We use Mössbauer spectroscopy in combination with atomic scale modeling in order to gather a comprehensive understanding of the growth and the dynamics of cobalt nanoprecipitates in silver. The modeling makes use of classical molecular dynamics in the canonical ensemble by means of the Rahman-Parinello technique. Atomic interactions are governed by an embedded atom model, which is validated for the static Co-Ag interaction by means of a comparison with extended x-ray absorption fine structure measurements and for the dynamical interaction with Mössbauer spectroscopy data. This allows us to identify the cluster size dependent atomic arrangements at the cluster-matrix interface, where strong relaxation takes place. A detailed analysis of the Mössbauer spectra taken at two temperatures after annealing at different temperatures allows us not only to characterize the cluster size dependence of magnetic properties, but also to evidence a possible Ostwald ripening growth mechanism. The mean and interface Debye temperatures are deduced from the Mössbauer spectra and found quite consistent with the model predictions. On this basis, the atomic scale modeling allows us to identify detail of atomic vibrational properties as a function of distance from the cluster center and a discontinuity of the vibration amplitudes at the precipitate-matrix interface is evidenced.PRB 62 5121 GROWTH AND LATTICE DYNAMICS OF Co . . .

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Symmetrical and asymmetrical configurations of a 2? = 3 (111) twin boundary in Cu3Au are studied by means of energy minimization and Monte Carlo simulations of a rigid lattice, using an adapted n-body potential. We find that relaxation occurring in the neighborhood of the boundary is larger for asymmetrical than for the symmetrical minimum energy configurations. From energy minimization the latter are also shown to correspond to the lowest excess energy. For the asymmetrical twin structure, Monte Carlo calculations of the long-range order parameter on a local basis show the existence of prewetting effects, i.e., the appearance well below the critical temperature of several partially disordered layers at each side of the twin boundary. In contrast, the order is preserved in the symmetrical twin in the same temperature range. These results are consistent with experimental observations of wetting in 2? = 3 (111) twins in Cu3Au, and it is found to occur at lower temperature in asymmetrical than in symmetrical boundaries.

The thermal dependence of the relaxation, order and segregation in the vicinity of a = 5 (210) [001] tilt grain boundary in the Cu 3 Au L1 2 binary alloy is investigated by means of computer simulation with an empirical N-body potential. Energy minimization is performed in order to estimate the particularly strong relaxation effects in the vicinity of the boundary plane at 0 K. Monte Carlo simulations are carried out for constant chemical potential, number of particles, volume and temperature in order to study the thermal properties of the system. Detail is provided plane by plane, parallel to the boundary, which characterizes the temperature dependencies of the order, segregation, sublattice occupancy and relaxation. The vicinity of the boundary remains strongly affected by atomic relaxation and segregation at all temperatures, although no simple relation between relaxation and segregation is found. The evolution of longrange order and sublattice occupancy are strikingly different in the close vicinity of the boundary plane to those in the bulk. The boundary plane is fully disordered at all temperatures between 0.2 T c and 1.5 T c , where T c is the bulk temperature for the order-disorder phase transition. The transition to bulk properties with distance from the boundary is characterized quantitatively.The influence of the potential model is emphasized by means of a comparison between the results obtained with two somewhat different N-body potentials of similar nature.

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