Structure and microhardness of samples of composition Al-Mn obtained by the modified Stepanov technique

Microhardness of the structure of the cross section and longitudinal section of polycrystalline samples of composition Al-Mn obtained using Stepanov’s technique with the aid of a shaper which contains moving elements is investigated. It is discovered that a change in the value of the microhardness of the samples obtained by the modified Stepanov technique independently of the method of formation of structure does not exceed approximately 5% of the mean value. The obtained results support the expansion of the possibilities of this technique.     

Structure and physicomechanical properties of Al-Si alloys

Al-Si alloy samples with a silicon content of 8–15 wt % are grown by the Stepanov method at a solidification rate of 10² and 10³ μm/s. The microstructure of the samples is examined, and the stress-strain curves obtained during tension and bending at a strain rate of 10^?4 s^?1 are studied. The behavior of Young’s modulus, the modulus defect, and ultrasonic attenuation is investigated. The silicon content in a eutectic structure is found to increase with the solidification rate. The yield strength and the ultimate tensile strength increase with the silicon content up to a eutectic composition. The quality index (which characterizes the strength and plasticity of the material) of Stepanov-grown samples is higher than the quality indices of traditional modified ingots.     

Effect of the pressure on Al-Si alloys

Abstract

The aim of this communication is to study the effect of the high hydrostatic pressure on the phase diagram of an AS12UN commercial alloy by DTA. The high pressure range is 0-360 MPa. Microphotographies of the recovered samples are compared with samples produced by dynamic high pressure in a die-casting set in a 4000 T press on a industrial plant.     

Tribological properties of aluminum oxide single crystals grown by the Stepanov method

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.     

Faceting of sapphire crystals grown from a melt by the Stepanov method

The appearance of faces on cylindrical sapphire single crystals grown from a melt by the Stepanov method is studied by photogoniography and optical microscopy. The crystallographic indices of the detected faces are established, and the microstructure of the growth layers is investigated. A relationship between the faceting and the growth conditions is found. The experimental results are compared with data on the faceting of the sapphire crystals grown from a solution-melt and with the calculated specific free surface energies of the faces.     

Structure and properties of aluminum coatings obtained by the cold gas-dynamic spraying method

Structure and properties of aluminum coatings deposited onto steel substrates by the cold gas-dynamic spraying (CGS) method were examined. Aluminum CGS coatings fundamentally differ from their thermal counterparts as they enable the formation of heavy-duty layers of metal particles on substrates at temperatures below 500 K. A dense, low-porosity coating is found to form, tightly bound to the base metal. The adhesion strength is shown to weakly depend on the thickness of the sprayed coating due to the compressive stress present in the surface layer. A qualitative model for the coating formation process is proposed. This work was supported by the Russian Foundation for Basic Research (Grants Nos. 03-02-16329 and 05-07-90172).     

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.     

Thermoelastic stress in thin large-area windows grown from melt by the Stepanov method

The mathematical model for thermoelastic stress in a thin large-area window grown from melt by the Stepanov method is considered. The distribution and magnitude of stress are analyzed depending on the height of the radiative shield and on the distance between the shield and growing window. The model includes the heat-conduction equation and the integral equations relating the thermal radiation fluxes and the temperatures of the surfaces involved in heat transfer.     

Structure and optical and dielectric properties of barium titanate nanoparticles obtained by the mechanochemical method

The parameters of mechanochemical synthesis of barium titanate nanoparticles from barium titanyl oxalate and an oxide mixture are optimized. X-ray phase analysis, infrared and Raman scattering spectroscopy, differential scanning calorimetry, and dielectric spectroscopy are applied to demonstrate that the nanoparticles contain the cubic and tetragonal crystal lattice modifications. The impurity spectra for the particles obtained by the mechanochemical method from different initial substances are identical. The ferroelectric phase volume increases as hydroxyl groups are annealed from the crystal lattice and decreases upon annealing of metal atom vacancies.     

Properties of Al-Si alloys prepared by rapid quenching from the melt under high pressure

Abstract

The substantial improvement of the mechanical properties and increase of superconductivity temperature of Al-Si alloys is achieved by rapid quenching from the melt under high pressure. The preparation procedure and the main properties of these alloys are presented.

L'amélioration notable de propriétés mècaniques et un accroissement de la températur critique de supra-conductivitě des alliages Al-Si sont obtenus par trempe rapide depuis la fusion, sous HP. La procédure de prkparation et la propriétés principales de ces alliages sont présentěs.     

