On the decomposition of solid solutions of magnesium in aluminum

A nonequilibrium version of measuring the electromotive force during a continuous decrease in the temperature at a rate of 5–7 °C/min is used to study the thermodynamic properties of solid solutions of magnesium in aluminum. The studies show that the abrupt change in the potential of an alloy containing 20–30 mol % Mg at 370–380°C reliably correlates with the decomposition of the solid solution formed at 450°C.     

X-ray study of the decomposition of metastable Al-rich Al-Fe solid solutions

The extension of the solid-solubility limit obtained in Al-Fe alloys up to 4.4 at. pct Fe by rapid quenching from the melt and the appearance of metastable Al₆Fe phase for 4 to 20 wt pct Fe have been reported in the past. Present experiments with isochronal and isothermal annealing of Al-Fe solid solutions containing 3.6 at. pct Fe have shown that the solutions are very stable at room temperature, and less stable at 200°C, while at 300°C they decompose very rapidly and a metastable Al₆Fe phase appears. This phase is replaced by Al₃Fe after annealing for 10 min at 550°C or for 16 hr at 400°C. The Al₆Fe phase remains stable for 76 hr at 300°C, while taking more than 670 hr to be replaced by Al₃Fe. The author discusses the operation of the “two-piston” quenching apparatus and concludes that the sample thickness cannot serve as a sufficient criterion for quenching effectiveness.     

An investigation into the kinetics of formation of inclusions during the decomposition of supersaturated solid solutions

The features of the formation of a system of microinclusions during the decomposition of supersaturated solid solutions are analytically described taking into account the size factor of interfacial surface energy. The suggested computation discloses the physical meaning of phenomena corresponding to phase transformation during the homogeneous and heterogeneous modes by which it proceeds. The supersaturation of the matrix phase is the determining characteristic of such a possibility at the stage of nanostructured nucleation. Unimodal and bimodal systems of microparticles can form at definite values of this quantity. Reliably established experimental facts confirm the conclusions of this theoretical analysis. The obtained solutions can be spread to the various bulk and surface systems of dispersed particles.     

Thermodynamics of Pd-Cu-H solid solutions

Solubility isobars at various temperatures in the range 625–1250 K. have been measured for Pd-Cu-H solid solutions containing 0–20 at.% Cu. The H-concentration was always less than ~ 10⁻⁴ at.%. The variation with Cu-concentration of the partial molar thermodynamic functions of the H-atoms, deduced from the measurements, exhibit singularities not observed in other Pd-noble metal-H systems. The observed behavior has been explained using a statistical mechanical model for the solid solution. The contribution of effects related to changes in the specific volume of the solution with Cu-concentration has been calculated from elastic data measured in the same range of temperature and composition.     

Thermodynamics of molybdenum-nitrogen solid solutions

Recent measurements of the temperature and pressure variation of the equilibrium between N₂-gas and N-Mo solid solutions extending up to 2700 K have been analyzed. It has been shown that the enthalpy of solution varies strongly with temperature above ~2100 K. This phenomenon has been attributed to the simultaneous occupation of both octahedral and tetrahedral sites in the metal lattice by nitrogen atoms. The relevance of this concept to the observation of non-linear Arrhenius plots for the diffusivity of the interstitial species in such systems is discussed.     

Interdiffusion in copper-base Cu-AI solid solutions

Interdiffusion in couples consisting of pure copper and a Cu-12.2 at. pet A1 alloy has been studied in the temperature range between 977 and 1277 K. Concentration of aluminum was determined by EPMA. The interdiffusion coefficient increased with the atomic fraction of aluminum, N_ai, in solid solution: D₀(m²/s) = 0.43 × 10^-4 and Q (kJ/tool) =194000 — 180000 N_a1 The Kirkendall effect has also been studied in the temperature range from 977 to 1277 K. The markers moved toward the aluminum-rich side. The ratio of the tracer-diffusion coefficient of aluminum, D _* ^Al , to that of copper, D _* ^Cu , at the marker position where N_a1 = 0.073, was estimated at 1.7 to 3.7; this ratio showed a tendency to increase with temperature.     

Thermal diffusion in concentrated solid solutions

The kinetic model of thermal diffusion in crystals is considered for a concentrated multicomponent system. The dependence of the heat of transport upon thermodynamic variables introduces corrective terms which are capable of explaining many of the large heats of transport found in the experimental literature. These terms also appear in the diffusion coefficients for substitutional as well as interstitial solutes; the form for the diffusion coefficient would suggest that diagonal terms in the matrix of diffusion coefficients would be larger than off-diagonal terms in agreement with experiment.     

Thermodynamics of interstital solid solutions

Conclusions. (2) On the assumption that within the bounds of one sublattice the interstitial atoms and vacancies are arranged in a disordered manner, while the larger atoms are in complete order, we have compiled an equation for the entropy of formation of an interstitial solid solution with two interstitial sublattices.Equations have been compiled for the free energy of formation and activities of the components and they contain a small number of constants. As a first approximation the activity of a component possessing a large atomic radius is only a function of the crystal structure and composition. The homogeneity of boundaries of isostructural interstitial solid solutions formed with the participation of the same components with a large atomic radius are close to one another, provided the phases in equilibrium with them are also isostructural. A method of using the phase diagram to find the constants in the free energy and activity equations is demonstrated.Briefly considered are the energy and formation entropy of two types of ternary solid solutions (depending on whether two similar components have a large or small atomic radius).Conclusion. For beginning see No. 3, 1965.     

Diffusion in ternary Ag-Zn-Cd solid solutions

Diffusion in the Ag-Zn-Cd system was investigated at 600°C with semi-infinite, vapor-solid couples in order to evaluate intrinsic diffusion coefficients and interdiffusion coefficients in theα andβ phases. Intrinsic diffusion coefficients were determined at several composition points in theα-phase region along concentration lines of 11 at. pct Zn, 18 at. pct Zn, 5 at. pct Cd, and 9 at. pct Cd. Interdiffusion coefficients in theα phase were determined at several points of intersection of various composition paths; these composition points of intersection were along lines of constant zinc-to-cadmium concentration ratios of 3.8, 1.8, and 1.2. All two-phase diffusion couples developed planar β/α interfaces. Intrinsic coefficients were computed at three composition points in the β region corresponding to a silver concentration of approximately 55 at. pct. Intrinsic coefficients for β alloys are two to three orders of magnitude larger than those for α alloys. P. T. CARLSON, formerly Donnan Fellow, School of Materials Science and Metallurgical Engineering, Purdue University, Lafayette, Ind., This paper is based on a thesis submitted by P. T. CARLSON in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Purdue University.     

Interdiffusion in copper-rich cu-ag solid solutions

Interdiffusion was studied in the temperature between 974 and 1273 K, using conventional sandwich-type diffusion couples consisting of pure copper and Cu-2.1 at. pct Ag alloy. The interdiffusion coefficient, , increased slightly with increasing the atomic fraction of silver, N_Ag. Values of were well represented by the parameters of ₀ = 0.21 x 10^-4 (m²/s) and ≈ = 184.5 - 143.0 N_Ag (kJ/g-atom) up toNAg = 0.02. The Kirkendall effect during diffu-sion treatments at 1174 K for up to 1.4724 x 10⁶ s was also studied. The marker moved always toward the silver-rich side. The ratio, D*_Ag / D*_Cu,was 5.6 for N_Ag = 0.011, while it was 4.2 for N_Ag = 0.