On the Fit between Precipitates and Matrix in Al–Zn Alloys

The quality of the fit between the precipitates occurring in Al–Zn alloys and the matrix of the Al host is considered taking into account the positions of the atoms in the lattice planes forming the possible faces of the precipitates and the neighboured lattice planes of the matrix. The quality of the fit depends on the number of (pseudo-)coincidence sites of both planes, the density of Al atoms in the matrix plane and the misfit in the spacings of the atoms in the adjoining planes in consideration. The effect of the misfit is taken into account by means of two different models. Based upon the obtained results many of the experimentally observed features about the preferred shapes and orientations of the faces of the growing precipitates can be explained. It turned out that in both of the applied models most probably the effect of the density of the coincidence sites on the interfacial energy is underestimated.     

Determination of the Effective Vacancy Concentrations and the Vacancy Losses after the Quench of Al–Zn–Mg Alloys

On the basis of

• 1 the experimental observation of dislocation loops by means of TEM,
• 2 the conclusion from this that the concentration of vacancies in the dislocation loops exists at the cost of the vacancy concentration primarily present at T_q, and
• 3 the inversion of the decomposition kinetics of Al–Zn–Mg alloys found out by means of resistivity and microhardness measurements, the effective vacancy concentration c_v.eff being at disposal for the GP-zone formation was found out by means of the relation t_R,max (t_R,max being the time required to reach the maximum of resistivity) and the Arrhenius plot In t_R,max = f(1/T_q) of the isothermal resistivity measurements. From the comparison of c_v.eff with the theoretical V concentration at T_q it is possible to derive statements about the loss of vacancies during and after the quench through loop formation, building-in of vacancies in GP-zones and annealing of vacancies at lattice defects. From this comparison follows:
  • The ideal ZnV concentration at T_q (calculated by means of the Lomer equation) is sufficiently great to account for the effective vacancy concentration.
  • The part of c_MgV in the loss concentration is rising with increasing c_Mg.
  Both these statements support the hypothesis that the initial process of the decomposition in Al–Zn–Mg alloys is caused by the ZnV diffusion.

     

Dissolution of Si in Al–Si Melts. Effect of Sb on the Crystallization Process and As-cast Structure of Al–Si Alloys

The effect of Sb on the dissolution of Si single crystals (face {111}) in Al–Si melts is studied. The dissolution isotherms at three temperatures and fixed undersaturation are obtained. The experimental data are treated according to the Bliznakov's ratio. Some conclusions are made with reference to the crystal growth process. A probable mechanism of the influence of Sb on the structure of Al–Si alloys is given.     

On the Determination of the Size Distribution Functions of Spherical Precipitations in Al–Zn Alloys by Neutron Small-angle Scattering

The size distribution of approximately spherical incoherent precipitations of Zn in AlZn-alloys is calculated and experimentally tested by means of neutron small angle scattering (NSAS) at a double-crystal diffractometer (DCD). The scattering curves can be approximated by a Gaussian function, so that the size distribution is directly calculated for the experimentical scattering curves under application of a particular LAPLACE-transformation. The size distributions of the α′_m-phase of AlZn(16)-samples of different averaged radii of precipitations are calculated by means of FWHM which are determined by the fitting procedure of a computer programme and compared with TEM micrographs.     

Kinetics of Structural Coarsening of Microcrystalline Al—Si—Mg Alloys during Thermal Treatment and Compacting

The structures of microcrystalline AlSiMg alloys during heat treatment were studied using optical microscope Reichert MeF₂ equipped with a monochrome TV camera VS-700. The average size of the silicon particles was determined. The significant influence of magnesium on the microstructure at high temperatures was demonstrated. Magnesium accelerates the structure coarsening process and a decrease of the effective activation energy of the silicon particles growth was found.     

Effect of Cyclic Stress Reduction on the Creep Behaviour of Al‐Ag and Al‐Ag‐Zr Alloys Containing γ′ and γ Precipitates

Creep tests under the effect of cyclic stress reduction were conducted to Al‐16 wt%Ag and Al‐16 wt%Ag‐0.1 wt%Zr alloys aged at 548 and 673 K. The steady state creep rate was found to be higher under cyclic stress reduction condition than that under static creep condition for the same maximum stress. An expression for the temperature dependence of the threshold stress for cyclic creep acceleration is offered. Cyclic creep acceleration is interpreted using the dislocation cross‐slip model. TEM investigations confirmed that the addition of Zr to Al‐Ag alloy accelerates the formation and coarsening of γ′ and γ‐phases. The mean value of activation energy for both alloys aged at 548 K was found to be 35±0.3 kJ/mol, and 58±0.4 kJ/mol for alloys aged at 673 K.     

