Mechanisms and kinetics of θ' dissolution in Al-3% Cu

In situ hot stage studies of θ' dissolution at 370°C in Al-3% Cu, bulk aged before thinning, have been carried out with the aim of determining the mechanisms and kinetics of θ' dissolution. An important observation concerns the presence of dislocations linking semicoherent θ' precipitates to both other θ' precipitates and to the growing incoherent θ phase. The dislocation distribution changes continuously during dissolution, with climb excursions taking place as the θ' fragments and dissolves. In situ studies on specimens having undergone deformation prior to thinning reveal that the dissolution rate of θ' was reduced, suggesting that dissolution is controlled by a vacancy flux to accommodate the large local volume change accompanying the transformation. This mechanism is in accord with the observed linear relationship between precipitate radius and dissolution time. The addition of 0.05% Cd substantially reduces the θ' dissolution rate, and possible reasons to account for this observation are discussed.     

Solubility,diffusivity and trapping of hydrogen in dilute alloys. Deformed and amorphous metals—II

The chemical potential and diffusivity of interstitial solute atoms was calculated for disordered materials in a general way. Disorder may be of chemical nature as in alloys or caused by a strain field as around dislocations and a crack tip or disorder may be as total as in an amorphous alloy. For these examples of disordered systems relations were derived from a general expression and compared with corresponding special theoretical results which are mostly related to attractive interaction (trapping) and repulsive interaction (antitrapping) between interstitially and substitutionally dissolved impurity atoms. A comparison with experimental results is namely made for the solubility and diffusivity of hydrogen in dilute alloys, deformed Fe and Pd and amorphous Pd_77.5Si_16.5Cu₆ and Ni₆₄Zr₃₆.     

Shock synthesis of silicides—II. Thermodynamics and kinetics

A thermodynamic and kinetic analysis of shock-induced reactions in the (Nb or Mo)-Si systems provides a framework for the extraordinary high reaction rates and a quantitative interpretation of the experimental results obtained in Part I. The thermodynamic analysis is conducted by adding the heat of reaction to the shock energy; increases in shock pressure, temperature, and velocity are predicted. At the particle level, melting at the silicon-metal interface is found to be a necessary condition for the initiation of reaction; heat conduction calculations enable the prediction of a critical molten (Si) region size for which the heat generated through the reaction exceeds the heat lost to the unreacted regions. The calculation of melt fraction (of Si) as a function of shock energy coupled with critical melt pool sizes enables the determination of a minimum shock energy for the initiation of shock-induced reaction. At the local level, the reaction kinetics can be rationalized through the production of a liquid-phase reaction product (NbSi₂), the formation of spherical nodules (∼2 μm diameter) of this product through interfacial tension and their subsequent solidification (in times of 1–5 ns). The heat generated by the reaction is sufficient to melt niobium along the interface which facilitates both the expulsion of the NbSi₂ nodules into the liquid Si, and the generation of fresh Nb interface for further reaction. In addition, the dissolved Nb enriches the surrounding Si liquid, promoting more NbSi₂ reaction and formation.     

Slag–graphite wettability and reaction kinetics Part 1 Kinetics and mechanism of molten FeO reduction reaction

Abstract

The present paper reports results relating to the kinetics and mechanism of FeO reduction by graphite, the data being obtained from experimental investigations into the wettability of graphite by molten slag containing FeO. The rate of FeO reduction was determined by measuring the volume of CO gas formed as a result of the reduction of FeO in experiments conducted in the same sessile drop apparatus. The reduction reaction initiated by direct slag–graphite contact produces CO gas which spreads into the molten slag droplet causing foaming of the slag; further reduction of FeO proceeds mostly via indirect reduction. The rate of reduction was found to depend directly on the initial FeO content. An increase in temperature improves the rate of reaction, which has an activation energy of 112·18 kJ mol^-1. These results indicate that transport of FeO (Fe²⁺, O^2- ) in the liquid slag phase is probably the slowest step.     

