Grain boundaries bonding

An alternative, but to some extent complementaty approach to the structure of grain boundaries notes that as the tilt angle between the crystals forming the grain boundary increases, planes of lower atomic concentrations, the high index planes, such as (221), (331) and (115) in the face-centred strucmre, become parallel to the grain boundary. There is therefore a decrease in the number of metal-metal bonds at the boundary as the tilt angle increases. 

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. 

The crystals, or grains, in a polycrystal fit together exactly but their crystal orientations differ (Fig. 10.4). Where they meet, at grain boundaries, the crystal structure is disturbed, but the atomic bonds across the boundary are numerous and strong enough that the boundaries do not usually weaken the material. 

First, I shall describe the hydrogenation method I used and then consider the passivation of surface states and that of bulk dangling bonds, including grain boundaries, dislocations and point defects. 

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