Showing papers by "Fabrizio Cleri published in 1993"

Tight-binding potentials for transition metals and alloys.

Abstract: The parameters of many-body potentials for fcc and hcp transition metals, based on the second-moment approximation of a tight-binding Hamiltonian, have been systematically evaluated. The potential scheme, cast in analytical form, allows us to reproduce correctly the thermal behavior of transition metals making use of a small set of adjustable parameters. The large cutoff, which extends the range of the interactions up to the fifth-neighbor distance, ensures good quantitative agreement with the experimental data up to temperatures close to the melting point. The ability of the potentials to describe real systems has been checked by calculating point-defect properties, lattice dynamics, and finite-temperature behavior, and by comparing the results with other potential schemes. Application of this scheme to bcc transition metals has proved unsuccessful. Examples of derivation of many-body potentials for a few transition-metal alloys with cubic structure are also reported.

Experimental and theoretical investigation of the order-disorder transformation in Ni3Al

Abstract: The crystalline disordered phase obtained by mechanical alloying of elemental 75 at. % Ni and 25 at. % Al powders has been investigated. The stability of this phase with respect to the thermal reordering process leading to the L1 2 structure has been analyzed by means of x-ray diffractometry, scanning electron microscopy, and differential scanning calorimetry. Atomistic simulations on an Ni 3 Al model, reproduced via molecular dynamics using a many-body potential, have been used to interpret experimental data. The ordering transformation takes place in an extended range of temperature (from 320 to 600 °C) and occurs simultaneously with the release of internal strain. Numerical simulations performed under different conditions show that the activation energy of the Ni-vacancy migration mechanism responsible for the ordering process depends on the local state of strain, thus suggesting an explanation for the considerable lowering of this energy in samples obtained by ball milling.

Order-disorder transition in Cu3Au : a combined molecular-dynamics and cluster-variation-method approach

Abstract: The order-disorder transition in ${\mathrm{Cu}}_{3}$Au has been investigated using a combined molecular-dynamics and cluster-variation-method approach. These techniques allow an accurate estimate of, respectively, the configuration-dependent enthalpy and the configurational entropy. Vibrational entropy has also been included in the calculations. The transition temperature is accurately reproduced. The behavior of long-range and short-range order parameters is consistent with the hypothesis that the disordering process occurs via the creation of out-of-step domains.

Lattice dynamics of ordered and disordered Cu3 Au with a tight-binding potential model

Abstract: The phonon dispersion curves for both ordered and configurationally disordered cubic phases of Cu3Au are calculated using force constants derived from a many-body tight-binding potential. The derived phonon frequency spectra are used to estimate the vibrational contribution to the entropy difference between the two phases. A brief discussion concerning the relative values of interatomic force constants is included.

Phase stability of Ni-Al solid solutions

Abstract: We have studied the stability of Ni(1−x), -Alx solid solutions at different stoichiometries (x = 0.25 and x = 0.5) via molecular dynamics simulations of models based on an n-body potential. The face-centred-cubic solid solution with x = 0.25 is stable at room temperature against the ordering transformation which occurs at T=360°C via the activation of vacancy diffusion with a release of a disordering enthalpy of 8 kJ(g-atom)−1. The results obtained from the simulated model show an intrinsic instability of the x = 0.5 solid solution which is consistent with experimental results that have revealed the direct formation of NiAl (B2) intermetallic phase accompanied by a considerable heat release.