Nucleation of Al3Zr and Al3Sc in aluminum alloys: from kinetic Monte Carlo simulations to classical theory

Zr and Sc precipitate in aluminum alloys to form the compounds Al3Zr and Al3Sc which for low supersaturations of the solid solution have the L12 structure. The aim of the present study is to model at an atomic scale this kinetics of precipitation and to build a mesoscopic model based on classical nucleation theory so as to extend the field of supersaturations and annealing times that can be simulated. We use some ab-initio calculations and experimental data to fit an Ising model describing thermodynamics of the Al-Zr and Al-Sc systems. Kinetic behavior is described by means of an atom-vacancy exchange mechanism. This allows us to simulate with a kinetic Monte Carlo algorithm kinetics of precipitation of Al3Zr and Al3Sc. These kinetics are then used to test the classical nucleation theory. In this purpose, we deduce from our atomic model an isotropic interface free energy which is consistent with the one deduced from experimental kinetics and a nucleation free energy. We test di erent mean-field approximations (Bragg-Williams approximation as well as Cluster Variation Method) for these parameters. The classical nucleation theory is coherent with the kinetic Monte Carlo simulations only when CVM is used: it manages to reproduce the cluster size distribution in the metastable solid solution and its evolution as well as the steady-state nucleation rate. We also find that the capillary approximation used in the classical nucleation theory works surprisingly well when compared to a direct calculation of the free energy of formation for small L12 clusters.Comment: submitted to Physical Review B (2004

THERMOTRANSPORT OF 85Sr AND 54Mn IN POTASSIUM CHLORIDE SINGLE CRYSTALS

On a mesuré le thermotransport de 85Sr et 54Mn dans des monocristaux de KCl dans le domaine 950-1 025 K au moyen de l'effet Soret. La chaleur de transport de l'impurité est — 3,18 ± 0,03 (1 ± 0,25) eV pour Sr en KCl pur et en KCl dopé par Sr, et il est — 5,45 ± 0,29 (1 ± 0,27) eV pour Mn en KCl pur et en KCl dopé par Mn. Il n'y a aucune différence significante entre les cristaux purs et dopés. On a discuté les limites expérimentales et théoriques des mesures pour étudier les chaleurs de transport et l'on a comparé les résultats avec les mesures antérieures dans les cristaux ioniques.The thermotransport of 85Sr and 54Mn tracers in single crystals of KCl was measured in the temperature range 950-1 025 K, by means of Soret effect measurements. The mean value of the impurity heat of transport was — 3.18 ± 0.03 (1 ± 0.25) eV for Sr in pure or Mn-doped crystal, and was — 5.45 ± 0.29 (1 ± 0.27) eV for Mn in pure or Mn-doped crystals. There was no significant difference between values from pure and doped crystals. A discussion of the experimental and theoretical limitations of such measurements as a source of information about the heats of transport for individual jumps was given, and a cornparison made with earlier thermotransport measurements in simple ionic solids

Diffusion kinetics in dilute binary alloys with the h.c.p. crystal structure

In this paper, an extended version of the matrix method is derived in order to address diffusion kinetics for the full anisotropic three-dimensional h.c.p. structure. It is shown that the diffusion anisotropy can be properly addressed with a model of 13 atom-vacancy frequencies which is an extended version of the well-known 5-frequency model for the f.c.c. lattice. Both tracer and phenomenological diffusion coefficients are calculated using this new approach. Extended Monte Carlo simulations are performed in order to cross-check some of the results of the matrix method. Applications of the proposed model to experimental diffusion data are discussed