On the kinetics and mechanism of extraction of copper with β-hydroximes in the absence of mass transfer limitations

For a variety of experimental systems, the dependence of the rate of extraction of copper with pure, β-hydroximes on reagent concentration changes as a function of that concentration. Under experimental conditions where mass transfer limitations can be considered absent, the dependence can be explained using the Gibbs isotherm. The limiting conditions under which the dependence assumes certain specific forms are discussed. Finally, a mechanism is proposed which can explain the chemistry of the extraction for a wide range of reagent concentrations and experimental conditions.     

Precipitation and dissolution kinetics in AlLiCuMg alloy 8090

Resistivity and differential scanning calorimetry (DSC) techniques are used to study the kinetics of precipitation and dissolution of GPB zones and metastable phases (δ′ and S′) in AlLiCuMg alloy 8090. Three stages of the precipitation sequence during aging have been analyzed. A new analytical method is developed for the DSC technique, which requires only one heating rate to obtain the kinetic parameters, and the results show good agreement with the more conventional method of varying heating rate. The low-temperature endothermic peak in the DSC thermogram is interpreted as the dissolution of Li-bearing zones, which is supported by the hardness results. The activation energy, Q, and the growth parameter, n, determined by resistivity and DSC techniques are in good agreement with previously published data.     

Synthesis and dissolution of slag minerals - a study of β-dicalcium silicate,pseudowollastonite and monticellite

Leaching of slag must be limited when slag is used in other areas than landfill. As slag is composed of different minerals, the properties of slag, inter alia the dissolution, depend on these minerals. If the leaching properties of each mineral are known, counter measurements can be taken to prevent the leaching of unwanted elements. In this study, the dissolution of three common slag minerals will be examined. The three minerals; boron-stabilised β-dicalcium silicate, pseudowollastonite and monticellite were synthesised. The dissolution was measured by setting pH to 4, 7 or 10 and adding 0.05 g mineral in the size range 20-38 mm and the HNO₃ consumption required to maintain the pH level was recorded during 40 h. As expected, the dissolution increases for all minerals when pH decreases. The boron-stabilised β-dicalcium silicate was the only mineral fully dissolved at all pH levels, while pseudowollastonite and monticellite only dissolved completely at pH 4.

On doit limiter le lessivage de la scorie lorsqu'elle est utilisée en d'autres endroits que la decharge publique. Puisque différents minéraux composent la scorie, ses proprieties, parmi d'autres la dissolution, dependent de ces minéraux. Si l'on connaTt les propriétés de lessivage de chaque mineral, on peut utiliser des contre-mesures pour empecher le lessivage d’éléments indésirables.

Dans cette étude, on a examine la dissolution de trois minéraux communs de la scorie. On a synthétisé les trois minéraux, soit le silicate β-dicalcique stabilise au bore, la pseudowallonite et la monticellite. On a mesuré la dissolution en établissant le pH à 4,7 ou 10 et en ajoutant 0.05 g de minéral dans la gamme de taille de 20 a 38 mm. On a ensuite enregistré pendant 40 heures la consommation d'HNO₃ requise au maintien du niveau de pH.

Comme on s'y attendait, la dissolution de tous les minéraux augmente avec la diminution du pH. Le silicate β-dicalcique stabilise au bore était le seul minéral complétement dissous a tous les niveaux de pH, alors que la pseudowallonite et la monticellite étaient complétement dissoutes seulement au pH de 4.     

Kinetics and mechanism of oxidation of β-sialons

Conclusions We examined the behavior of two materials during high-temperature oxidation: the singlephase -sialon and the material based on -sialon containing up to 8% residual -Si₃N₄ and the glass phase produced by reaction sintering of the charge based on -modification of silicon nitride. The results show that the maximum oxidation resistance in the temperature range 800–1300°C is exhibited by the single-phase -sialon Si_5.5Al_0.5O_0.5N_7.5. The oxidation resistance decreases in the presence of the residual -Si₃N₄ and the glass phase in the material. The oxidation kinetics are also determined by the structure and composition of the oxide film. The kinetic curves are governed by a parabolic law which indicates that oxidation of the sialons is limited by the diffusion processes. No marked changes were detected in the lattice spacing of the sialon after oxidation.Translated from Poroshkovaya Metallurgiya, No. 6(306), pp. 69–73, June, 1988.     

