Band 103, Heft 8

A model for the thermodynamics of and strengthening due to Cu–Mg co-clusters in Al–Cu–Mg based alloys is analysed and tested. The formulation uses a single interaction enthalpy between dissimilar alloying elements (e.g. Cu and Mg atoms in an Al–Cu–Mg based alloy) combined with the configurational entropy. The metastable solvus in Al–Cu–Mg based alloys is calculated. Recently published small angle X-ray scattering experiments, 3-dimensional atom probe and yield strength data on these type of alloys support the model. The small angle X-ray scattering and hardness experiments, as well as calorimetry experiments, are sensitive to the main free energy (or enthalpy) changes, which are dominated by Cu–Mg bonds formed by the dimers and the local electron densities related to these bonds. 3 dimensional atom probe is less sensitive to dimers, and will detect agglomeration of dimers to form larger clusters.

The effect of natural ageing time before artificial ageing has been investigated in four Al–Mg–Si(–Cu) alloys, with 0.4% Mg + 0.8% Si and 0.8% Mg + 0.4% Si, both with and without 0.14 at.% Cu. The precipitate microstructure was quantified by means of transmission electron microscopy. Varying the storage time before ageing for 170 min at 200°C, we observe an initial hardness increase after minutes, a decrease after several hours and another increase after weeks. The hardness decrease was most pronounced in the Mg-rich Cu-free alloy, caused by a reduced precipitate volume fraction. Adding Cu produces finer microstructures, higher hardness and reduces the negative effect of natural ageing regardless of the Mg/Si ratio of the alloy. With 1 week storage, an increase in the fraction of the Cu-containing precipitates L and Q′ was observed in the Cu-containing Si-rich and Mg-rich alloys respectively.

The clustering kinetics of Al–Mg–Si alloys are studied by means of differential scanning calorimetry. Five alloys with different Mg and Si contents from 0.4 to 1.1 at.% are used. Two overlapping cluster peaks are observed in the heat flow curves; Cluster 1 (C1) situated at ≈50°C and Cluster 2 (C2) situated at around ≈80°C when heating at 10 K min −1 . It was found that the heat effect of clustering is largest in alloys with Mg/Si close to 1, while the C1/C2 heat effect ratio depends on the Si content. Activation energies for the two clustering processes C1 and C2 are obtained by applying different evaluation methods. The alloy having an Mg/Si ratio close to 1 has the smallest initial activation energies for C1, while samples with higher Mg content have lower initial activation energies for C2, inferring that C1 is Si-related, whereas C2 is Mg-related. However, the entire clustering process requires both Mg and Si to proceed.

Tensile tests have been carried out on a commercial Al–Mg–Si-alloy after simultaneous deformation and artificial ageing. Extruded and solution heat treated specimens were deformed 2%, 5% and 10% (engineering strain) and simultaneously artificially aged at two different temperatures (175 and 190°C) for various times (depending on the strain rates used for pre-deformation) before tensile testing. A comparison with previously obtained tensile properties following sequential pre-deformation and artificial ageing shows that the simultaneous mode generally displays better mechanical strength and after shorter ageing times. The best performing alloy has been obtained by using a split artificial ageing procedure for 10% deformed sample simultaneously aged for 40 min at 175°C followed by additional ageing for 60 min at the same temperature.

Aluminum 7000-alloys depend primarily on precipitation hardening for their strength. Precipitate phases may form through complex precipitation sequences, in which a precipitate gradually evolves towards a thermodynamically stable state via intermediate thermodynamically metastable phases. During the precipitation sequence, precipitates may change not only size but also shape, composition and coherence with the matrix. Despite the industrial significance of aluminum-7000 alloys, the precipitation process remains poorly understood, as evinced by a large amount of scatter in the experimentally observed data on precipitation. In this work, we employ density functional theory first principles calculations to study the energetics of precipitation. It is shown that for the various precipitate families, a monotonic decrease in bulk formation enthalpy occurs during the precipitation sequence and various proposed structures are discarded based on unreasonable energetics. Further, the influences of strain and interfacial energy are quantified and a structural sequence is proposed for the important η precipitate family based on energetics.

