On the mechanism of Al?Si alloy structure modification by metal antimony

In the process of Al Si alloys solidification in presence of Sb the intermetallic compound AlSb is taking obviously a significant part. An adsorptional incorporation mechanism of AlSb action is proposed. It depends strongly on the antimony concentration in the melt. The modifying effect is influenced also by a possibility of nucleation and growth of the AlSb phase itself.     

Bridgman and Czochralski growth of Ge?Si alloy crystals

Ge_1–xSi_x crystals were grown with the Bridgman and the Czochralski method over a wide concentration range. With the Bridgman process single crystals up to 40 at.% Si were possible. The reasons for polycrystalline growth were the permanent contact of the interface with the crucible wall in combination with curvature towards the crystal and inclusions of high Si concentration. Analysis of striations in Czochralski grown material showed that in this process the crystal does not continuously grow in one direction but consists of piece-wise grown crystals in which the composition permanently changes. According to that fact the main reason for polycrystalline growth in the Czochralski process is common due to lattice mismatch between the seed and equilibrium concentration transients in the crystal corresponding to the microscopic growth rates.     

Influence of crystallisation conditions on the microtexture of the directionally solidified eutectic of the MgF2?CaF2 system

The influence of high-frequency (HF) and resistive (R) heating of a vertical crystallizer furnace on the directional solidification process of the MgF₂ CaF₂ system eutectic has been compared. The turbulent flow induced by HF heating given rise to inhomogeneities in microtexture, which do not occur under R heating conditions. Variations in the growth rate (v) and temperature gradient (ΔT) lead to characteristic changes of the eutectic microstructure: from lamellar to rod-like. Quantitative analysis of these changes was advantageously performed using a digital image analysis of the area (AR), maximum cross-section (D-MAX), direction of the maximum cross-section (ANGLE D-MAX) of MgF₂ particles of the microtexture. This was recorded on SCAN images of transverse cuts through the given directionally solified samples as well as the analysis of the shape of MgF₂ particles.     

On the quench-sensitivity of Al?Zn alloys during RT-ageing in dependence on alloy composition

The quench-sensitivity (variation of T_q) of the decomposition rate at RT was studied for four Al Zn(X) alloys (X = 3.0; 4.5; 8.0, and 12.0 at. %) by SAXS. It was found that the occurring “inversion temperature” depends on alloy composition and, particularly for the two alloys with the highest zinc content, on ageing time.     

Stabilization of the melt composition in Ga?Al?Sb?As

In this work we present a new effect of stabilization of the Ga Al Sb As melt composition when it is in contact with the GaAs substrate. It was found that the As content in the Ga Al Sb As melt did not depend on the initial Sb concentration when the liquid phase was formed by saturating the Ga Al Sb melt from the GaAs substrate. This effect is supposed to be due to the change in phase equilibria conditions caused by large lattice mismatch between the substrate and the solid in equilibrium with the liquid.     

Study of the reversible martensitic transformation in Cu?Zn?Al alloy by positron annihilation

The positron lifetime measurement to investigate phase transformations: martensitic phase (MP) ⇔︁ parent phase (PP) in Cu Zn Al alloy has been used. We have observed the generation of new defects by MP → PP transformation; these defects are liquidated by martensitic transformation. This effect is only observed in case of high concentration vacancies in alloy. In order to explain the nature of these defects we take into consideration the supervacancies idea.     

Preparation of Al?Ga?P?As heterostructures

The present paper reports on the results of an investigation of crystallization of multilayer structures with a narrow-gap semiconductor surrounded by two wide-gap ones in the Al-Ga-P-As system on GaAs and GaP substrates. Influence of the lattice parameters' difference on the value of thermal stress in structures with a composition gradient was determined by the bend of structures separated from the substrate. It has been proved that with the P concentration in solid solutions being less than 1% heterojunctions with less tensions than in the Al–Ga–As system are available. A new method of obtaining Al_xGa_1–xP_yAs_1–y solid solutions with the concentration of P proportional to that of Al has been worked out on the basis of which multilayer heterolight-emitting structures have been prepared.     

Rheological behaviour of lamellar eutectic Al?Al2?Cu elaborated by induction heating

This paper presents solutions to the experimental problems encountered in the elaboration of lamellar Al Al₂Cu or fibrous Al Al₃Ni eutectic samples using the Bridgman technique. The objective is to prepare samples for use in mechanical tests. The investigation of the elaborated structures of Al Al₂Cu (orientation, grain selection, shape of the solid-liquid (S.L.) interface, interlamellar spacing λ…) can then be used to confirm the predicted behavior. Creep tests under constant applied stress σ (from 2 to 5 · 10⁷ N/m²) at temperatures from 300 to 450 °C show:

• – that a stationary state exists (with constant strain rate )
• – the influence of an imperfect orientation
• – the existence of a Bailey-Norton creep law = kσⁿ. This law leads to the possiblity of finding a constitutive equation for high temperatures tests.

