Solid-solid and solid-liquid phase equilibrium in the Ni-Ti system is studied within the framework of the cluster variation method (CVM). The energy parameters entering the free-energy description of each phase are determined by tight-binding energy calculations using the cluster Bethe lattice method (CBLM). For simplicity, the pair interactions are restricted to first-nearest neighbours in FCC-based structures and to first-and second-nearest neighbours in BCC-based structures. The configurational entropy for disordered solutions and for ordered compounds are obtained using the tetrahedron approximation of the CVM. The three compounds NiTi, Ni3Ti and NiTi2 compounds are treated as stoichiometric compounds. The calculated diagram agrees reasonably well with that determined experimentally.

A tight-binding bond model is used for a quantitative study of electronic interactions in nickel-titanium compounds. The model is not an ab initio calculation but it requires only the data on the elemental properties of pure metal constituents that are readily available in the literature. Combined with an extended cluster Bethe lattice method (CBLM) for FCC- and BCC-based solid solutions and the B2 phase, and the recursion method for NiTi2 and Ni3Ti stoichiometric compounds, this approach allows the energy of formation of these alloys to be calculated and the chemical trends in the ordering effects to be analysed.

Abstract A new mechanism is proposed for homogeneous nucleation of crystalline phases via density fluctuations in undercooled melts. This process leads to the formation of crystalline nuclei of the critical size by a diffusionless transformation during relaxation of density fluctuations in the undercooled liquid. The proposed mechanism avoids the postulation of a distribution of energetically unfavourable sub-critical nuclei, as was required by earlier formulations.

Small-angle neutron scattering (SANS) studies have been reported in various metallic glasses /1-4/, and it appears that SANS arises from different kinds of heterogeneities, such as density or concentration fluctuations, segregation, impurities (oxide, gas) or surface states. Here we present a series of SANS experiments on as-quenched and in-situ annealed melt-spun ribbons of Ni^^Ygy and CujjY^y using the isotopie substitution method. The anisotropy of SANS and the evolution of the scattering law with temperature and surface states are more particularly discussed.

The thermodynamic properties of amorphous phases are of great interest, especially for the prediction of amorphous phase formation obtained by solid state reaction. We present microscopic calculations of thermodynamic data for amorphous NiZr, CoZr and FeZr alloys; our approach is based on a model one-band tight-binding hamiltonian and a thermodynamic variational technique founded on the Gibbs-Bogolyubov inequality and a hard-sphere Yukawa fluid as a reference system. According to our calculations, we are able to predict the composition range in which a single amorphous phase is expected.

Nous etudions par une approche simple en structure de bande electronique l'adhesion thermodynamique a l'interface entre un metal normal (electrons sp) liquide et un oxyde ionocobalent en nous limitant au cas ou le metal ne reagit pas avec l'oxyde. Nous calculons le travail d'adhesion W ad en supposant que la liaison interfaciale resulte d'un transfert d'electrons du metal vers la bande de conduction initialement vide de l'oxyde. Le travail d'adhesion peut alors se decomposer en deux contributions: l'une negative, provient de la perte en energie cinetique du gaz d'electrons presques libres cote metal; la deuxieme, positive, resulte de la creation d'etats electroniques liants a partir de la bande de conduction de l'oxyde. Nous avons applique ce calcul a une serie de metaux sp (groupes IIB, IIIA, IVA et VA du tableau periodique) sur l'alumine. L'ordre relatif des valeurs experimentales de W ad pour les differents couples metal-alumine est correctement prevu par le modele

The three Faber-Ziman partial structure factors as well as the Bhatia-Thornton (1970) structure factors of the Ni33Y67 and Cu33Y67 metallic glasses have been evaluated by neutron diffraction using the isotopic substitution method and X-ray diffraction. The chemical ordering is described by a function alpha (R) related to the partial coordination numbers zij(R). The relevant CSRO parameter alpha 1 in the first coordination shell is chosen equal to the value of alpha (R) at the position of the first minimum of GNN(R). Thus, the parameters alpha 1 show a strong chemical ordering in Ni33Y67 and, in contrast, a tendency towards random mixing of both constituents in Cu33Y67. The evaluation of alpha 1 by the same method in several NixMt1-x glasses (x approximately=0.33, Mt=Y, Ti, Zr, Dy) gives evidence of a relation between the strength of the bonding and the nature of the d transition element alloyed with Ni.

