SIGMA = 99 AND SIGMA = 41 GRAIN-BOUNDARIES

The structure and crystallography of two special high angle grain boundaries which have been observed in spinel are discussed in terms of coincident site lattice concepts. The two grain boundaries correspond to relative high Σ values; the value of Σ=99 is higher than that which is usually considered to be significant for grain boundaries. It is proposed that it is not the actual value of Σ which is important for these interfaces but rather the fact that the misorientations which are responsible for producing the grain boundaries cause several pairs of low index crystal lattice planes to be nearly parallel to one another at the interface. The relationship between this interpretation and the frequent observation of both low Σ interfaces and asymmetric grain boundaries in this and other materials is emphasized. The material examined was a polycrystalline magnesium aluminate spinel which had been prepared by hot pressing a powder at 1800°C in a graphite die.