Amorphous eutectic-type

In the ribbon with x=5 the amorphous structural component was absent, X-ray study registered the existence of a-Al together with Al4Ce and Al3Sc crystalline intermetallics (Fig. 4c). However, ribbon hardness remained at almost the same level (Fig.7). Evidently, it can be explained by the formation of a very fine-scale structure of a eutectic type with a relatively small amount of a-Al grains (Fig. 6b). Small plates of both intermetallics of 20-60 nm in thickness were found in the eutectic by dark-field TEM investigation. A further increase of x caused a drop of hardness (Fig. 7), and in the ribbon Al91Ce2Sc7 no areas with eutectic structure were found (Fig. 

It is instructive to consider the free-energy hierarchy and the metastable phase equilibria when crystallization of an amorphous material is discussed. Koster and Herold [56] discussed these aspects of crystallization and showed that crystallization reactions of amorphous alloys can be classified into the following three types polymorphic, primary and eutectic crystallization reactions. Among these three types, the slowest crystal growth process is expected for primary crystallization and thus, primary crystallization is ideal for tailoring fine microstructures upon decomposition of amorphous alloys. 

Chiou and Riegelman classified solid dispersions intothe following six representative types 1) simple eutectic mixtures 2) solid solutions 3) glass solutions and glass suspensions 4) amorphous precipitations in a crystalline carrier 5) compound or complex formation and 6) combinations of the previous five types. 

This type of solid dispersion is distinguished from a simple eutectic mixture by the fact that the drug is precipitated out in an amorphous form. In a simple eutectic mixture, the drug is precipitated out in a crystalline form. An example of this is the precipitation of sulfathiazole in the amorphous form in crystalline urea. It is postulated that a drug with a propensity to supercooling has more tendency to solidify as an amorphous form in the presence of a carrier. 

Three types of crystallization reactions that may occur during devitrification can be classified, depending on their chemical compositions (Koster Herold, 1981 Lu, 1996) polymorphous, eutectic and primary crystallization. Fig. 2 shows a hypothetical free energy diagram to illustrate the crystallization reactions during crystallization. This schematic is essentially a representation of the variation of free energy with the chemical compositions of the amorphous phase and various crystalline phases (in this case, two crystalline phases, a solid solution a and a compound p, are included) at a chosen annealing temperature. 

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