The effect of strain rate and temperature on the yield point of hydrolytically weakened synthetic quartz

The 5-kbar 500°C CO2 apparatus developed by Griggs and Heard in the late 1950's has been renovated and adapted for deformation of single-crystal samples of synthetic quartz. The O+ cores were taken from the seed area of crystal Xo (8515H/106Si) and deformed at 500°C at strain rates of 7 × 10−3 to 5 × 10−8 s−1 in a systematic investigation of the relationship of (upper) yield stress and strain rate. A secondary series of experiments at 6 × 10−6 s−1 examined the temperature dependence of (upper) yield stress from 250° to 500°C. The yield stress-strain rate relationship was found to follow a power flow law, = A σn. Values of A = 5.82 (±1.87) × 10−7 and n = 3.64 ± 0.18 were determined at 500°C. Yield stress and temperature showed an exponential relationship of the form σy = [α exp (Q/RT)]1/n. Average activation energy over the range studied was Q = 31.6 ± 3.5 kcal/mol. The data seemed to indicate that either Q decreased or n increased with decreasing temperature. No sharp discontinuity in the σy versus T relationship was noted in contrast with previous observations of the ‘critical weakening temperature.’ The observed temperature and strain rate relationships were consistent with the Weertman power law and the Alexander and Haasen dislocation multiplication model. Stress-strain curves obtained in this study exhibited yield point and strain-hardening characteristics consistent with observations on hydrolytically weakened quartz published by others. Deformation features showed a dependence on strain rate, temperature, and variations in OH content within the sample consistent with previous observations.