The heats of formation of disordered transition metal alloys are related to the moments of the density of states within an analytical tight-binding model for the d band. We show that the values of the bandwidths of the elements and the difference between the energy levels are two important parameters in the evolution of the heats of formation of these alloys. These two major effects are discussed as functions of the average number of d electrons; their influence on the charge transfer, determined in a self-consistent way, is analysed.

We present a simple model for the calculation of the partial excess entropy of a transition metal at infinite dilution in a liquid polyvalent metal. We show that the large negative values observed experimentally arise from the electronic and packing effects.

We present a first-principles study of chemical short-range ordering in liquid (s,p)-bonded alloys. Our approach is based on an optimized pseudopotential technique for the construction of the interatomic potentials and a thermodynamic variational technique based on the Gibbs-Bogoliubov inequality and hard-sphere Yukawa reference potentials (we use the analytical solution of the mean spherical solution for the equal-diameter case). The analysis of the redistribution of the valence electrons upon alloying allows us to elucidate the electronic origin of the ordering potential. In the case of a moderately strong ordering interaction, the application of the Gibbs-Bogoliubov variational technique yields a reasonably accurate prediction of the structure factors and of the thermodynamic excess functions. For very strong ordering potentials, a free minimization of the variational upper bound to the exact free energy gives unrealistic results. This is a consequence of the complete decoupling of number-density and concentration fluctuations in the mean-spherical approximation to the equal-diameter hard-sphere Yukawa mixture. We find that realistic solutions may be found by imposing the condition that the exact and the reference-system ordering potentials be the same at the mean effective atomic diameter. This constrained minimization of the variational free energy yields good results for the structure factors, but rather bad ones for the thermodynamic excess functions. We are able to show that this is due to a neglect of the finite electronic mean free path of the electrons in those concentration regions where it is comparable to the mean interatomic distance.

The strong chemical interactions, that occur in some alloy systems as a function of composition, have been investigated using band-structure concept. The occurence of a gap or pseudogap in the density of states seems to be one of the major effects for the determination of critical composition.

The authors present a simple model for the calculation of the entropy of mixing of liquid transition-metal-based alloys with strong chemical interactions. They show that the thermodynamic 'anomalies' observed in these systems arise from the interplay of electronic and packing effects.

The partial structure factors of Ni 33 Y 67 and CU 33 Y 67 glasses were evaluated by neutron diffraction using the isotopic method and X-ray diffraction. The main difference between both systems results from the arrangements of NiNi and CuCu atomic pairs, which are similar respectively to those in the stable 3 Ni> (or 3 N 2 >) and crystalline compounds.

We present simple band-models able to calculate the thermodynamic data for transition metal based alloys in a tight-binding approach. In the case of d metal-p metal alloys the highly negative values of thermodynamic data have been explained from the filling of the d-band of the transition metal by the sp electrons of p metal. For binary transitional alloys, the heats of formation are studied as a function of differences between elemental bandwidths and atomic energy levels.

A simple electron band theory model of the enthalpy of formation ΔH of transition metal alloys is used to predict ΔH for 210 transition metal alloys as a function of composition. Some of the input parameters of the model namely bandwidths, atomic energy levels are allowed to vary within certain constraints to closely approximate any known value of ΔH. For this purpose experimental values of the enthalpy of formation are listed for binary transition metals alloys. The resulting predictions of the model are compared with the experimental data as well as with the calculated values from various models.