Abstrakt
Quartzite samples of high purity with a grain size of similar to 200 mu m have been experimentally deformed by coaxial shortening in a solid medium apparatus at 900 degrees C and at confining pressures ranging from 0.6 to 2 GPa. Most samples have been shortened by similar to 30% with 0.1 wt% added H2O.
The samples deformed dominantly by crystal plasticity (dislocation creep), and there is a systematic decrease of flow stress with increasing confining pressure. Strain rate stepping tests yield stress exponents of n approximate to 1.4.
The strain determined from individual grain shapes matches that determined from bulk shortening. In addition to plastic strain, mode I cracks developed in all samples, principally in the grain boundary regions.
Recrystallized material, visible through cathodoluminescence colours, forms by two mechanisms: (1) progressive subgrain rotation and (2) cracking, nucleating small new grains. After high-angle boundaries have been established, grain boundary migration takes place, and a distinction of new grains nucleation origin (subgrain rotation or cracking) is impossible.
At higher pressure, there is more recrystallized material forming in the deformed samples, and it is inferred that the inverse pressure dependence of flow stress is caused by enhanced grain boundary migration at higher pressure, consistent with previous studies.