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The effect of bulk radiative heat transfer during the growth of profiled sapphire crystals from the melt is studied by the numerical simulation method. The peculiarities in the structure of a lightguide flow in the crystal and in the melt, as well as the possibility of radiation instability of the interface, are considered. It is shown that overcooled regions with the size determined by the growth rate appear under the crystallization front. It is found that the use of a conical shaper ensures a more stable growth.

The influence of thermomechanical cycles (compression along the [011] direction and recovery of the shape memory strain upon crystal heating) on the form and characteristics of the compression diagrams of Ni49Fe18Ga27Co6 alloy crystals not subjected to high-temperature annealing and quenching in water after the growth has been investigated. It is found that these characteristics change during the first nine cycles. Starting from the tenth thermomechanical cycle, they become stable and the deformation properties of the crystals become similar to those of quenched crystals of this alloy. This means that antiphase nanodomains are dispersed by dislocations during the thermomechanical cycling, as a result of which the disordered B2 crystal structure is transformed into the ordered L21 structure, which is characteristic of quenched crystals.

Stress–strain diagrams (σ–e curves) have been studied during uniaxial compression of Ni49Fe18Ga27Co6 alloy single crystals in the [011] axis direction for the initial samples and those after 15-min annealing at 1373 K with subsequent quenching in water. It was established that the σ–e curves of initial crystals exhibited a one-stage character and showed a single stress decay, whereas the curves of heat-treated and quenched crystals displayed two stress decays. The shape-memory (SM) strain recovery on heating the compression-strained crystals showed an anomalous (burstlike) character. Investigation of the reproducibility of burstlike recovery of SM strain in a series of thermomechanical treatment cycles showed that the recovery took place in quenched crystals, whereas the temperature interval of SM recovery in the initial crystal grew from 1 to 12 K with increasing number of thermomechanical treatment cycles.

We have performed experimental and theoretical investigation of the anomalous form of the compression diagrams and shape memory restoration curves in Ni49Fe18Ga27Co6 alloy crystals deformed by uniaxial compression along the [011]A crystallographic direction (A-austenite) in the temperature range of 200–350 K. It is found that in the investigated temperature range, all compression diagrams contain anomalous segments of smooth and sharp decrease in deforming stresses. It is shown that the segments of a smooth decrease in stress are associated with peculiarities in martensite reaction L12 → 14M, while segments of a sharp drop are due to instability of martensite reactions 14M → L10 and L12 → L10. A possible source of reaction instability is associated with interfacial stresses at the interfaces between the martensite and austenite phases (lamellas) due to different elastic moduli of contacting phases. The magnitude of these stresses is significant in the case of 14M → L10 and L12 → L10 transformations, which induces a sharp drop of the deforming stress, while the restoration of the shape memory effect is of a burst nature. It is established that the contribution of interfacial stresses to the free energy of martensite transformation is smaller than the dissipative (entropy) contribution to this energy; however, interfacial stresses higher than a certain threshold strongly affect transformation kinetics and, hence, determine the strongly anomalous shape of pseudoelastic deformation curves and burst restoration of the shape memory effect.

We have studied the influence of thermal treatment on the dynamic Young’s modulus, linear and nonlinear internal friction, and Young’s modulus defect of Ni55Fe18Ga27 alloy single crystals. Strong variations of all these parameters were observed and explained by changes in the concentration of vacancies in the crystal lattice. It is concluded that vacancies form atmospheres near twin boundaries, the density of which (but not their extension) strongly depend on the concentration of vacancies.

Uniaxial stress–strain diagrams of a cubic Ni49Fe18Ga27Co6 single crystal with the shape memory effect have been investigated under successive loads in three directions perpendicular to its faces. The crystal is found to behave radically differently during deformations along the 〈110〉A and [001]A directions. At room temperature, the crystal compressed along the [001]A direction behaves as a superelastic, irrespective of the strain value, whereas under compressions along the 〈110〉A directions, the superelastic behavior is replaced with shape memory strain at strains of more than 4%. The measured stains in the directions orthogonal to the compression are indicative of anomalous behavior of the Poisson ratio.

