Parabolic intermetallics

Consider the results obtained by E.M. Tanguep Njiokep et al. with Al-Mg couples as an example of diffusional parabolic growth kinetics. Consideration will mainly be restricted to a temperature of 400°C at which two intermetallic compounds, Al3Mg2 (also designated as Al8Mg5) and Al12Mg17, are known to exist and which is well below both eutectic temperatures.142 193207... 

The layer-growth kinetics were found to be parabolic for both compounds (Fig. 2.18), indicative of diffusion control. This is an expectable result since the layer thickness varied from about 10 pm to 300 pm for the Al12Mg17 intermetallic compound and from about 80 pm to more than 900 pm for the Al3Mg2 intermetallic compound. Diffusional constants were calculated using parabolic equations of the type x2 = 2k t. The temperature dependence of the diffusional constants was found to obey the Arrhenius relation ... 

This binary system is worth further investigation, especially in the region of non-parabolic layer-growth kinetics. Marker experiments are also desirable, with inert markers embedded in both intermetallic layers. 

Note that even in those cases where multiple compound layers were present at the A-B interface, two layers were dominating. For example, G. Hillmann and W. Hofmann and O. Taguchi et al. observed the formation of all six intermetallics shown on the equilibrium phase diagram in the reaction zone between zirconium and copper, with two Cu-rich compounds occupying more than 90 % of the total layer thickness and layer-growth kinetics deviating from a parabolic law. When investigating... 

Diffusion processes taking place in such systems have to be regarded as processes that take place in thin Aims, and not in bulk samples. Reactive diffusion in thin Aims differs from that in the bulk case IMCs are formed in sequence ( one by one ), so that the phase spectrum of the diffusion zone differs from the full list of stable intermediate phases. Besides, the growth of new phase layers very often demonstrates a linear time dependence rather than a parabolic one. The best known theory providing an explanation for this fact was offered by Gosele and Tu [9]. However, it was shown that the linear stage of intermetallic layer growth may also be caused by a finite relaxation rate of nonequilibrium vacancies at these interfaces [lOj. 

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