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Processes at the crystal—solution interface attract attention due to their great role, in particular, in the growth and dissolution of crystals in solution. The overwhelming majority of studies of dissolution and precipitation of a dissolved substance follows a classical scheme. The change of volume and shape of a crystalline substance dissolved in the surrounding liquid is followed for the given degree of nonequilibrium (unsaturation or supersaturation). In certain studies, the procedure described involves an artificial leveling of the concentration inhomogeneity at the crystal—solution interface that accompanies dissolution and growth of a crystal. In particular, the leveling is achieved by forced stirring of the solution surrounding the crystal.

Mechanisms and kinetics of crystal formation from a gas-saturated melt comprise a special area of crystal growth. Besides theoretical interest, applications are also important, in particular since gas inclusions cause various imperfections in single crystals grown from the melt with a gaseous impurity [1–3]. Therefore, formation and behavior of macroscopic gas inclusions formed during crystallization from the melt are widely studied in actual specimens and in models.

Theoretical considerations, experimentally established facts, and elementary estimates lead to the conclusion that the initial stage of the sintering process may be accompanied by plastic deformation, while a later stage is controlled by diffusion-dislocation flow, in which mass transport responsible for growth of the contact neck takes place under conditions of self-consistency of the dislocation loop generation and dissolution processes. This article is concerned only with monocrystalline objects, but the conclusions arrived at in it can be extended also to real polycrystalline systems. This will necessitate regarding the structural elements of a polycrystal as obstacles to dislocation motion characterized by different degrees of penetrability. An opportunity will thus arise in particular to obtain fuller information on the initial stage of sintering of powder compacts, which are characterized by high sintering activity.

The results of model experiments confirm the validity of the hypothesis that the plastic flow of material in the contact zone is a consequence of conservative motion of dislocations in the contact region. Before the capacity for mass transport by this mechanism and hence its contribution to “activity” can be reliably assessed, it will be necessary to examine the problem of possible dislocation multiplication in the peripheral zone of the contact. At present we can only assert that this capacity is not less than that due to dislocations linked with the previous history of a material. Under conditions of a model experiment this capacity is determined by the density of dislocations generated in the vicinity of the crack mouth during cleavage. In the case of ideally brittle fracture this density might prove to be equal to zero.

A systematic review is made of the experimental and theoretical investigations of the mechanism and kinetics of mass transfer in a system of islands of substance A deposited on a solid substrate of substance B in which A is insoluble. The analysis is carried out on the assumption that the total amount of matter present jointly in the islands and in a two-dimensional gas of adsorbed atoms is constant. Behavior of systems of islands at rest and in motion is discussed. Coalescence of islands is considered in the case of diffusive mass transfer between islands at rest and also in the case of direct collisions between moving islands. "Dispersion hardening" of the surface by a system of islands, which hinders evaporation and growth of a crystal, is described. Considerable attention is devoted to reporting experimental investigations carried out mainly by electron microscopy. Some new effects are predicted, including a possible phase transition during coalescence of islands, size effect in recrystallization, one-dimensional coalescence, etc.

The history of the discovery and investigation of ascending diffusion and the diffusion aftereffect is described. The role of V. S. Gorskiĭ (1905-1941) in the discovery and study of these effects is discussed. A brief presentation of V. S. Gorskiĭ's biography and the content of his pioneering work (1934-1935) on this problem is given. Diffusion inelasticity and equilibrium segregation are discussed within the framework of V. S. Gorskii's ideas. Kinetic segregation, which appears in situations when an alloy contains an active source of vacancies and the diffusion of the components of the alloy is directly proportional to the different partial diffusion coefficients is discussed in the concluding section.

1. We have analyzed the possibility of applying metallographic inspection to solids with defects which have been subjected to high-temperature annealing for the accurate determination of the densities of such solids (microscopic pycnometry). 2. As an example, we have considered the problems concerning the defects appearing in metals which have been subjected to cyclic heat-treatment and in metals obtained by electrolytic deposition. 3. The energy of vacancy formation in copper was determined with respect to data on the actual density of copper which was subjected to cyclic heat treatment.

A detailed analysis is made of the mechanism of sintering of coarse (from the powder metallurgy viewpoint) powder particles. Phenomenological quantitative functions are derived involving the kinetics of variation of contact size, and experimental results are presented on the sintering together of copper hemispheres and a copper plate. A relationship between the effective viscosity and the compressing force (hemisphere weight) is determined.

I. Introduction 129 II. Methods for Determining the Surface Energies of Solids 130 III. The Natural Roughness of the Surfaces of Crystals 136 IV. Change of Surface Profile in Single Crystals and Polycrystalline Materials 139 V. The Development of Intergrain Thermal-etching Grooves 147 VI. Liquid Films on Surfaces of Crystalline Materials 151 VII. Conclusion 153 Literature Cited 153

1. It was shown that, at an early stage of annealing of a porous solid with a boundary lattice, there occurs an interaction between the boundaries and the pores, which consists in the stage of the separation of boundaries from the pores being replaced by the stage of the transfer of a portion of pores to the boundaries, which become stabilized in time. 2. The coalescence of pores in a porous polycrystal was investigated experimentally. At a remote stage, this process obeys the kinetic law R3 ≈ t, which was established earlier in [2]. 3. It was shown that, in regions with a large number of pores, the coalescence process may not be accompanied by diffusion-mechanism sintering, i.e., a reduction in the over-all volume of pores.

A description is given of the correct procedure for processing results of model sintering experiments for the purpose of finding coefficients of self-diffusion. It is shown that the large quantity of published data on X vs t relationships has been processed without taking into account the strong influence of the initial stage of rapid contact growth, which is followed by a stage of slow growth, on the value of D determined from the X vs t relationship.

A study has been made of the shrinkage characteristics of compacts obtained by pressing: 1) “active” powders of electrolytic origin, 2) powders thoroughly “deactivated” by high-temperature annealing, and 3) a mixture of equal amounts of the above two powders. It is shown that the observed shrinkage characteristics can be qualitatively explained by assuming that, in addition to the usual mechanism of “deactivation” operating during annealing, there is also a mechanism of “activity” loss resulting from defect migration from “active” to “deactivated” particles.

The details of the high-temperature relief on a macroscopically smooth surface should move systematically during evaporation and condensation because they act as sources or sinks for the atoms involved.

The migration of inclusions in two-component crystals (solid solutions, ionic crystals, and other compounds) is, in many respects, different from their migration in crystals which contain atoms of one kind only. In the case of two-component crystals, an inclusion may migrate under the influence of diffusion fluxes of atoms of both types, and the mobilities of these atoms in an external force field may be different. Therefore, gradients of concentrations of atoms or defects (or an additional inhomogeneous force field) are needed to ensure continuous diffusion fluxes of different kinds of atoms so that the rate of arrival or departure of atoms of different kinds on the surface of a moving dislocation are equal.

When external forces are applied to a solid or a liquid, second-phase particle or to a bubble of gas in a liquid, the particle or the bubble begins to move at a velocity determined by the Stokes formula. The moving inclusion forces the liquid apart and sets up viscous flow.