Constitutive relationships for AlZnMg, AlZnMgCr, and AlZnMgZr alloys

The effects of zinc, magnesium, chromium, and zirconium on the steady-state flow stress during hot working of both as-cast and homogenized AlZnMg(Cr/Zr) alloys were determined by means of torsion testing. The equivalent strain rates varied between 0.01/s and 10/s and the temperatures ranged from 450 °C to 560 °C. The zinc and magnesium concentration varied from 4.5 to 7.5 wt pct and from 0.8 to 1.8 wt pct, respectively. In addition, alloys containing typically 0.15 wt pct zirconium or 0.20 wt pct chromium were investigated. Magnesium, zirconium, and chromium were found to increase the flow stress, whereas zinc had practically no effect. The flow stress in the homogenized material was in most cases higher than in the as-cast material. Fitting of the coefficients in the hyperbolic sine constitutive equation to the experimental results showed that some of the coefficients could be related to concentrations of magnesium and zinc in solid solution, whereas others might be regarded as constants. The following relationship was determined between the coefficients α and n and the magnesium and zinc concentration: α=a·[Mg] ^b and n=c·[Mg] ^d +e·[Zn]+f. The coefficients a, b, c, d, e, and f were determined by fitting of these relationships to the experimental data. The steady-state flow stress calculated by means of the constitutive equations was in good agreement with the experimental steady-state flow stress.     

Melting of secondary-phase particles in Al-Mg-Si alloys

The melting of secondary-phase particles—or, more precisely, the melting of such particles together with the surrounding matrix—in two ternary Al-Mg-Si alloys has been studied. In the quasi-binary Al-Mg₂Si alloy, one melting reaction is found. In the alloy with an Si content in excess of that necessary to form Mg₂Si, three different melting reactions are observed. At upquenching temperatures above the eutectic temperature, the reaction rates are very high, and it is assumed that they are controlled by diffusion of the alloying elements in the liquid. Melting is also observed after prolonged annealing at temperatures below the eutectic temperature in these alloys, which is explained by the different diffusion rates of Mg and Si. The rate of the melting reaction is in this case assumed to be controlled by diffusion of the alloying elements in the solid α-Al phase. It is shown that calculation of the particle/matrix interface composition, which determines when melting is possible, cannot be made solely on the basis of the phase diagram, but must also include the rate of diffusion of Mg and Si. The melting temperatures observed differ somewhat from the accepted eutectic temperatures for these alloys. On prolonged annealing, the liquid droplets formed dissolve into the surrounding matrix and their chemical composition is found to change during dissolution. The resulting eutectic structure after quenching of a droplet is explained by the phase diagram and the different diffusion rates of Mg and Si as well as by the nucleation conditions of the constituents involved.     

The relationship between microstructural and plastic instability in AI-4.0 Wt Pct Cu alloy

The investigation of the effect of plastic deformation on the stability of theθ′ precipitates in an aluminum-4.0 wt pct copper alloy was performed. The alloy was produced by directional solidification, with Ti added as a grain refiner. Hot compression tests were performed at 200 °C in the strain rate range of 10_-3 to 10_-5 s₁ and equivalent strain up to 0.7 on specimens that had been initially heat treated, also at 200 °C, in order to obtain a uniform distribution of theθ0′ precipitates within the matrix. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of the plastically deformed specimens revealed a very heterogeneous distribution of strain. Also, the regions with localized strain contained randomly distributedθ precipitates of nearly equiaxed shape without any preferred orientation relationships to the matrix. Thus, the plastic deformation initiated the transformationθ′ →θ. The flow stress was reduced in the regions in which this transformation had occurred, which further accentuated the localization tendency of the strain. The combined process,θ′ →θ transformation/strain localization, thus developed in an avalanching way.     

Dynamic simulation of grain boundary migration

Dynamic simulation of grain boundary migration in two dimensions requires the combined movement of grain boundaries and three-junctions. In this work, a method of dynamic simulation is described where the migration is broken down into two steps: I. The grain boundaries migrate with frozen threejunctions. II. The three-junctions are released in order to obtain equilibrium. In principle, the development of any initial two-dimensional grain-structure can be studied. The CPU-time and memory requirements increase with increasing number of grains.     

