Kinetics melt coupling reactions

The melting point of titanium is 1670°C, while that of aluminium is 660°C.142 In kelvins, these are 1943 K and 933 K, respectively. Thus, the temperature 625°C (898 K) amounts to 0.46 7melting of titanium and 0.96 melting of aluminium. Hence, at this temperature the aluminium atoms may be expected to be much more mobile in the crystal lattices of the titanium aluminides than the titanium atoms. This appears to be the case even with the Ti3Al intermetallic compound. The duplex structure of the Ti3Al layer in the Ti-TiAl diffusion couple (see Fig. 5.13 in Ref. 66) provides evidence that aluminium is the main diffusant. Otherwise, its microstructure would be homogeneous. This point will be explained in more detail in the next chapter devoted to the consideration of growth kinetics of the same compound layer in various reaction couples of a multiphase binary system. 

The melting points of the components of a reaction couple are most frequently different. Therefore, there is a certain range of temperature, in which one of the components is in the solid state, while the other in the liquid state. If soluble, the solid substance will dissolve in the liquid phase. The dissolution process should clearly affect the growth kinetics of a chemical compound layer at the solid-liquid interface. 

The transformation of the crystalline into the glassy state by solid-state reactions is extensively reviewed in its theoretical and experimental aspects. First, we give some historical background and describe the thermodynamics of metastable phase formations, adding as well the kinetic requirements for the amorphization process. Then we discuss the different experimental routes into the amorphous state hydriding, thin diffusion couples, and other driven systems. In the discussion and the summary, we close the gap between the melting phenomena and the amorphization and provide a tentative outlook. 

In an A/E polymer blend, the reactive additive can react with both polymers as a coupling agent. The advantages of this approach are the availability of many cheap reactive products and also the capability to diffuse in the melt as they possess a low molecular weight. The latter property makes the reaction kinetics suitable for extrusion. On the other hand, the disadvantage of a specific reactivity of the additives can lead to side reactions as the undesired polymer branching. 

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