Grain boundary effects

There is a large literature discussing the effects of grain boundaries on plastic deformation. The essential effects for clean boundaries have just been discussed, but there are many additional effects when the boundaries are contaminated with impurities and precipitates. All this will not be discussed further here. Books that have differing viewpoints on grain boundary effects are Baker (1983), and Meyers and Chawla (1998). 

Single crystals are of critical importance to the study of complex solids such as the new high temperature superconductors. Single crystals are free of grain boundary effects and allow measurements of physical properties as a function of crystal orientation. 

Another circumstance is when deterioration becomes apparent under fields in excess of 0.5 MVm-1 at temperatures above 85 °C, and occurs more rapidly the higher the field or the temperature. The fall in resistance has been observed in single crystals of rutile and barium titanate and so must be assumed to be a bulk rather than a grain boundary effect, although there is evidence that grain boundaries play a part in degradation processes in ceramics. 

Figure 3 shows the enthalpy reduction experimentally observed in three different powders made with MgHa nanoparticles [7]. It also shows the relative importance of the three different nanostructures studied in this paper. It is clear from the data that the particle size and size distribution are not enough to predict the enthalpy of formation and that deformed regions with excess enthalpy can explain the experimental observation. Figure 3 also illustrates that surface and grain boundary effects do not reduce the enthalpy sufficiently and that the excess volume effect is needed to explain the observed reduction in the enthalpy of formation. 

Vitek, J. M., Riihle, M. (1986). Diffraction effects from internal interfaces - 1. general considerations and grain boundary effects. Acta. Metall., 34, 2085-94. 

T. Van Dijk and A.J. Burggraaf, Grain boundary effects on ionic conductivity in ceramic GdxZri-x02-x/2 solid solutions. Phys. Stat. Sol (A), 63 (1981) 229-240. 

Single crystals are the most valuable subjects for investigating the bulk properties of materials. In particular, they clearly demonstrate the conduction anisotropy (if any), and are free from grain-boundary effects. For technical utilization, however, they are usually impractical due to their unsuitable shape and size, and high cost of production. 

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