Applications of HRTEM in Materials Science Problems and Dislocation Simulations

Based on Pirouz's twinning model in Si [ 46] which has been successfully applied to other materials (35,24) as well, polytypic transformations in SiC can be explained (47, 51, 50). In order to get evidence for this model a study of dislocation core structures using HRTEM is performed. As a preliminary study, the glide and shuffle question in Ge is investigated. From experimental HRTEM, and image simulations including image processing, it is shown that perfect 60^circ dislocations in Ge occur predominantly in the shuffle type configuration. Simulations show that the core structure of 30 ^circ glide partials in SiC cannot be characterized using HRTEM. But 90^ circ partials can be characterized. Pirouz's cross slip mechanism has also been applied to basal twinning in sapphire (alpha - Al _2O_3) [ 49]. The HRTEM investigation of twins in Al_2O_3 reveal that basal twins are type II twins with a stacking sequence of ... ABCBAC ... for the Al sublattice which is in compliance with the proposed mechanism of formation and that rhombohedral twins are type II glide twins with a 1/6< 0111> shear. A zonal dislocation mechanism for the formation of rhombohedral twins is proposed which explains the structure of rhombohedral twins as well as the shear. The latter involves only very small shuffle motions of the anions and cations and also correctly predicts the size of rhombohedral stacking fault with local twin symmetry. Simulations of dislocation multiplication by the Frank-Read source for a 60^circ dislocation in Si reveal that the Orowan stress, i.e. the maximum line tension stress, is temperature sensitive. For an applied stress slightly higher than the Orowan stress, the back stress due to a small number of already existing loops upon a nascent dislocation (the back stress is much smaller than the line tension stress) brings the operation of the Frank-Read source to a stop. So the number of loops which stop the operation of a Frank-Read source is highly stress and temperature sensitive. The multiplication simulation may be extended to partial dislocations in polytypes which cross slip when the Frank-Read source stops due to the back stress of already existing loops. From this it may be concluded that polytypic phase transformations are also highly temperature and stress sensitive.