Aftereffect in microhardness testing of corundum

Cleavage near microhardness indentations on the surface of corundum is found to appear long (20–30 days) after unloading.     

Anisotropy and scale effect in the microhardness of crystals of Bi-based superconducting phases

Microhardness studies are carried out on the (001) plane of faceted crystals of the Bi₂Sr₂CuO₆ (2201) and Bi₂Sr₂CaCu₂O₈ (2212) phases with the help of a Knoop indenter. Peculiarities in the effect produced on the microhardness by the indenter orientation relative to crystal faces, the presence of lead in the structure, and the force applied to the indenter (scale effect) are detected. It is found that crystals of the 2212 phase exhibit the strongest dependence of the microhardness on the above factors.     

Microhardness anisotropy and the density of atoms in the unit identity volume of crystals

A new criterion of the reticular and polar anisotropy of microhardness is proposed: the density of atoms in the unit identity volume, i.e., the ratio of the number of atoms of one species in the atomic planes (plane) contained within the limits of the unit identity volume to the sum of the areas of these planes (plane) containing atoms of that species. It is shown that these densities of lanthanide, actinide, and boron atoms correlate satisfactorily with the reticular and polar microhardness anisotropy of their tetraborides and hexaborides. This criterion can be used to predict the nature of the variation of the reticular and polar microhardness anisotropy for other classes of compounds as well. Fiz. Tverd. Tela (St. Petersburg) 40, 481–486 (March 1998)     

Alteration of the lattice constant and the composition of KCl and KBr crystals under macrogravity

In KCl and KBr crystals grown from supersaturated aqueous solutions under centrifuging (3g, 7g, and 12g) the lattice constant and the halogen content are altered significantly in comparison with original compounds, and the microhardness is increased by 1.5–2 fold. Fiz. Tverd. Tela (St. Petersburg) 39, 1792–1793 (October 1997)     

Parameters and properties of crystals of ionic compounds obtained by centrifugal separation of supersaturated aqueous solutions

The change in the chemical composition, structural parameters, and properties of Ba(NO₃)₂, PbCl₂, and KI crystals and the binary alkali-halide systems LiCl-KCl (eutectic) and LiCl-NaCl and KCl-KI (solid solutions) after centrifugal separation under acceleration (1.3, 6.2, 11.8)×10³ g is investigated. The composition variation was monitored by the methods of chemical gravimetric and titration analyses, as well as of flame-photometric and spectrophotometric analyses. Acceleration causes a change in the chemical composition, lattice parameter, refractive index, and microhardness in all systems as compared to the initial values. It is found that centrifugation affects the systems with different masses of cations more strongly than the systems with different masses of anions. A criterion of the ratio of the masses of ions (cations and anions) rat i/i is proposed for predicting the effect of centrifugation on the change in the chemical composition and properties of compounds and their systems (a larger value of this ratio leads to a stronger variation of the composition and properties). The crystals of all compounds and solid solutions display an increase in microinclusions (visible only in transmitted light) upon an increase in the acceleration. The lattice parameters and microhardness measured in KCl and KBr crystals four years after their fabrication by using centrifugal separation indicate a considerable return to their initial values.     

Hydrogen sensitivity of sensors based on Co x O y /SnO2/Sb2O5 nanomaterials obtained by the sol-gel method

The sensitivity to H₂ at 40 ppm concentration of adsorption-semiconductor sensors with a gassensitive layer based on SnO₂/Sb₂O₅ nanomaterial obtained by the sol-gel method was studied. The hydrogen sensitivity was shown to increase for materials with Co additions and smaller particles.     

Structure and mechanical properties of Al-Si(Ge) alloys upon melt centrifugation and quenching

The structures of eutectic binary Al-12.7 at. % Si and Al-29.7 at. % Ge alloys and a ternary Al-10 at. % Si-10 at. % Ge alloy produced by quenching levitated melts or through solidification either in the presence or in the absence of a centrifugal acceleration of 7g are studied. Centrifugation is found to cause an increase in the silicon content in the Al-Si alloy in the direction opposite to the direction of centrifugal acceleration and an increase in the germanium content in the Al-Ge alloy in the direction of centrifugal acceleration. These differences are explained by the fact that the densities of silicon and germanium clusters and solidification centers differ from the liquid-phase density at temperatures of solidification. The related changes in the values of the Young’s modulus and in the stress-strain curves can be due to sedimentation-induced changes in the composition of samples cut from the middle part of an ingot. The processes of decomposition and recovery are shown to have a substantial effect on the elastic moduli of these alloys.     

Effect of the gaseous medium in the process of rolling on the microhardness of aluminum and iron

The load dependence of the microhardness of polycrystalline aluminum and iron specimens produced by rolling in a nitrogen, helium, or air medium has been investigated. It has been found that nitrogen and helium have different effects on the microhardness of these metals in the low-load range. This difference is associated with the specific features in the intensity of dynamic penetration of nitrogen and helium into the surface layer of aluminum and iron, which depends on the initial defect crystal structure of the metals, as well as on the type of bonding of helium atoms and nitrogen molecules with metal atoms. It has been shown that the effect of the gaseous medium of the rolling on the microhardness manifests itself only in a very thin surface layer of metal specimens, where the microhardness exhibits a size effect, and an increase in the microhardness indentation depth remains unchanged with an increase in the load and does not depend on the gaseous medium of the prerolling of the specimens.     

Synthesis and Growth of CrSi2 Crystals from a Zn Solution–Melt during Spontaneous Crystallization in Noninertial Systems in Conditions of Space Station and in Terrestrial Conditions

Physics of the Solid State - The effect of the terrestrial gravitation field on crystal growth from a solution–melt during spontaneous crystallization is considered, taking into account that...     

Structural and physicomechanical properties of directionally crystallized aluminum-silicon alloys

Aluminum-silicon alloys (from 8 to 25 wt % Si) have been prepared by directional crystallization of shaped samples by the Stepanov growth at a solidification rate of 10³ μm s^?1. The dependences of the microhardness, Young’s modulus, internal friction, yield stress, and ultimate tensile stress of the alloys on the silicon content have been studied. It has been shown that the ultimate tensile stress has a maximum, and the yield stress has a kink at 15 wt % Si; the composition corresponds to the eutectic composition at the solidification rate used. The silicon content in the eutectics increases with an increase in the solidification rate. The increase in the ultimate tensile stress is explained by an increase in the volume fraction of the more strength fine-crystalline structure of the eutectics as a result of the decrease in the volume fraction of more plastic dendrites of the primary crystals of the α-Al solid solution. The decrease in the ultimate tensile stress of the hypereutectic alloy is determined by the increase in the volume fraction of brittle primary silicon crystals of various shapes.