Band 99, Heft 2

The isothermal section of the Fe–Zn–Cr ternary system at 600 °C was determined using equilibrated alloys with the aid of a diffusion couple approach. The specimens were investigated by means of scanning electron microscopy equipped with energy dispersive X-ray spectroscopy, electron probe microanalysis and X-ray diffraction. The results showed that (Cr) is in equilibrium with all phases in the system except (α-Fe). A ternary phase with an approximate formula of Fe 18 Zn 79 Cr 3 was found. The composition ranges of all intermetallic phases in the system were determined. The solubility of Zn in (Cr) is about 2.5 at.%, and that of Cr in the δ, Γ and liquid zinc is 3.1, 5.4 and 1.4 at.%, respectively.

The thermodynamic properties of the liquid Pd–Sn–Zn ternary system were investigated, because the Sn–Zn alloy is a promising lead-free solder and Pd can be used as a substrate material. Using an appropriate galvanic cell, the partial free energies of Zn in liquid Pd–Sn–Zn alloys were determined as a function of concentration and temperature. Thermodynamic properties were obtained for 18 alloys. Their composition was situated on two cross-sections with the constant molar ratios of Pd: Sn = 1:3 and 1:9. The integral Gibbs free energy and the integral enthalpy for the ternary system at 1000 K were calculated by Gibbs–Duhem integration. For the liquid binary Pd–Sn system, some inconsistencies were found in previous CALPHAD results. Considering the latest experimental information, the interaction parameters of the liquid phase were reevaluated. The liquidus temperatures were also determined from EMF measurements.

Different iron–chromium alloys (4, 8, 13 and 20 wt.% Cr) were nitrided in NH 3 /H 2 gas mixtures at 580°C. The nitrided microstructure was investigated by X-ray diffraction, light microscopy, hardness measurements and scanning electron microscopy. Composition depth-profiles of the nitrided zone were determined by electron probe microanalysis. Various microstructures develop, depending on the nitriding conditions and the alloy composition (chromium content). The initial development of coherent, sub-microscopical CrN nitrides leads to a state of hydrostatic stress allowing the uptake of excess nitrogen dissolved in the ferrite matrix. It is shown that the outcome of the subsequent discontinuous coarsening process, which leads to a lamellar microstructure, has a close relation to the nitrogen supersaturation. As a result, the occurrence of a distinct gradient in hardness across the nitrided zone can be understood.

The sessile drop method has been used to measure the density and surface tension of liquid Ag–Bi–Sn mixtures. The density of these alloys shows linear dependence on temperature for all the compositions investigated. However, the surface tension shows curvilinear dependence on temperature for all of the compositions, except for those of a low silver content. The experimental values of the surface tension of Ag–Bi–Sn alloys are compared with those computed from the model, and relatively good agreement is observed.

Densities and thermal expansion coefficients of liquid Al–Ag and Al–Cu alloys were measured using the technique of electromagnetic levitation. This technique involves producing shadow images of the sample from which the volume is calculated by an image processing algorithm. Data was obtained at temperatures above the melting point. The concentration dependence of density and thermal expansion was studied for both systems. It was found for Al–Ag and Al–Cu that both density and thermal expansion can be related to a negative excess volume and its temperature coefficient.

Precipitation microstructures and mechanical properties of two new Mg–Mn–Y–Gd alloys have been studied. Small Angle X-ray Scattering and Transmission Electron Microscopy carried out on homogeneous and aged states (200°C and 250°C) have revealed Guinier–Preston zones formed from the homogeneous state at ambient temperature and first stages of precipitation consisting in DO 19 type monolayer precipitates. These ultrafine precipitates show further organisation leading to the orthorhombic β′ phase. Peak-hardness microstructure is characterised by a high density of connected β′ precipitates. Overaged samples show a high level of mechanical strength and creep resistance. The related microstructure consists of stable β′ phase precipitates forming connected strings giving rise to a local strain within the matrix. Comparison with previous studies confirms the strong relationship between rare earth elements choice, precipitation microstructure and mechanical resistance.

The texture and microstructure transformations during the annealing of highly cold-rolled silver, α-brass and phosphorus copper were studied using electron microscopy and calorimetry. The results imply that recrystallization is a superposition of several local processes which proceed in two stages. In all materials analyzed, the first stage is governed by oriented nucleation, comprising the nucleation of new grains in the areas of localized strain and their further growth accompanied by the generation of recrystallization twins. The orientation distributions of nuclei and the new grains of all materials analyzed have certain features in common at the beginning of recrystallization. The second stage in phosphorus copper is governed by oriented nucleation, while in silver and in α-brass, it is dominated by oriented growth of grains, characterized by low orientation pinning.

By incorporating the constitutive equations based on classical plasticity and the mechanism-based strain gradient plasticity with finite element software, the stress–strain relationships and uniform elongations of Al polycrystals with different grain sizes were studied numerically. The calculation results indicate that grain refinement cannot substantially improve the uniform elongation but can increase the yield strength of Al polycrystals when the grain size is of the order of the micron and submicron scale. The Hall–Petch relationship for yield strength holds and the uniform elongation decreases with decreasing grain size. The calculation results in general agree well with the experiment data.