The kinetics of clustering in Al–Mg–Si alloy is studied by means of Kinetic Monte Carlo simulations. The simulations take into account the probability of vacancy jumping to nearest neighbour sites. This probability is calculated by considering both the activation energies for single vacancy migration and the difference of interaction energies before and after jumping. The simulations show that clustering in Al–Mg–Si is fast and takes place in three stages. In the initial stage, dimers, trimers and small co-clusters form. The number density of such clusters increases rapidly and solute atoms aggregate to those clusters until a maximum number density value is reached after 2 min. In the second stage, from 2 min to around 100 min, a decrease in the number density of clusters is accompanied by an increase in the fraction of solute contained in all the clusters. Finally, a further reduction in the cluster number density is observed, while the amount of solute atoms in the clusters remains constant. We discuss how robust the results are with respect to changes in the input parameters.

This paper describes an approach to simulate the incomplete bainitic transformation observed in high-Si steels. The model can capture the incomplete transformation when a simple procedure accounting for carbon enrichment in the remaining austenite is applied to calculate the change in the zero transformation temperature, which is a key parameter controlling the transformation kinetics. The carbon concentration in the remaining austenite is determined by the volume fraction of bainitic ferrite and the effective carbon concentration of the bainitic ferrite. Comparison with experimental data for Fe-0.3C-2.4Mn-1.8Si is made, which demonstrates that the isothermal kinetics of bainite formation in the range 370–480°C can be satisfactorily described with the model by using a single set of model parameters and only adjusting the carbon concentration of the bainitic ferrite. A good agreement is found between predicted final carbon contents of retained austenite and those measured using X-ray diffraction, which indicates that a plausible carbon dependency of the zero transformation temperature has been assumed in the calculations.

The impact of interannealing on the recrystallization process in a twin-belt cast AlFeSi alloy was investigated by means of calorimetry, microstructural analysis, electrical conductivity, hardness measurements and cupping tests. The AlFeSi hot band cold-rolled to 1 mm without interannealing offers a higher precipitation capacity than that interannealed at 4 mm. The response to annealing of the former is therefore dominated by precipitation activities. Recrystallization is sluggish, lasts until 623 K and produces relatively coarse grains. The interannealed sheet, on the other hand, softens fully at 573 K via a typical two-stage, recovery and recrystallization reaction. The limited precipitation capacity in the latter allows recrystallization to occur primarily in a discontinuous fashion. This helps to balance the rolling texture inherited from the prior cold-rolling passes. In sheet samples processed without interannealing, the nucleation rate and the volume fraction of the discontinuously recrystallized grains are greatly reduced, owing to extensive concurrent precipitation. This reduces the strength of the annealing texture components which thus fail to balance the rolling texture after annealing.

A preparation route of ferrite magnetic material by leaching of Jinchuan nickel sulfide concentrates in hydrochloric acid was developed. Pre-roasting of the concentrates could significantly improve the leaching efficiency of nickel. X-ray diffraction measurements, thermogravimetric, and differential thermal analyses were used to determine the phase transition after pre-roasting. The leaching could extract almost 100% nickel and 89% iron. The present process involved pre-roast of nickel sulfide concentrate, hydrochloric acid leaching at ambient pressure, solid–liquid separation, co-precipitation, and calcination. The nickel ferrite doped with Cu 2+ and Mg 2+ could be obtained by co-precipitation and calcination. Further study confirmed that ferrite products were of spinel structure and exhibit good ferromagnetism.

Microwave dielectric properties of La 3+ -doped (Pb 1− x Ca x )(Fe 0.5 Nb 0.5 )O 3 ( x = 0.5–0.6) ceramics were investigated, and a superstructure was detected in these ceramics. The dielectric constant (∊ r ) and the temperature coefficient of the resonant frequency ( TCF ) decreased, while the Qf value of the specimens increased with the increase in Ca 2+ content. At the same Ca 2+ content, the ∊ r and Qf values of La 3+ -doped specimens were much higher than those of undoped ones. This is attributed to the elimination of oxygen vacancies by the solid solution of surplus La 3+ with (Pb, Ca) 2+ . Excellent microwave dielectric properties of Qf = 6795 GHz, ∊ r = 96.41 with TCF = −4.99 ppm · K −1 were obtained at x = 0.55.

