Steady-State Microstructures of Quartz Revisited: Evaluation of Stress States in Deformation Experiments Using a Solid-Medium Apparatus

Dynamically recrystallizing quartz is believed to approach a steady-state microstructure, which reflects flow stress in dislocation creep. In a classic experimental study performed by Masuda and Fujimura in 1981 using a solid-medium deformation apparatus, two types of steady-state microstructures of quartz, denoted as S and P, were found under varying temperature and strain rate conditions. However, the differential stresses did not systematically change with the deformation conditions, and unexpectedly high flow stresses (over 700 MPa) were recorded on some experimental runs compared with the applied confining pressure (400 MPa). Internal friction in the sample assembly is a possible cause of reported high differential stresses. Using a pyrophyllite assembly similar to that used in the previous work and setting up paired load cells above and below the sample assembly, we quantified the frictional stress acting on the sample and corrected the axial stress. The internal friction changed in a complicated manner during pressurization, heating, and axial deformation at a constant strain rate. Our results suggest that Masuda and Fujimura overestimated the differential stress by about 200 MPa in their 800 degrees C runs. Crystallographic fabrics in the previous experimental sample indicated that the development of elongated quartz grains, which are characteristics of Type-S microstructures, was associated with preferential growth of unfavorably oriented grains during dynamic recrystallization.

Automated summary

Dynamically recrystallizing quartz has a steady-state microstructure that reflects flow stress in dislocation creep. In a classic experimental study, two types of steady-state microstructures, S and P, were found under different temperature and strain rate conditions. However, the differential stresses did not systematically change with the deformation conditions, and unexpectedly high flow stresses were recorded in some experiments. Internal friction in the sample assembly is a possible cause of the reported high differential stresses. Further experiments suggest that the previous researchers overestimated the differential stress by about 200 MPa.