The Gibbs energy minimiser incorporated in the thermodynamic subroutine library ChemApp and a thermodynamically consistent, ten-component Bayer liquor model were applied to a simulation of the Bayer process employed worldwide for the refining of alumina from bauxite ores. As well as the liquor cycle from digestion, clarification, precipitation, evaporation and back to digestion (including flash cooling and heating steps), calcination to alumina in a circulating fluid bed furnace was simulated. The present study not only covers temperature and concentration ranges from boehmite digestion to gibbsite precipitation, it also permits exploration of boehmite precipitation as a potential energy saving modification of the process. Furthermore, the simulation is able to predict the accumulation and precipitation of undesirable impurities in various stages of the liquor circuit. It also identifies possible reductions in the overall energy consumption of the Bayer process.

In the present study, it is found that one-pass friction stir processed AZ61 alloy exhibits inferior deformation ability along the longitudinal direction compared with the transverse direction at high temperatures. Based on observation of microstructure and fracture surface, premature onion ring splitting is suggested to be mainly responsible for this anisotropy of deformation behavior. Furthermore, the homogeneous microstructure from multi-pass friction stir processing can cure the premature onion ring splitting and then exhibit isotropic high-temperature superplastic deformation behavior.

The microstructure in modified semi-solid thixocast (A356) alloy consists of an α-Al phase with an Al/Si eutectic structure. The silicon phase in the eutectic structure is coral-like with fine fibers. The lack of good ductility in this alloy is related to the presence of silicon fibers. Accordingly, many investigations have been recently conducted to modify the Si morphology through thermomechanical processing on this alloy. In the present work, a thixocast (A356) alloy was deformed at 450 °C, 500 °C and 540 °C through different strain rates. Consequently the silicon fibers were broken into small particles. The results indicated that the Si particles might coarsen at 540 °C when deformed by rates of 0.0005 s −1 and 0.0001 s −1 .

Hydrogen absorption of aluminum melt during melt treatment is one of the important factors that affect the hydrogen content of aluminum melt. In this paper, the effects of several factors (melt temperature, atmosphere humidity, and rotary speed in the rotary impellor degassing process) on hydrogen absorption are investigated quantitatively. It is found that, the higher the melt temperature, the higher the atmospheric humidity, and the higher the rotary speed in the rotary impellor degassing process, the more serious the hydrogen absorption of melt will become.

To investigate the reliability of the laser welding process for the magnesium alloy ZE41A-T5, eight butt joints were welded using the same processing parameters. These joints were tensile tested in the as-welded and aged conditions and the tensile data were analyzed from work-hardening characteristics. The flow curves cannot entirely be satisfactorily described by the Kocks–Mecking model; however, the model is still applicable to the high strain zone of the flow curves where work-hardening rate decreases linearly with flow stress. The reproducibility of the initial work-hardening rate and saturation stress is statistically analyzed. The initial work-hardening rates for the base castings and welded joints vary from approximately 4000 to 7000 MPa, i. e. 1/4 to 1/3 of the base material shear modulus. The as-welded joints have slightly higher initial work-hardening rates than the base castings. Artificial aging produces lower initial work-hardening rates compared with the base material. The saturation stress ranges approximately from 260 to 320 MPa, i. e. about 2 % of the shear modulus. The saturation stress for the welded joints is lower than that for the base material. Compared with the as-weld joints, aging decreases initial work-hardening rate but slightly increases saturation stress. Both initial work-hardening rate and saturation stress become more scattered after aging.

Aluminum alloy matrix samples reinforced with short carbon fibers or hybrid SiC p composites were prepared, and friction and wear tests were carried out on a wear test machine using a rotation ring on a stationary disk at high load. Optical microscopy, scanning electron microscopy and energy-dispersive analysis of X-rays were used to investigate the composites, worn surface, wear debris and composition changes after wear. By comparison with Al alloy, Al/SCF and Al/SCF/SiC p , the results indicate that the incorporation of SCF into Al alloys improves the wear resistance significantly, the combination of a mechanically mixed layer formed on the worn surface and hard SiC p are responsible for the good tribological properties of the hybrid Al/SCF/SiC p composites at high load. During sliding, an obvious decrease in the temperature rise with SCF addition is attributed to the good anti-friction of SCF.

The microstructure and elevated temperature compressive behavior of Nb-22Ti-16Si-7Cr-3Al-3Ta-2Hf alloys with and without 0.1 at.% Ho were investigated by using scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and compression tests. The results show that the addition of 0.1 at.% Ho to the matrix alloy significantly increases its elevated temperature strength at a higher strain rate, while the strengthening effect becomes weak at a lower strain rate. The elevated temperature compressive deformation behavior of the Ho-doped alloy can be adequately described by the power-law equation. The mechanism of Ho addition on elevated-temperature compressive behavior of the alloy is discussed in the paper.