We present the results of experimental studies of the return of Ni–Fe–Ga–Co and Cu–Al–Ni single crystals to the initial form at the macro- and microlevels after their deformation by compression and subsequent restorative heating. It is found that the geometric dimensions of the sample are reproduced with high accuracy (up to 20 μm). The microrelief appearing on the initially polished surface during crystal compression (the arithmetic mean profile deviation Ra ≈ 0.25–0.75 μm, the average spacing of local protrusions of the profile S ≈ 0.027–0.11 mm) is not removed by subsequent heating and remains with further multiple repetition of deformation–restoration cycles.

Long crystals of NiFeGaCo alloy with shape memory effect, including magnetically controlled ones, were obtained by the methods of Czochralski and Stepanov. A strong influence on the properties of crystals of dendritic formations, especially noticeable in the initial part of the crystal, has been revealed. In order to optimize the growth experiments, the heat transfer process in the thermal growth zone was simulated. It is shown that the formation of dendrites is due to a change in heat transfer during growth, which leads to an increase in the axial temperature gradient near the crystallization front as the crystal grows. This fits into the framework of the classical concepts of the transition from dendritic growth to normal growth.

The formation of the protective and hardening oxide coatings on the surface of the single-crystal sapphire and sintered corundum (ceramics) is studied. Corundum systems with oxide coatings are as yet unknown and our studies are original. The compositions of the coating based on the mixture of Al2O3 and B2O3 oxides and a technique of their deposition on the samples are worked out. The strength of the samples during the bending test is studied. It is shown that the hardening of several samples achieves ~30% and microhardness (HV) reaches 30 GPa. Basing on the results of an investigation of the significant number of the samples obtained under different conditions, it is concluded that the developed method of the protection and hardening of the corundum products is promising.

The reactive stresses induced in Ni49Fe18Ga27Co6-alloy single crystals during martensitic transformations with a limited possibility of shape-memory-strain recovery have been experimentally studied. The data on these crystals are compared with the results obtained previously for Cu–Al–Ni, Ni–Ti, and Ni‒Fe–Ga crystals. The potential of application of the Ni49Fe18Ga27Co6 single crystals in designing drives and power motors is demonstrated.

The formation of blocks and residual stresses in shaped sapphire crystals grown from the melt by the Stepanov method (EFG) has been studied. The probability of block formation is higher for the growth along the c axis compared to that grown in the a -axis direction. The distribution of residual stress in sapphire crystals of tubular, rectangular and round cross section was measured by the conoscopy method. It was found that the magnitude of the residual stress increases from the center to the periphery of the crystal and reaches up to about 20 MPa. Residual stress tensor components for solid round rod and tubular single crystals were determined by numerical integration.

The paper considers the experimentally observed instability of capillary shaping during the growth of thick-walled sapphire tubes by the Stepanov method. The explanation of this phenomenon is based on the theoretical model of radiative-conductive heat transfer in a crystal. An algorithm is developed for the asymptotic expansion of the problem based on the presence of two small parameters. It is shown that the spatial density of the radiation is inhomogeneous along the cross section of the tube and is maximum near its inner walls. This leads to their overheating and the meniscus separation from the inner edges of the shaper.

Shape recovery in conventional shape memory alloys is typically a relatively slow process showing low heating rate sensitivity. In contrast to that, some shape memory alloys such as Ni–Fe–Ga–Co, Cu–Al–Ni, Cu–Al–Fe–Mn can exhibit the effect of burst-like shape recovery when the original shape is restored almost instantaneously in a very narrow temperature range. Here we report on an abnormal stress-strain behavior of Ni49Fe18Ga27Co6 single crystals upon uniaxial compression along the [110] axis and caloric effects during burst-like recovery of shape memory strain in the samples. Differential scanning calorimetry (DSC) study revealed that DSC peak shifts to lower temperatures with increasing heating rate.

