Nature of the Grain Boundaries

Before discussing the possible accommodation processes controlling superplasticity, it is important first to discuss how the nature of the grain boundaries can influence superplastic behavior. 

It is well known that grain boundaries are sinks/sources of point defects, and that the addition of secondary phases, which normally are used as sintering aids, are distributed along the grain boundaries and triple-point junctions of the grains. Consequently, the nature of the grain boundary may either enhance or retard the diffusion coefficients of the process controlling superplasticity. 

Many investigations have been conducted on the influence of grain boundary segregation on superplasticity in ceramics. Indeed, it has been shown successively that the yttrium in YTZP polycrystals segregates at the grain boundaries, and that this segregation was the possible cause of the threshold stress (Oq). It also helped to explain (quantitatively) the dependence of Oq with temperature and grain size [23]. A detailed analysis is provided later in the chapter. 

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Another type of nickel alloy with which problems of intergranular corrosion may be encountered is that based on Ni-Cr-Mo containing about 15% Cr and 15% Mo. In this type of alloy the nature of the grain boundary precipitation responsible for the phenomenon is more complex than in Ni-Cr-Fe alloys, and the precipitates that may form during unfavourable heat treatment are not confined to carbides but include at least one inter-metallic phase in addition. The phenomenon has been extensively studied in recent years . The grain boundary precipitates responsible are molybdenum-rich M C carbide and non-stoichiometric intermetallic ix... 

The nature of the grain boundaries when segregation or glassy phases exist... 

It is also established that water is able to enter a vitreous phase and, acting as a glass network modifier , enhance ionic mobilities. There is no doubt that the nature of the grain boundary phase, coupled with the presence of water vapour, influences the diffusion of silver from electrodes and terminations in multilayer capacitors leading to eventual inter-electrode shorting and component failure [9]. 

Y NMR has also been reported extensively from yttrium sialon ceramics. Yttria is an important sintering aid for densifying silicon nitride. The properties of the sialon produced are determined by the nature of the grain boundary phase, which is usually a crystalline or glassy yttrium sialon. The often disordered nature of such phases can mean that broader 89Y resonances are... 

We have discussed the packing of ions in terms of coordination polyhedra. When we create defects in a crystal we can create new polyhedra that are not found in the perfect crystal. Pauling s rules were developed for perfect crystals, but the principles still apply when we examine defects. One complication is that as we introduce grain boundaries, for example, new sites are produced that depend on the detailed nature of the grain boundary. Amorphous materials present a new challenge when describing point defects. Two amorphous materials can have different structures that depend on the processing history even if the chemistry is the same. 

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