has been reported to depend in a peculiar manner on the initial stress levelσ ₀ at which relaxation is allowed to start. For large grains (D>75μm),s varies withσ ₀ linearly over the entire stress strain curve. For small grains (D<75μm),s-σ ₀ curve undergoes a change in its slope at a critical value of plastic strainɛ, which decreases as grain size increases. The observation referred to are found to correlate well with the work-hardening behaviour of the nickel polycrystals.     

On additional aging-induced hardening of textured ribbon substrates prepared from binary copper-based alloys

The structure and microhardness of textured ribbon substrates made of binary copper-based alloys and annealed in the temperature interval 400–600°C is investigated. The optimal temperature of additional annealing at which the strength of Cu–Fe and Cu–Cr alloys reaches a maximum is determined. From experimental data, recommendations on the optimal deposition temperature of epitaxial buffer layers and superconducting films can be developed.     

Cellular dislocation substructure in polycrystals of FCC solid solutions: quantitative characteristics,laws of formation,and role in hardening

A cycle of investigations carried out by the authors and devoted to the most important cellular dislocation substructure is generalized. Laws of formation of this substructure upon plastic strain of FCC Cu–Mn and Cu–Al alloy polycrystals are considered. The influence of the grain size, strain temperature, and alloy concentration on the parameters of evolving cellular dislocation substructures (DSS) is quantitatively analyzed by the transmission electron microscopy (TEM) method. Special attention is given to the kinetic phase transition in the defect subsystem leading to the formation of the cellular DSS. Based on modern dislocation models, it is demonstrated that hardening by the cellular DSS obeys the main dislocation laws.     

Fast solute diffusion in solid and supersaturated solid solutions

In this paper, we review the experimental data concerning fast diffusion of solute in Pb matrix and Pb (solute) solid solutions. Using both calorimetric and radiotracer techniques the temperature dependence of solute diffusion coefficients in Pb (solute) supersaturated solid solutions is determined. On the basis of experimental results it is shown that these coefficients can only be understood by assuming different defect states for solutes in solid and supersaturated solid solutions.     

Grain-boundary fluctuations in two-dimensional colloidal crystals

We study grain-boundary fluctuations in two-dimensional colloidal crystals in real space and time using video microscopy. The experimentally obtained static and dynamic correlation functions are very well described by expressions obtained using capillary wave theory. This directly leads to values for the interfacial stiffness and the interface mobility, the key parameters in curvature-driven grain-boundary migration. Furthermore, we show that the average grain-boundary position exhibits a one-dimensional random walk as recently suggested by computer simulations [Z. T. Trautt, M. Upmanyu, and A. Karma, Science 314, 632 (2006)]. The interface mobility determined from the mean-square displacement of the average grain-boundary position is in good agreement with values inferred from grain-boundary fluctuations.     

On solid solution hardening in crystals with randomly distributed solute atoms

The temperature dependence of the critical resolved shear stress (CRSS) of solid solution single crystals is explained on the basis of random distribution of solute atoms in the alloy. The calculated temperature dependence of CRSS in a crystal with both isolated solute atoms and their pairs is very similar to the experimentally observed one, i.e., the pronounced temperature independent region, so called plateau, appears.