Twinning as a possible mechanism of stress relaxation accompanying the formation of refractory crystalline oxides: Polycrystalline ceramics and single crystals

Abstract

Recent results of studies dealing with the formation of polycrystalline laser ceramics based on cubic oxides such as yttrium aluminum garnet (Y 3 Al 5 O 12 ) [1‐3] have demonstrated that a plausible mechanism of the plastic strain giving rise to crystal lattice rotations and to the matching of grains can be related to mechanical twinning along the (111) plane in the [112] direction. It would be natural to suggest that such a mechanism of plastic strain can also manifest itself in the growth from the melt of bulk Y 3 Al 5 O 12 single crystals. This suggestion provoked study of the problem that we solve in this paper. In the processes of crystal growth, difficulties are mainly related to inhomogeneities at the crystallization front, which lead to defects in the crystal structure. The stresses created by these defects can stop the crystal growth. Therefore, the main issue that should be addressed for the successful growth of crystals, especially of large ones, is ensuring the timely relaxation of arising stresses. An optimum mechanism of such relaxation could be related to twinning, which provides a symmetrical reorientation of some domains in the crystal lattice or of the crystal as whole. According to book [4], the twinning arising under the effect of mechanical forces acting upon the crystal does not always change the shape of the distorted samples. The mechanical stresses can also give rise to another type of twinning when the lattice transformation does not result in changes in the shape of the crystal. The twinning at the fixed shape is accompanied by changes in the elastic constants of the distorted samples, which are related to the reorientations in the crystal lattice. However, there exists also the third type of twinning, which does not require any changes either in the shape changes or in elastic constants. All inversion twins and some kinds of rotation and mirror reflection twins belong to such a type [5]. In general, it is rather difficult to observe these twins in crystals. To reveal them, one needs to study thoroughly the structure and properties of the crystal, that is, to determine the structural features allowing the formation of such twinning types and to find out their possible manifestations in the properties of the samples.