Structure of the grain boundary region in an Al–15 at.% Zn and Al–2.0 at.% Zn–1.3 at.% Mg alloy aged at elevated temperatures

The results of TEM investigations are presented of the effects of various solutions and ageing treatments, cooling and quenching rates as well as of tensile stress at elevated temperatures on the microstructure of the grain boundary (GB) region of the examined Al Zn and Al Zn Mg alloys. A pronounced influence was found of partial coherency of GB stable β(Zn) precipitates with the α matrix in the Al Zn alloy on the asymmetry of Zn depleted GB region and on the shape of GB precipitates. The role of GB's as sources and sinks of vacancies and the effect of stress gradients in GB regions were emphasized in the establishment of differences in the nucleation and decomposition characteristics within the GB region as compared to those within the grain interior. The existence of a negative stress gradient towards the GB has been suggested to lead to the observed gradual retardation of transformation kinetics within the GB region in Al Zn alloys. A rapid quench from annealing to ageing temperature introduced large local stresses close to GB giving rise to “band of precipitates” on subsequent ageing within the former precipitate free region (PFZ) in Al Zn Mg alloy. Similar enhancement of local stresses accompanied by acceleration of decomposition kinetics was also observed in the aged Al Zn sample subjected to a prior tensile creep exposure within the one-phase region.     

Phase‐field simulation of dendrite growth under forced flow conditions in an Al–Cu welding molten pool

A quantitative phase‐field model for directional solidification is applied to study dendrite growth under forced flow conditions in an Al‐Cu Gas Tungsten Arc (GTA) welding molten pool. Evolution of the dendrite morphology and the solute field under forced flow conditions is simulated. Growth of columnar grains goes through three periods, including the initial instability period, the competitive growth period and the relatively stable period. The solute segregation, the solute redistribution and the solute concentration in the liquid side of the interface are investigated, respectively. For the given conditions, simulation results are in good agreement with experimental findings.     

Observation of the periodic fluctuant dendritic structure in an Al–38 wt% Cu hypereutectic alloy processed by ACRT-B method

The microstructure of periodic fluctuant dendritic θ in a matrix of +θ eutectic was obtained in an Al–38 wt% Cu alloy processed by ACRT-B method grown at growth velocities (V) ranging from 5 to 60 μm/s and crucible rotating in a trapezoidal way with maximum rotation rates (Ω_max) ranging from 100 to 400 rpm. Formation of this structure is explained by the influence of the periodical Ekman flow on the growth of dendritic θ during the spin-up and spin-down process. It was also observed that the +θ eutectic between the primary dendrites of θ (CuAl₂) is not periodic and fluctuant during ACRT-B process. This is quite different from our previous observation of periodic eutectic in Al–Cu eutectic processed by similar ACRT-B method.     

Effect of a vertical magnetic field on the dendrite morphology during Bridgman crystal growth of Al–4.5 wt% Cu

The effect of a vertical high magnetic field (up to 10 T) on the dendrite morphology has been investigated during Bridgman growth of Al–4.5 wt%Cu alloys experimentally. It is found that the field causes disorder in dendrites and their tilt in orientation. Along with the increase of the magnetic field and decrease of the growth velocity, the dendrites became broken and orientated in 1 1 1 along the direction of solidification instead of 1 0 0. The field also enlarged the primary dendrite spacing and promoted the branching of the dendrites to form high-order arms. Above phenomena are attributed to the thermoelectromagnetic convection effect and orientation caused by the high magnetic field.     

Influence of source composition on the properties of flash-evaporated thin films in the Cu–In–Se system

Using CuIn₂Se_3.5 as source material epitaxial layers are obtained on (111)A- und (100)-oriented GaAs substrates by flash evaporation in the substrate temperature range T_sub = 720—870 K. The composition of the films is between CuInSe₂ and CuIn₂Se_3.5. The structural properties of the films are similar to those for CuInSe₂ epitaxial layers and refer to a chalcopyrite-like structure. The films are always p-type conducting but different acceptor states are found in dependence on the substrate temperature.     

On the curious structural phases in Al-deficient (Al62Cu23Cr15) and Al-rich (Al68Cu17Cr15) quasicrystalline alloys

In the present work investigations on the effect of stoichiometric variations on the Occurrence and stabilization of quasicrystalline (qc) and related phases have been carried out. Based on the explorations of several Al-deficient and Al-rich versions of the ideal nominal composition i.e. Al₆₅Cu₂₀Cr₁₅, it has been found that the alloy compositions corresponding to Al₆₂Cu₂₃Cr₁₅ (Al-deficient) and Al₆₈Cu₁₇Cr₁₅ (Al-rich) exhibit several structural subtleties. The Al-deficient alloy, has been found to exhibit crystalline bcc and fcc phases with a = 8.90 Å and a = 17.98 Å respectively. In addition to these phases, a new crystalline bcc variant (a = 15.42 Å) originating from the bcc (a = 8.90 Å) phase has been found. Also a curious superstructure of the fcc (a = 17.98 Å) has been observed. The Al-rich alloy typified by Al₆₈Cu₁₇Cr₁₅, on the other hand, does not exhibit any structural variants, instead it shows nearly pure i-phase.     

