Evolution of c axis pole figures and grain size during dynamic recrystallization: Results from experimentally sheared quartzite

Quartzite samples experimentally sheared at conditions where subgrain rotation and grain boundary migration recrystallization are active show a striking change in the c axis crystallographic preferred orientation (CPO) with increasing shear strain (up to γ = 8) and degree of recrystallization (up to 100%). We used optical methods to determine the c axis CPO for the bulk samples and for porphyroclasts and recrystallized grains separately and to track the strengths of the CPOs in five different orientation domains with increasing γ. The c axis pole figure evolves from a broad peripheral maximum indicative of basal <a> slip, to an inclined single girdle with two maxima indicative of rhomb <a> slip, and finally an elongate single maximum at the girdle center indicative of prism <a> slip; throughout this sequence the fabric skeleton rotates with the sense of shear. The bulk preferred c axis orientation results from growth and shrinkage of differently oriented crystallographic domains. Grain boundary density analysis indicates that the size of prism <a> recrystallized grains is 1.5 times that of average and 2 times that of rhomb grains, indicating that prism <a> slip is significantly easier at our experimental conditions. This inference is supported by autocorrelation function analysis of the various orientation domains and how they evolve with strain. Similar progressive changes in c axis CPO are observed in quartzites naturally sheared at conditions where grain boundary migration recrystallization operates. These results indicate that achievement of microstructural and mechanical steady state requires high strain and complete recrystallization.