The kinetics of dislocation climb over hard particles—I. Climb without attractive particle-dislocation interaction

The effect of hard particles of low volume fraction on the creep strength is studied theoretically in a two-part paper. Here in part I, the kinetics of dislocation climb over a particle is modelled assuming that the particle does not exert an attractive force on the dislocation (“non-interacting” particle). The shape of the climbing dislocation is given by a minimum energy condition, which is shown to rule out purely “local” climb (as considered in previous models). A natural power-law dependence of the dislocation velocity on the applied stress, with n ranging from about 3 to 4, is obtained, and only a small threshold stress can be identified. The results appear compatible with the creep behaviour of alloys strengthened by coherent precipitates, but are totally at variance with experimental threshold stress data for materials with incoherent particle dispersions.     

The isothermal reduction kinetics of chromium-bearing vanadium–titanium magnetite sinter

The reduction of chromium-bearing vanadium–titanium magnetite sinter (CVTMS) by CO was investigated at 1123–1223?K. The reduction degree increased with increasing temperature. The isothermal reduction kinetics of CVTMS was analysed, according to Sharp analysis and ln–ln analysis, the kinetic mechanism of reduction process for all samples in different basicity can be represented as f(α)?=?1.61(1?α)[?ln(1?α)]^1–1/1.61. The reaction activation energy of all samples (R?=?1.9, 2.1, 2.3, obtained according to the components of the materials burdening used in sintering process) at different reaction degrees were calculated by the model-free method. And the pre-exponential factors of reaction also were calculated by the mathematical method. The rate controlling step for the reduction process under the present reduction condition was chemical reaction.     

Kinetics of discontinuous precipitation and dissolution in Cu–Ag alloys

The kinetics of discontinuous precipitation and dissolution of the cellular precipitate have been studied in Cu-3 at.% Ag and Cu-4 at.% Ag alloys. The growth rates of the cells were measured using optical microscopy. The interlamellar spacings of the primary cells and compositions of the depleted matrix were measured using scanning electron microscope and X-ray diffraction, respectively. The Cu–Ag alloys were observed to decompose into a lamellar structure consisting of alternate lamellae of the α (Cu-rich) and β (Ag-rich) phases when a solid solution of the alloy was aged below the solvus temperature. The rod shaped morphology of the β phase dispersed in the matrix of α was observed at all temperatures. The primary cell growth data were analysed using the theories of Cahn, Hillert, Sundquist, Turnbull and Petermann and Hornbogen. From the diffusivity values, it has been shown that the growth of primary cells occurs by the diffusion of Ag along the grain boundaries. The results are consistent with the diffusivity values reported in the literature in the same temperature range.The discontinuous dissolution of the primary cells occurred above the solvus temperature as well as 30–40 K below it. The dissolution occurred primarily at the prior positions of grain boundaries as well as at the primary cells intersection, at least in the initial stages of dissolution. The discontinuous dissolution occurred by the diffusion of Ag along the interface boundary between the primary and dissolution cells. The diffusivity and mobility values obtained during dissolution are one order of magnitude smaller than those obtained for the discontinuous precipitation. This has been explained by volume diffusion ahead of the interface and not all of the free energy being used to drive the interface boundary.     

An in situ study of prismatic glide in magnesium—I. The rate controlling mechanism

In situ experiments have been performed in magnesium single crystals, oriented so as to inhibit basal glide, in order to study the glide mechanism of a dislocations in prismatic planes, between 50 and 650 K. In the first part of this study, the observation of rectilinear screw dislocations, submitted to a strong friction force, leads to the conclusion that the strain rate is controlled by the Friedel-Escaig mechanism, in the whole temperature range. The corresponding macroscopic activation area is discussed, and the athermal temperature is estimated as higher than 700 K. Dislocation multiplication occurs like in b.c.c. metals, on fixed sources, and on closed and open dislocation loops. Creep of magnesium is then reinterpreted in terms of the present results.     

