The phase transitions and the annealing processes simulated by the mol
ecular-dynamics technique for KSCN and YBCO crystals are presented. Th
e KSCN and YBCO model crystals show co-elastic and ferroelastic phase
transitions, respectively. On quenching from disordered phases and sub
sequent annealing, tweed microstructures are obtained, which evolve to
strip phases with well defined domain walls. Ln KSCN the antiphase bo
undaries are arbitrarily oriented. A number of dielectric and ferroela
stic crystals possess incommensurately modulated phases, which could b
e seen as a sequence of discommensuration planes. The same simulation
technique has been used to study the nucleation of topological defects
responsible for creation of discommensurations. It is proved that the
elastic domain walls favour nucleation of discommensuration planes.