The thermodynamics properties of liquid Cu–Sn–Sb alloys were determined using solid-oxide galvanic cells with zirconia electrolyte:(1)Re+kanthal,Cux−Sby−Sn(1−x−y),SnO2//ZrO2+(Y2O3)//NiO,Ni,Pt in the temperature range from 998 to 1223 K. The Gibbs free energy of formation of pure solid SnO 2 from pure elements was derived. Using electromotive force values measured for the cell with x Sn = 1.0, the following temperature dependence was obtained:ΔGf,SnO20=−567903+200.33T(±1000J)[Jmol−1] Activities of tin were determined along three sections of the ternary system specified by the ratio of mole fractions x Cu / x Sb = 1:3, 1:1, and 3:1. Thermodynamic properties of the liquid phase were described by the Redlich–Kister–Muggianu formula. Using commercial software, phase relations in the ternary system were calculated. The results of the calculations are compared with experimental data available from literature.

The control of the particle size distribution of metal nano-particles produced via aerosol condensation is a technologically relevant open problem, constrained by the limits of the construction materials and by the desire to minimise carrier gas consumption. As reactive impurities can be present in the carrier gas, we have reviewed the physical chemistry and reactivity of nano-particles and developed a quantitative, continuous model of the nucleation and particle growth processes in the presence of a reactive impurity. The model has been applied to nickel as the metal and oxygen as the impurity, but could equally well be applied to other metals and other impurities such as nitrogen, sulphur, or the halogens. In certain conditions oxygen is only stochastically present in the nucleating nickel droplets, resulting in oxide inclusions that cannot be described by a continuous model. In other conditions the oxidation occurs simultaneously with the metal condensation and can be studied qualitatively using the developed continuous model. The model indicates that the concentration of a reactive contaminant in the gas phase could be used as an additional operating parameter to control the final particle size.

A distinct concentration of copper was embedded into silica matrix to form xerogel using Cu(NO 3 ) 2 · 3H 2 O as the copper source. The xerogel samples were prepared by hydrolysis and condensation of tetraethyl orthosilicate (TEOS) by the sol–gel method. The new molar ratio of H 2 O/TEOS was determined to be 11.67. Also the necessary amount of trihydrated copper nitrate was added into the solution such that the concentration of the copper oxide in the final solution reached 10 wt.%. After ambient drying, the xerogel samples were heated to 100, 400, 600, 800, and 1000°C at a slow heating rate (50 K h −1 ). The structural properties were characterized by means of transmission electron microscopy, surface analysis and thermal program reduction methods at different temperatures. Finally, the catalytic behavior of nanocomposites was studied for nitrogen oxide (NO) oxidation and di-nitrogen oxide (N 2 O) decomposition reactions. The results present a systematic reactivity study of NO oxidation and N 2 O decomposition on the dispersed copper oxide nanocatalysts over silica supports in order to determine the ability of these materials to convert nitrogen oxide and di-nitrogen oxide into harmless species at different temperatures.

The manufacturing process, electrochemical property, and microstructure evolution of as-cast, rolled, and annealed Mg-4.1% Ga-2.2% Hg anode sheet are studied in this investigation. The results show that the Mg-4.1% Ga-2.2% Hg anode sheet obtains an average discharge potential of −1.7924 V (vs. saturated calomel electrode) with an electric current density of 100 mA · cm −2 and a rapid activation time of less than 10 s in a half-cell test. The Mg–Ga–Hg sheet can be used as an anode material for seawater activated batteries. The Mg 3 Hg and Mg 21 Hg 5 Ga 3 phases in the anode influence the electrochemical corrosion of the Mg–Ga–Hg anode sheet and provide a stable corrosion potential during discharge.

