The reactions of metal layers with their silicon substrates resulting in the formation of various silicides are considered generally not only as phenomena common to all diffusion couples where new phases are formed, but also as typical of all transitions from two to three phases. The conditions under which such transitions will display the same characteristics as encountered in the usual one-to-two phase transitions (condensation, crystallization, boiling) are analyzed by comparison to the classical theory of nucleation. Because of the lack of knowledge about the exact values of the relevant parameters, the discussion is carried out mostly in descriptive thermodynamic terms. Although nucleation effects are analyzed in general terms, the main focus of attention is a class of reactions where nucleation dominates the formation of a new phase; a salient feature of these reactions is the absence of any equilibrium temperature, although the nucleation temperatures are relatively well defined within narrow limits. Nucleation effects are correlated to such material characteristics as the stability of the nucleated phases, and to such kinetic characteristics as the sequence of phase formation. The modification of the energy levels of the different phases brought about by stress, ion bombardment, or the replacement of usual phases by metastable ones, are considered with respect to their effect on nucleation processes. The nearly total absence of literature references to nucleation in metal-metal diffusion couples is discussed with respect to some specific aspects of the metal-silicon reactions.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400002557

Nucleation of a new phase from the interaction of two adjacent phases: Some silicides

A revised calculation of the Ni-Nb (nickel-niobium) phase diagram includes realistic enthalpy of formation values for the intermetallic phases that compare with experimental values, a more accurate crystallographic description of the μ phase, and a better fit to the experimentally determined invariant equilibrium temperatures and liquidus-solidus boundaries between liquid and (Ni). The set of parameters describing the Gibbs energy of each phase is given.

A reassessment of the calculated Ni-Nb phase diagram

The Nb−Ni (Niobium-Nickel) system

A reassessment of the Nb–Ni system is proposed taking into account new experimental data. According to recent crystallographic work, the μ phase has been modelled using three independent sublattices of variable occupation. This has been made possible by including experimental site fractions in the different sublattices as data for parameter optimization. A consistent set of parameters is proposed. Good agreement is obtained between calculated and experimental values for all kinds of data (thermodynamic, crystallographic and phase diagram data).

Assessment of the niobium-nickel system

The Nb–Ni system is reassessed on the basis of a critical literature review involving recent experimental data. These newly published experimental data include the phase relation associated with the NbNi8 phase, phase transition temperatures resulting from selected alloys, all invariant reaction temperatures, and enthalpies of mixing of liquid, as well as the crystallographic data on the μ (Nb7Ni6) phase. A consistent thermodynamic data set for the Nb–Ni system is obtained by optimization of the selected experimental values. The calculated phase diagram, crystallographic properties and thermodynamic properties agree reasonably with the experimental data. Noticeable improvements have been made, compared with the previous thermodynamic optimizations.

Yuji Muramatsu

Papers

Influence de la Pression sur la Croissance des Couches des Composés Intermétalliques dans les Couples de Diffusion Niobium/Nickel