Grain boundary weakness

Ag-doped Cu. In all the boundaries examined by Bruley et al. (1999), Ag segregation did not lead to any observable effect on the Cu L2>3 edge, either in the as-recorded spectra or the difference spectra. Ag segregation does not embrittle Cu, and so the absence of a detectable effect is consistent with the suggestion that electronic factors are responsible for grain boundary weakness. 

Fig. 3.89 Low angle grain boundaries Weak beam DF images showing dislocation structure in SIAION a in a tilt boundary b in a twist boundary [48], With kind permission of Professor Riihle...

As the substrate temperature increases, the surface mobUity increases and the stmctural morphology first transforms to that of Zone T, ie, tightly packed fibrous grains having weak grain boundaries, and then to a full density columnar morphology corresponding to Zone 2 (see Fig. 7). 

Corrosion is a complex phenomenon tnat may take any one or more of several forms. It is usually confined to the metal surface, and this is called general corrosion. But it sometimes occurs along grain boundaries or other lines of weakness because of a difference in resistance to attack or local elec trolytic action. 

If the weak bonding model is valid for high angle grain boundaries (>20°) it follows that diffusion in amorphous preparations of a given material should also show this measure of enhanced diffusion, when compared widr die crystalline material. 

In summary, a simple chemical picture of surface recombination is presented. The surface or grain boundary recombination velocity decreases when the appropriate surface species is reacted with a strongly chemisorbed species. It increases when a species is weakly chemisorbed. We shall now illustrate this concept for six extensively studied semiconductors, Ge, Si, GaP, GaAs, InP and InSb. 

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