Dissolution of particles in binary alloys: part I. computer simulations

A detailed numerical study has been made of the dissolution kinetics of particles in binary alloys during isothermal annealing. In earlier models, the assumption was made that the dissolution reaction could be described by the dissolution of only one particle in an infinite matrix or the dissolution of a regular array of particles of equal size. This assumption has been relaxed and a log-normal size distribution of particles has been introduced instead. The calculations have been done numerically by applying a finite difference technique to a spherical particle in a spherical cell of finite size. The presence of a size distribution of particles was found to have a great effect on the dissolution kinetics and, therefore, must be included in a reliable model for the dissolution of particles. The results have been presented in diagrams, giving the volume fraction as a function of the dimensionless annealing time with the geometrical standard deviation as a parameter, and thus should be useful in making accurate predictions of the dissolution kinetics of binary alloys. The curves can be used for all volume fractions provided that all of the particles can be dissolved completely at the temperature considered. Also, equations have been derived that can easily be used to give an estimate of the annealing time to dissolve 90 pct of the initial volume fraction.     

Crystallography of directionally solidified magnesium alloy AZ91

The crystallographic direction of growth in directionally solidified magnesium alloy AZ91 has been studied by TEM and EBSP techniques in SEM. The main direction of growth is found to be . The dendrites have sixfold symmetry around the main direction, with secondary arms lying along the traces of the (0001), , and -planes, respectively. The secondary arms lying in the basal plane are crystallographically of the same type as the main direction: and .     

The role of transference work, the therapeutic alliance, and their interaction in reducing interpersonal problems among psychotherapy patients with Cluster C personality disorders.

The aim of the present study was to examine whether transference work, the therapeutic alliance, and their interaction predicted a reduction in interpersonal problems at treatment termination. Forty-nine patients with Cluster C personality disorders from a randomized controlled trial investigating the effectiveness of short-term dynamic psychotherapy and cognitive therapy were included. Transference work was measured with the Inventory of Therapeutic Strategies (Gaston & Ring, 1992), while the therapeutic alliance was measured with the Helping Alliance Questionnaire (Luborsky, Crits-Christoph, Alexander, Margolis & Cohen, 1983). Less emphasis on transference work predicted overall reduced interpersonal problems, whereas the effects of the therapeutic alliance did not reach statistical significance. An interaction effect was also demonstrated, indicating that greater emphasis on transference work performed on patients with lower therapeutic alliance ratings was associated with a smaller reduction in interpersonal problems at termination. However, the results also indicate that a low dose of transference work may be beneficial in reducing interpersonal problems. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A new parameter extraction technique for small-signal equivalentcircuit of polysilicon emitter bipolar transistors

We propose a new parameter extraction method for advanced polysilicon emitter bipolar transistors. This method is based on the predetermination of equivalent circuit parameters using the analytical expressions of de-embedded Z-parameters of these devices. These parameter values are used as initial values for the parameter extraction process using optimization. The entire device equivalent circuit, containing RF probe pad and interconnection circuit parameters extracted by test structures, is optimized to fit measured S-parameters for eliminating de-embedding errors due to the imperfection of pad and interconnection test structures. The equivalent circuit determined by this method shows excellent agreement with the measured S-parameters from 0.1 to 26.5 GHz     

Development of a qualitative evaluation framework for performance shaping factors (PSFs) in advanced MCR HRA

Human reliability analysis (HRA) is performed as part of the probabilistic risk assessment to identify and quantify human actions and the associated impacts on structures, systems, and components of complex facilities. In performing HRA, conditions that influence human performance have been analyzed in terms of several context factors. These context factors, which are called performance shaping factors (PSFs) are used to adjust the basic human error probability (BHEP), and PSFs have been derived in various ways depending on the HRA methods used.