The formation of the block structure and residual stresses in sapphire single-crystal tubes grown from the melt by the Stepanov method has been studied. The distribution of the difference in the residual stress tensor components (σφ–σr) in a thick-walled [0001] sapphire tube has been measured by the conoscopy method. It has been shown that circumferential tensile σφ and compressive stresses act on the outer and inner tube surfaces, respectively. The maximum stress is ∼20 MPa. It has been demonstrated that the development of the block structure begins from the outer tube surface; as the crystal length increases during growth, blocks propagate over the entire cross section.

The results of an experimental investigation of the relative lengthening of samples of single-crystal aluminum oxide for cyclic thermal loadings in the 1200–1860 K temperature range are presented.

The formation of blocks in shaped sapphire rods of two crystallographic orientations has been investigated. It is shown that, when growth occurs in the direction of the optical c axis, blocks are formed with a higher probability than in the case of growth in the a direction. A model of formation of blocks in rods of different orientations is proposed. The distribution of residual stresses over sapphire rod cross sections is measured by conoscopy. It is found that stresses increase from the middle of a rod to its periphery and reach 20 MPa.

The residual stresses in cylindrical [0001] sapphire crystals have been studied using the polarization-optical method. The angle between the optical axes 2V and the difference in the normal components of the tensor of elastic residual stresses (σφ − σ r ) have been determined from the isogyre divergence. It has been found that a tangential tensile stress of no more than 20 MPa acts on the ingot surface. The residual stresses have been compared with the calculated thermoelastic stresses generated during the crystal growth in a given heating zone. It has been shown that the determined pattern of residual stresses can be caused by thermoelastic stresses developing in the immediate vicinity of the crystallization front.

It is lead numerical modeling of processes of heat exchange at growth of crystals of sapphire from a melt by Stepanov and Czochralski methods. The influences of volumetric radiating overcooling of a melt and of growth speed on the form of front of crystallization and distribution of temperature near to it is studied. It is shown, that the account of absorption of thermal radiation in a melt can lead to formation of volumetric supercooled areas near to crystallization front and to appreciable change of its shape.

The internal friction of the hyperstabilized martensite demonstrates very low values, both above and below the nominal martensitic transformation temperature, due to a pronounced pinning effect. Over a wide temperature range it is comparable with the level of damping in the parent phase. A study of the temperature dependence of the non-linear ultrasonic internal friction and its strain amplitude hysteresis indicates that the diffusion, assisted by dislocations/interfaces, is quite pronounced and in Ni-Fe-Ga and Cu-Al-Be alloys it operates at temperatures around 20 K. The renucleation of the lamellar parent phase during the reverse martensitic transformation close to 600 K is accompanied by an internal friction peak which demonstrates a substantial transitory contribution. After renucleation of the parent phase the samples recover a conventional martensitic transformation with the internal friction level in the martensite comparable to the one in non-stabilized samples. Observations of a relaxation peak in the parent phase of different alloys for temperatures just below the renucleation stage of the reverse transformation point to the essential role of diffusion in the nucleation of the parent phase in hyperstabilized martensites.

The applicability of several nonnoble materials for growing of β-Ga2O3 crystals using free crystallization in a crucible has been studied. The possibility of growing β-Ga2O3 crystals in crucibles made of single crystal sapphire has been demonstrated. Main features of the growth and properties of the obtained crystals have been investigated.

The ways in which a block structure is formed in shaped sapphire single crystals grown from melt by the Stepanov method are considered. The measured temperature distributions and results of a mathematical modeling of the heat exchange in the growth zones, as well as the calculated thermoelastic fields and measured residual stresses, are reported. The possibility of effectively controlling the thermal fields and growth of block-free crystals by choosing optimal screening is shown for single crystals in the form of tubes and basal-plane-faceted ribbons.