Solvent Extraction of Cobalt and Nickel—The Chemistry and its Application*

Abstract

The paper presents a review of the chemistry of solvent extraction of cobalt and nickel with reference to the application in the industry. The electronic configuration and the coordination chemistry of the cobalt and nickel ions provide fundamental basis relating to the extraction and separation.     

Dynamic simulation of crystal growth by Monte Carlo method—I. Model description and kinetics

Previous Monte Carlo methods used in the crystal growth area are briefly reviewed and a new Monte Carlo simulation method for crystal growth from the melt (solidification) is presented. In this, a Monte Carlo simulation is coupled with traditional continuum equations, i.e. heat transfer and solute transfer equations. The material properties of interfacial energy and volumetric free energy are also taken into account. Finally, the kinetics of crystal growth are examined. The validity of the model is confirmed by its prediction of the Johnson-Mehl-Avrami (JMA) equation in the case of equiaxed grain formation.     

Effect of Milling Parameters and Cr Content on Morphology and Solubility of Nanostructured Cu–Cr Solid Solutions

In the present work, a set of Cu-based powder mixtures containing up to 6 wt% Cr has been processed through mechanical alloying for a range of milling times up to 96 h. The mixtures were processed in a ball mill with ball to powder ratio of 10:1 and the equal numbers of 1 and 2 cm balls. The processed powder mixtures were investigated by scanning electron microscope, optical microscopic equipped with image analyzer, X-ray diffraction technique and micro hardness in order to determine the particles morphology, distribution of chromium, mean crystallite size, lattice parameter and hardness after milling, respectively. Crystallite sizes were measured by Williamson–Hall method and lattice parameters were determined using an extrapolation function. Results show that the powder behavior varies with milling time, and powder composition.     

The Syntheses,Characterization and Crystal Structure of Extraction Complexes of Rare Earths

The syntheses,characterization and crystal structure of rare earth coordination compounds of five typicalextraction systems commonly used in the separation technology were reported.They are:(1)carboxylic acidsystem,(2)organophosphorous acid system,(3)quarternary ammonium salt system,(4)neutralorganophosphonate system and(5)synergistic system.     

Equilibrium kinetics of manganese-ore carbon-reduction between molten steel and slag

The equilibrium condition between molten steel and slag during manganese-ore carbon-reduction was studied using both resistance furnaces and induction furnaces. The resistance furnace experiment shows that it is difficult to further increase the yield of manganese (maintained at around 37%) without stirring. The bottom blowing and top slag stirring were strengthened in the induction furnace test, where white slag was continuously produced to promote the carbon reduction of manganese dioxide in slag between molten steel and slag. Under these conditions, the yield of manganese between molten steel and slag can reach greater than 90%. The main factors affecting the manganate capacity are the carbon and silicon content in molten steel. The limiting process of manganese mass transfer is mainly the mass transfer of manganese in molten steel. Under carbon reduction in molten steel the limiting factor affecting the mass transfer of manganese is the mass transfer of manganese in slag.     

The Friction Stress of the Hall–Petch Relationship of Pure Mg and Solid Solutions of Al,Zn, and Gd

Temperature and solute concentration effects on the friction stress, σ_o, of cast (texture-free) polycrystals of pure Mg, and of Mg-Al, -Zn and -Gd binary solid solutions are discussed using phenomenological arguments. The temperature effects on the pure metal suggest that σ_o relates to the ratio between the CRSS of prism and basal slip, against early suggestions that it should only relate to the CRSS for basal slip. Solid solution softening upon prism slip accounts for the minima in σ_o at ~ 0.5 at. pct in Mg-Zn and Mg-Gd alloys. In the concentrated alloys, solute-specific hardening effects upon slip and twinning lead to diverging behaviors: in Mg-Al and Mg-Zn, σ_o remains below that of pure Mg. Strong short-range order by Gd leads to a steep monotonic increase, and to a value larger in compression than in tension due to the activation of {10-11} twinning at high concentrations. The negative σ_o of the dilute Mg-Zn alloys is an artifact created by the tension/compression asymmetry stemming from the polar character of {10-12} twinning.