STABILITY OF THE AMORPHOUS INTERLAYERS DURING S.S.A.R. AGAINST HOMOGENEOUS AND HETEROGENEOUS NUCLEATION

A partir d'arguments thermodynamiques simples, nous démontrons que les forts gradients de concentration ∇C présents dans les multicouches facilitent l'amorphisation par reaction en phase solide en réduisant ou supprimant le moteur thermodynamique ƊGac de la cristallisation. Lorsque ces gradients baissent avec la croissance de la couche amorphe, le moteur pour la cristallisation est rétabli ; ce qui donne une épaisseur critique maximum pour la couche amorphe comme on observe expérimentalement. Les mécanismes de nucléation homogène et hétérogène sont considérés, l'effet mis en évidence croît avec la courbure dans l'espace concentration de l'énergie libre ƊG de la formation de l'alliage amorphe. Dans le cas de la germination hétérogène de composés de concentration ayant un écart ƊC* avec celle de l'amorphe à l'interface site de germination, nous traçons la région dans l'espace (ƊC, ∇C) dans laquelle le moteur thermodynamique pour la cristallisation est nul. Le développement est applique au système modèle NiZr.Using simple thermodynamic arguments, it is shown that sharp concentration gradients ∇C occuring during diffusive mixing in multilayers facilitate amorphisation by solid state reaction by reducing or suppressing the driving force for crystallisation ƊGac. As the concentration gradients diminish during the amorphous layer growth, the driving force for crystallisation is restored. This evolution yields a critical maximum amorphous layer thickness as observed experimentally. Both homogeneous and heterogeneous nucleation of compounds are considered. It is shown that the effect increases with the second concentration derivation of the Gibbs free energy ƊG of formation of the amorphous phase. For heterogeneous nucleation of a compound of nominal concentration differing by ƊC* for that of the amorphous phase at interface serving as nucleation site, the contours of the domain in (ƊC, ∇C*) space where no driving force for nucleation is available have been derived out and applied to the Ni-Zr system

NEUTRON DIFFRACTION BY LIQUID SEGREGATED COPPER-LEAD ALLOYS

Neutron scattering experiments were performed on liquid Cu-Pb alloys above the miscibility gap. Two different enrichments in Cu65 were used in order to extract two of the three BHATIA-THORNTON pertial structure factors SNN and SNC. The variations of SNN and SNC with atomic concentration (xcu = 0.50, 0.64 and 0.80) and deporture from the demixtion temperature (950 < t < 1200°C) were determined and attributed to size effects

A thermodynamic study of liquid Fe-Si-B alloys : an influence of ternary associates on a liquid → glass transition

Knudsen-cell mass spectrometry and the integral variant of effusion method have been applied to investigate the thermodynamic properties of liquid Fe-Si-B alloys at temperatures from 1423 to 1894 K over concentration intervals of 9-88.8 at. pct Fe. 2.6-81.4 at. pct Si, and 7.5-50 at. pct B. The activity values of the components and the Gibbs energy of the Fe-Si-B melt formation have been determined over a wide temperature-concentration range. The concentration and temperature dependences of the thermodynamic functions of liquid Fe-Si-B alloy have been described adequately within the ideal associated-solution model under the assumption that binary and ternary associates exist in the melt. The relevance of the model was tested in the undercooled region. The glass-forming ability of the melt as shown could be interpreted in terms of the thermodynamic parameters of the association reactions. A specific role of ternary interaction was clarified