EQUILIBRIUM VACANCY CONCENTRATION AND THERMODYNAMIC QUANTITIES OF FCC DEFECTIVE ALLOYS AuCuSi AND PtCuSi UNDER PRESSURE AND TEMPERATURE

We present the analytic expressions of the cohesive energy, the alloy parameters, the equation of state, the mean nearest neighbor distance, the Helmholtz free energy, equilibrium vacancy concentration, and thermodynamic quantities such as the isothermal compressibility, the thermal expansion coefficient, the heat capacities at constant volume and constant pressure for facecentered cubic (FCC) defective ternary substitutional and interstitial alloy ABC derived by the statistical moment method (SMM). The obtained thermodynamic quantities depend on temperature, pressure, the concentration of substitutional atoms, the concentration of interstitial atoms, and equilibrium vacancy concentration. Thermodynamic quantities of FCC defective metal A, FCC defective substitutional alloy AB, and FCC defective interstitial alloy AC are specific cases for thermodynamic quantities of FCC defective ternary substitutional and interstitial alloy ABC. The theoretical results are calculated numerically to alloys AuCuSi and PtCuSi. Our calculated results of thermal expansion coefficient and heat capacities at constant pressure for main metals Au, Pt are in good agreement with experimental data. Our other calculated results for thermodynamic quantities of alloys AuCuSi and PtCuSi at different temperatures, pressure, the concentration of substitutional atoms, and concentrations of interstitial atoms orient and predict new experimental data in the future.

The Influence of Surface Orientation and Sample Thickness on the Substitutional Bulk Diffusion Coefficient in Thin Samples

Model calculations are presented to show that for thin samples the vacancy formation energy is determined by the concentration of vacancies in the bulk. The equilibrium vacancy concentration is calculated and its effect on the substitutional bulk diffusion coefficient is discussed. It is shown that the diffusion coefficient D exhibits abrupt transitions as a function of temperature but that these transitions occur at temperatures which are independent of the surface orientation. The effects are confined to thin samples and for thicker samples the model predicts the well-established bulk diffusion values independent of surface properties.

Strength, Thermal Defects, and Solid Solution Hardening in Nickel-Containing B2 Iron Aluminides

ABSTRACTNickel-containing ternary iron aluminides with an aluminum concentration of 45 at. % were investigated with respect to room temperature strength, equilibrium vacancy concentration, and the kinetics of vacancy removal. As compared to binary iron aluminides with the same Al concentration, nickel additions reduce the thermal equilibrium vacancy concentration at 1273 K, whereas they increase this concentration at 973 K. Furthermore, at low temperatures such as 673 K, nickel additions increase dramatically the time needed to reach vacancy equilibrium. During prolonged annealing at 673 K, the density of <001> dislocations in Fe-45Al-3Ni (at. %) increased by an order of magnitude. This suggests that dislocations act as sinks for vacancies. At the same time, the number density of small (20–50 nm) voids decreased, indicating that they were not stable in the absence of substantial vacancy supersaturations. Our findings show also that the solid solution strengthening of iron aluminides due to Ni is much weaker than previously thought.

Asymptotic analysis of some two-dimensional diffusion problems

In this paper we discuss two-dimensional surface source and implant problems for a substitutional-interstitial diffusion model. We present asymptotic solutions in the limit of the surface concentration of impurity (or peak concentration of the implant) being far greater than the equilibrium vacancy concentration. Using leading order composite solutions we plot contours of constant impurity concentration. Some of these contours differ markedly from those of the corresponding linear problem, having the ‘bird’s beak’ shape which is frequently observed in experiments. We also discuss a two-dimensional surface source problem for a va­cancy model.