The development of faces on cylindrical and ribbon-shaped sapphire single crystals grown from a melt using the Stepanov method is investigated by optical and electronic microscopy. The crystallographic indices on the detected faces are established, and the microstructure of the growth layers is studied. The X-ray spectral microanalysis of foreign particles on the sapphire surface confirms the presence of molybdenum single crystals on it. Cathodoluminescence spectra of surface layers of sapphire single crystals are obtained.

The dependence of the step parameters on the surface of base-faceted sapphire ribbons on the angle of deviation of crystallographic orientation has been measured. A model of step formation on the lateral ribbon surface is proposed. Hysteretic character of the dependence of the lateral ribbon surface inclination on the meniscus angle at the triple point is shown. The shapes of the lateral surfaces are calculated and the range of variation in the meniscus angle is determined.

The global heat transfer during growth of 50-mm basal-plane-faced sapphire ribbons in a cylindrical heat zone has been numerically simulated for different heat shield configurations. The ribbon thermoelastic strains were computated to estimate the heat zone quality. It is shown that shield adjustment and redistribution of radiative heat fluxes inside the heat zone make it possible to essentially decrease the thermal field curvature in the ribbon around the crystallization front and, therefore, decrease the thermoelastic strains by a factor of 2 to 3 (to 20 MPa).

The influence of different heat shielding constructions on the distribution of temperature and thermoelastic and residual stresses in growing basal-plane-faceted sapphire ribbons (Al2O3) is studied. It is shown that inclined shields decrease thermoelastic stresses owing to the redistribution of heat fluxes from a heater to a ribbon, which allows growing block-free basal-plane-faceted sapphire ribbons.

The structure of basal-plane-faceted sapphire ribbons with cross sections of 30 × 1.5 and 50 × 1.5 mm, grown by the Stepanov method, has been investigated using X-ray diffraction topography and selective etching. The model of block formation in such ribbons through interaction of dislocations of prismatic slip system is confirmed. The role of gas bubbles as stress concentrators and dislocation sources is established. It is shown that the dislocations of basal-slip system are involved in the development the low-angle boundaries and block formation.

The defect structure of basal-plane-faceted sapphire ribbons 10, 30, and 50 mm wide is investigated. It is shown that the high density of blocks in these crystals is associated with the specific features of the plastic deformation of the crystals with this orientation during crystal growth. In the basal-plane-faceted ribbons, the plastic deformation begins with the slip in the prismatic system. Dislocations of this system form characteristic low-angle boundaries, which then transform into block boundaries. It is revealed that the slip in the easiest basal system, which is characteristic of ribbons with all other orientations, is impossible because of the geometric constraints. Block-free basal-plane-faceted ribbons 50 mm wide are produced by decreasing thermal stresses in growing crystals.

For the first time corundum ceramics are treated with an electron beam, and they are also fired at high temperature with surface coatings made from high-melting and refractory compounds, i.e. titanium diboride, boron and zirconium carbides. Coatings are applied with magnetron deposition and a newly developed mechanochemical method. In all cases there is a considerable increase in the microhardness of treated materials and in some cases an increase in bending strength.

The effects of an anomalous biaxiality and a residual-stress field in basal-plane-faceted single-crystal sapphire ribbons were studied by an optical polarization method using a conoscopic light beam. Formulas that relate the difference between the principal stresses to the angle of optical-axis divergence are obtained. It is shown that the central regions of the ribbons undergo compressive stresses whereas their edges undergo tensile stresses. As compared to block-free ribbons, the stresses in block-containing ribbons increase more intensely with distance from the seed.

An analysis of possible reasons of block formation in sapphire ribbons with a basal faceted surface is performed. Capillary problem, radiative heat transport, dislocation structure and thermoelastic stresses are considered. It is shown that blocks arise as a result of dislocation generation in a prism slip under the action of thermal stresses. Basal faceted ribbons free of blocks are demonstrated.

