MEMORY EFFECTS OF DOMAIN-STRUCTURES DURING DISPLACIVE PHASE-TRANSITIONS - A HIGH-TEMPERATURE TEM STUDY OF QUARTZ AND ANORTHITE

Memory effects associated with the Dauphine twins in alpha quartz and the c-antiphase domains (c-APDs) in P (1) over bar anorthite have been investigated by in situ hot-stage transmission electron microscopy (T EM). In a set of kinetic experiments, specimens were cycled about thei r transition temperatures, and changes in the Dauphine twin and c-APD positions were analyzed as a function of maximum annealing temperature and annealing time. The results indicate that Dauphine twins are stro ngly pinned by extended defects, such as Brazil twin and grain boundar ies, dislocations, and surfaces. However, the memory displayed by Daup hine twin boundaries pinned by point defects degrades with higher anne aling temperatures and longer annealing times. An Arrhenius analysis o f this behavior yielded an average activation energy for point-defect diffusion of 68.6 kJ/mol. In contrast to the Dauphine twins of quartz, the c-APDs of anorthite did not appear to interact strongly with exte nded defects, and they exhibited a nearly perfect memory for all annea ling times and temperatures tested. This extremely high fidelity is in terpreted as evidence that the positions of c-APDs are fixed by locali zed Al-Si disorder, which remains unchanged when anorthite is heat tre ated at <1000 degrees C.