The work-hardening of the solution-strengthened aluminium alloy AA5182-O is studied as a function of strain rate at room temperature. The main aim is to characterize the influence of strain rate and dynamic strain ageing on the work-hardening of the material. Tensile tests with constant nominal strain rates in the range from 10 −6 to 1 s −1 were carried out; additionally, strain-rate jump tests were performed. The material exhibits a sharp yield point, a Lüders plateau, and serrated yielding within the actual range of strain rates. This indicates that the steady-state strain rate sensitivity is negative as a result of dynamic strain ageing. The serrations in the stress–strain curves and the work-hardening are markedly reduced with increasing strain rates. In most cases, the jump tests exhibit the standard behaviour, i.e., after a transient period following the jump in strain rate, the stress–strain curve converges to the steady-state stress–strain curve at the new strain rate.

The steady-state creep behavior in an isotropic functionally graded composite cylinder subjected to internal pressure has been analyzed in the presence of a thermal gradient. The cylinder is assumed to be made of a functionally graded composite containing silicon carbide particles in a matrix of pure aluminum. The effect of imposing a linear particle as well as a thermal gradient on the distribution of creep stresses and creep rates in the functionally graded cylinder has been investigated. The study shows that in the presence of both thermal and particle gradients, the radial stress decreases throughout the cylinder, whereas the tangential, axial, and effective stresses increase significantly near the inner radius but show a significant decrease towards the outer radius. The strain rates as well as strain-rate inhomogeneity in the composite cylinder could be reduced to a significant extent by imposing thermal and particle gradients alone or together, while keeping the same average amount of reinforcement.

A composite combining hydroxyapatite with varying ratios of 316L to form an asymmetrical functionally gradient material was successfully prepared using a hot-pressing technique. The results show obvious macroscopic gradient changes in the functionally gradient material. Although the components change continuously on a microscopic scale, the interfaces between gradient layers remain tightly integrated. The addition of 316L powder changes the fracture modes of the composites and improves their mechanical properties. The hydroxyapatite content in the composites varies with different gradient layers, which is in keeping with the composite design. The phases of hydroxyapatite and 316L dissolve into each other to some degree during hot-pressing and the combining mechanism of the two phases in the gradient layers is dissolving behavior. Thermal stress relaxation calculation indicates that the gradient structure of the asymmetrical HA/316L functionally gradient material has a significant impact on the relaxation of residual thermal stress.

In this study, the strength and water permeability of concretes containing various types of fly ashes and filler material were investigated. Five types of fly ashes with two levels of fineness and ground-river sand were used to partially replace Portland cement at the dosages of 20 and 40% by weight. Results indicated that pulverized coal combustion fly ash produced concrete with good strength and low permeability. Normal fluidized bed combustion fly ashes containing very irregular particles with pores produced concrete with low strength and relatively low permeability. Fly ash from fluidized bed combustion of coal with bio-mass materials and fly ash with high amount of CaO and SO 3 produced poor concretes with low strength and high permeability. The increase in fly ash fineness helped improve both strength and permeability of concrete due to the increase in the reactivity of fly ash. Filler material could also be used to replace Portland cement at low replacement level of 20% with small effects on strength and water permeability.

In the paper “Re-evaluation of activities of magnesium and zinc components in the magnesium–zinc binary system from very low to high temperature” by Morishita et al. [1], which recently appeared as a Feature Article in this journal, the authors made the following three statements, commencing with the first sentence of the abstract: 1. The activities of zinc, a Zn , and magnesium, a Mg , from very low to high temperatures in the Mg–Zn binary system were evaluated for the first time from the relationship between the Gibbs energies of formation, Δ f G ° T , of Mg 48 Zn 52 , Mg 2 Zn 3 , MgZn 2 and Mg 2 Zn 11 and their phase equilibria.” 2. It was found that the a Zn and a Mg values in the compounds steeply changed from the minimum to maximum as a function of the composition in a narrow solubility range.” 3. “…at very low temperatures, a Zn and a Mg were very small approaching to zero, …”; “The values of log a Zn and log a Mg in the two-phase regions were found to be decreased … and resulted in very small values”. These three statements are disputed and commented below. For convenience, let us start with the second statement.