The friction coefficient and relative wear rate of the aluminum oxide single crystals grown by Stepanov much have been determined for dry friction and water lubrication of different material combinations. A comparative evaluation of the surface fatigue strength of aluminum oxide singleand polycrystals is performed.

This paper presents results of experiments on in situ temperature measurements during sapphire shaped crystal growth. The temperature distribution difference between the crystals grown by EFG (edge-defined, film-fed growth) and GES (growth from an element of shape) methods is considered.

X-ray back-reflection topography with TV and photoplates recording as well as X-ray analysis technique have been applied to a number of sapphire single crystals shaped as 90° angles and tubes. It was done to study the formation and evolution of dislocation and mosaic structure in the shaped crystals with complicated cross-section. It has been shown that the real-structure formation in the origin part of the all shaped crystals, corresponding to its seeding and growing up to the steady-state dimensions has a principal significance for the next growth stages. The rotation of tube crystal orientation around its growth direction was first discovered and measured. The curl of tube crystals is discussed as one of the essential mechanisms of mosaic structure development and as some reaction of tube crystal to growth conditions.

The influence of the growth conditions and process parameters on the formation of blocks during growth of basal-plane-faceted single crystalline sapphire ribbons by the Stepanov/EFG technique was investigated. A number of modifications of the crystallization unit were used to vary systematically the crystal growth conditions. The results gave clearly the growth conditions which are favorable to get blocks-free single crystalline basal-plane-faceted sapphire ribbons.

Shaped single crystals of lithium fluoride have been produced by the Stepanov method using shapers which are wetted and unwetted by the melt. Dislocation structure, porosity, optical properties, ion conductivity, and distribution of magnesium impurities have been studied.

The crystallographic orientation of shaped crystals grown by the Stepanov method is determined by the seed orientation. The original orientation persists if the crystal grows ideally enough. However, crystals often contain structural defects such as dislocations and their pileups, slip bands, and intergrain boundaries. Each of these defects is connected with a certain distortion of the crystal lattice. As a result, the orientation of the grown crystal differs from the initial one and varies throughout the volume. The orientation changes little during growth if the crystal contains few defects. The crystal orientation can change quite significantly if the defect density is high. Until now, studies of the change of crystal orientation during pulling have been limited to an examination of twinning effects.

The paper is concerned with the formation of a dislocation structure due the thermal stresses in shaped single crystals with different degree of anisotropy grown by Stepanov's method. The calculation of root-mean-square tangential stresses allows to determine the regions in which the plastic deformation may occur. Analysis of thermal stresses in separate slip systems permits to explaine the slipe band distribution in shaped single crystals. [Russian Text Ignore]

The formation of the dislocation structure in germanium single crystal ribbons grown from melt by Stepanov's method has been studied. It is shown that the relatively high dislocation density in the ribbons is due to thermal stresses present during growth and that the dislocation density can be controlled by properly varying the temperature of a radiation shield. Data are presented on the temperature distribution along the ribbon pulling axis, the tangential thermoelastic stress pattern, and the distribution of dislocations for three different shield geometries. The distribution of dislocations over the ribbon width is shown to correlate well with that of the rms tangential stresses. Samples of low dislocation density germanium ribbon were produced.

The method and results of the calculation of the thermal stresses in Hg2Cl2 and PbCl2 monocrystals during crystal growth process are presented. The experimental technique is shown which enables us to minimize the nonlinearity of temperature distribution in crystals.

Standard measures of linear thermal expansion made of monocrystalline aluminum oxide have been developed. The measures have a 59° orientation relative to the trigonal axis of the crystalline lattice. The raw material for making the measures is obtained by the Stepanov's method of growing shaped monocrystalline semifinished products with a highly accurate crystallographic orientation from a melt. The parameters of the measures are optimized for testing high-temperature relative dilatometers with transducers made of polycrystalline corundum. The coefficient of linear expansion of the new measures was checked in the range 90–1800 K and exact values of the coefficient were obtained for a broad range of temperatures.