The Ln–Ba–Cu–O (Ln = Lanthanoid) systems: Compounds and compatibilities in air at 900–980°C

Comparison of phases observed in air at 900–980°C in the Ln–Ba–Cu–O (Ln = Lanthanoid) systems is reported. On the basis of the occurrence of compounds found in these Ln₂O₃–BaCO₃–CuO systems they must be divided into at least three subgroups: the first is characteristic of La, Pr and Nd, the second specific for Sm and Eu, and the third common to the smaller lanthanoids, the Y-type elements, with some variation within the groups.     

Al–O–Al and Si–O–Si sites in framework aluminosilicate glasses with Si/Al=1: quantification of framework disorder

We present for the first time, direct and clear experimental evidence of Al–O–Al and Si–O–Si linkages in charge-balanced aluminosilicate glasses with Si/Al=1, such as NaAlSiO₄ (nepheline) and LiAlSiO₄ (β-eucryptite) compositions using ¹⁷O triple quantum MAS (3QMAS) NMR and quantify the extent of disorder in framework cations (Si/Al). The degree of Al avoidance in NaAlSiO₄ glass is 0.942 at 1050 K, and in LiAlSiO₄ glass is 0.928 at 930 K (0.902 at 1050 K), which demonstrates the effect of cation field strength on the extent of disorder. In addition, we find a remarkable similarity between the ordering state of the liquid and that of the first, disequilibrium phase to crystallize.     

Influence of the temperature and the oxygen partial pressure on the phase formation in the system Cu – Fe – O

Copper iron oxides, Cu_1‐xFe_2+xO₄ (0 ≤ x ≤ 0.5), have been synthesized by thermal oxidation of copper ‐ iron mixtures. In this process, the phase formation and the phase stability were investigated as function of the temperature (800°C – 1200°C) and the oxygen partial pressure (1.013 x 10¹ – 1.013 x 10⁵ Pa). The phase formation starts with the reaction of the metallic components to simple oxides (Fe₃O₄, Fe₂O₃, CuO). From these simple oxides, the formation of complex oxides requires a minimum temperature of 800°C. The synthesis of single phase spinel compounds Cu²⁺_1‐2x Cu¹⁺_xFe_2+xO_4±δ is realized for 0.1 ≤ x ≤ 0.5, using specific temperature – p(O₂) – conditions for a given value of x. Remarkably, to achieve our goal, we found that the increase of x implies that of the reaction temperature and/or a decrease of the p(O₂) in the reaction gas stream. Besides, a single phase spinel CuFe₂O₄ does not exist in the temperature / p(O₂)‐field investigated. Using the results of XRD ‐ phase analysis, T ‐ p(O₂) – x – diagrams were constructed. These diagrams allow the prediction of phase compositions expected for different synthesis conditions. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the Rajagopal relaxation-time distribution and its relationship to the Kohlrausch–Williams–Watts relaxation function

A detailed analysis of the relationship of the Kohlrausch–Williams–Watts relaxation function to its recently proposed approximation resulting from the Rajagopal log-relaxation-time distribution is presented. Our considerations are based on the interpretation of the relaxation function as a survival probability of the initial state of a relaxing system expressed by means of the weighted average of an exponential decay with respect to the distribution of the effective relaxation time. Such an interpretation of the relaxation function enforces certain framework of analysis in which the basic probabilistic rules have to be fulfilled. In our approach, to compare the properties of the relaxation function resulting from the Rajagopal relaxation-time distribution and the Kohlrausch–Williams–Watts relaxation function, we use the strict relationship of the latter to the one-sided stable relaxation-rate distribution.     

Double Salt Formation in the Cu(HCOO)2–Sr(HCOO)2–H2O System

The solubility in the Cu(HCOO)₂–Sr(HCOO)₂–H₂O system has been studied by the method of physico-chemical analysis at 25 and 50 °C. It has been established that two double salts are formed in the system: CuSr₂(HCOO)₆ · H₂O at 25 °C and CuSr(HCOO)₄ · 4 H₂O at 50 °C. The latter salt has not yet been described in the literature. It has been characterized by X-ray powder diffraction and DT and TG analysis. CuSr(HCOO)₄ · 4 H₂O crystallizes in the triclinic system with lattice parameters a = 12.376(6) Å, b = 13.394(4) Å, c = 11.508(6) Å, α = 93.38(3)°, β = 94.01(3)°, γ = 75.04(3)°. Dehydration proceeds in two stages.