Calculation of thermal stresses in hypereutectic Al-Si alloys by means of a plastic-elastic model

In this paper an evaluation on basis of calculation is made of thermal stresses developing during gradual cooling and solidification in both phases of hypereutectic Al-Si alloys with different content of Si. On the basis of metallographic evaluation of sample surfaces it is assumed that the building cell of an alloy has a cylindrical form with a base parallel to the surface of the sample. The setting of individual phases inside this cell is such that silicon forms a cylindrical nucleus which is surrounded by a coaxial cylindrical shell of phase.The thermal stresses, calculated on these assumptions, are then compared with the values which result from the X-ray measurements of interplanar spacings of the systems of (024) planes in phase and the system of (135) planes in Si.     

The bond strength of Al-Si coating on mild steel by kinetic spraying deposition

Kinetic spraying (or cold gas dynamic spraying) works by accelerating small solid particles to supersonic velocities, and then impacting them onto a substrate. These high impact velocities, and low particle temperatures are the principal attributes of kinetic spraying technology. However, only recently has this technology's interfacial behavior, due to particle/substrate impaction, become well understood. In order to investigate the particle/substrate bond behavior, Al-Si feedstock was deposited onto mild steel, over a range of particle velocities; next, their respective coating bond strengths were measured by the stud pull coating adherence test. The effects of the particle velocity and the substrate surface roughness on the coating bond strength were presented, and a model of the particle/substrate bond generation was discussed in an effort to estimate the bond strength.     

The kinetics of solidification of Al-Si eutectic alloys under high pressure

Abstract

The kinetics of crystallization of eutectic alloys Al_100-xSi_x (χ=12, 18, 26 at.%) was investigated under pressures p=0.5, 2.5, 4.5 GPa. The values of supercooling and average grain size of silicon crystals were determined for alloys quenched from melt under different pressures with the cooling rate 10³ K/s. The data obtained were used to evaluate quantitatively the pressure dependences of surface tension (between melt and crystal) and activation energy of crystal growth which, in turn, have made it possible to determine the relative change of nucleation frequency and of the rate of crystal growth with pressure.

The possibility is shown, based on the investigation of mechanical properties of the samples obtained under high pressure, for improving the strength and the plasticity of A1-Si alloys by means of high pressure-high temperature treatment.     

Controlling mechanical and physical properties of Al-Si alloys by controlling grain size through grain refinement and electromagnetic stirring

The present study concerns the influence of a travelling magnetic field (TMF) on the hardness, tensile strength and electrical conductivity of directionally solidified grain-refined and non-refined Al-7 wt.% Si alloys. Upwards and downwards travelling fields have been applied to force convection within the solidifying melt. Modifications of the examined physical and mechanical properties depend on the formation of a fine equiaxed structure caused by both the addition of grain refining AlTi5B1-particles and by electromagnetic stirring as well. Electromagnetic stirring without grain refining particles leads to an increase in tensile strength. The addition of grain refiners into the melt leads to the highest reduction of the mean grain size and results in a decrease in electrical conductivity. A melt stirring by a sufficiently high magnetic field provides a homogeneous grain size distribution in the sample volume which impacts the distribution of hardness, tensile strength and electrical conductivity.     

Properties of YBCO obtained by modified hydroxycarbonate method

YBCO was synthesized by the co-precipitation of stoichiometric amounts of nitrate solutions of Y, Ba and Cu using a mixture of NaOH, Na₂CO₃and H₂C₂O₄in a molar ratio of 5:1:1 as a precipitation agent. Even after prolonged thermal treatment at 950 °C the samples exhibit a zero-resistance critical temperature of about 87 K. This fact indicates a copper deficiency of about 0.1, more reduced than in the case of hydroxycarbonate co-precipitation. The as-synthesized samples have a high density, reaching 94% of the theoretical density. The SEM analyses show a large grain distribution from microns to tens of microns, favourable for good stacking.     

A study of phase stability in invar Fe--Ni alloys obtained by non-conventional methods

It is known that thermodynamic equilibrium in Fe--Ni alloys, in the invar composition at temperatures below 450^°C, is difficult to achieve because of the slow diffusion rate at low temperatures. One of the ways in which we can study phase transformation which may be responsible for invar behavior is to investigate: (i) materials of similar composition obtained by non-conventional methods, known to allow the enhancement of diffusion at temperatures where atomic mobility is nil on the laboratory time scale; (ii) materials which have been treated for very long periods of time (geological time scale) in the same temperature range, such as in meteorites. In this context we have studied the phase stability of Fe--Ni phases in mechanically alloyed powders, in ion-beam mixed multilayers and in meteorites. This revised version was published online in August 2006 with corrections to the Cover Date.