     

On the kinetics of the decomposition of ternary Al-Zn(6at.%)?Mg(X) and Al?Mg(X)?Si(Y) alloys

Investigations on Al-Zn(3)-Mg(X) alloys gave the rather surprising result that the migration energies depend on the physical quantities (resistivity, Young's modulus) and the ageing time used. Therefore it is the aim of this paper to test, if similar effects occur using other ternary alloys and additional measuring methods. The ageing kinetics of 7 Al-Zn(6)-Mg(X) alloys are examined by isothermal measurements of resistivity, Young's modulus, change of length, and microhardness over a relatively long time interval of about 3 h within the temperature range from 20°C to 90°C.

• 1 The activation energies got for the Al-ZN(6)-Mg(X) alloys depend on the physical quantity as well as on the ageing time used within the temperature range from 20°C to 40°C as known from the Al-Zn(3)-Mg(X) alloys.
• 2 The activation energies obtained for the Al-Zn(6)-Mg(1.3) alloy in the temperature interval from 50°C to 90°C decrease monotonously with increasing time for the measuring methods considered.

These results different with respect to those two temperatur ranges are probably caused by a change of the decomposition mechanism. Below 40°C presumably several kinds of clusters are formed in the beginning of the decomposition, but above 50°C probably only one sort of clusters should grow. The late stage of ageing should be caused mainly by the diffusion of VZn₂ complexes (E^M = 0.21 eV) in the temperature range investigated, since 0.2 eV holds for that stage. – Finally the results of the investigations of the kinetics of some Al-Mg(X)-Si(Y) alloys are discussed by means of the jump model.     

Peculiarities in the X-ray Spectral Microanalysis of the Sn?Te?I System

This is an examination of the methodological specificities of the roentgenospectral microanalysis of the Sn Te I system, involving the application of a graphoanalytical method used in the analysis of ternary systems according to data calculated for the binary ones. The calculations were made on an ES 1060 computer in FORTRAN-4. The calculations are for different selection angles of the microanalyzer, and for a broad interval of values of the accelerating voltage. In this manner the method offered is applicable to any type of microanalyzer, and not only to the Cameca MS-46 used by us.     

Natural convection during directional solidification of an Al?Zn eutectic

A study was made of the buoyancy convection of melt provoked by temperature gradient of an orientation identical to that of the gravity vector. The convection was assessed on the basis of temperature fluctuations in the melt. The critical Rayleigh numbers for periodic and turbulent flow were determined. Both periodic and turbulent flow gives rise to transverse “bands” in the structure of the eutection obtained by directional solidification. The intensity of unstable flow affects the number of eutectic crystallites in the sample.     

The influence of small deviations from stoichiometry on the properties of Y?Ba?Cu?O superconductor ceramics

According to a central composite design samples with deviations from stoichiometry YBa₂Cu₃O_7−x of ±2.5 wt% in the ingredients were prepared. The influence of these deviations on phase content, superconducting properties and microstructure of the ceramics was studied.     

Effect of the relative content of Y,Ba, and Cu on the superconducting transition characteristics of the Y?Ba?Cu?O System

By varying the content ratio of the initial products, nine compounds of the Y Ba Cu O system have been produced. These were studied to find their T_c values, critical currents j_c at 77 K and effects of compression stress on the U(j)-curves. Mechanical loading has been found to result in a shift of the U(j)-curve towards a larger current. A correlation between the critical current value, the slope of the U(j)-curve, and the stress induced shift is shown to exist. Samples with equal absolute deviations from the stoichiometric composition reveal close properties, the better the smaller is the deviation.     

On the position of the coherency spinodal curve in an Al?Zn (18 at.%) alloy

The values of the wavelength of the dominating concentration fluctuations λ_f1 (analyzing the “inverse XSAS-spectrum”) and the kinetics of the decomposition process (using measurements of microhardness) as a function of the ageing temperature T_a are taken in order to get information on the position of the coherent spinodal curve T_c.sp. in an Al Zn (18 at%) alloy. According to these investigations T_c.sp. = (116  5)°C is likely to be obtained. That value is not far from the position of the coherent spinodal curve calculated by LÖFFLER and SIMMICH (1975) (130°C). Some arguments are given that this curve of Al Zn (6.8 at.%) should not be placed at 129°C but at much lower temperatures.     

A decomposition study of a eutectoid Cu?Al alloy

A study of the eutectoid decomposition of a Cu-11.8wt.%Al alloy was carried out by means of differential scanning calorimeter and an electron microscope. The activation energies of eutectoid and peritectoid reactions were determined.