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The variation of hardness with temperature was measured for olivine on a number of crystal faces by the Vickers diamond pyramid technique (up to 800°C) and by a mutual indentation technique (for temperatures up to 1500°C). A comparative review of hardness data and compressive creep measurements obtained under large confining pressures confirms the hypothesis of Rice 1971 that single‐crystal hardness measurements, corrected for elastic effects, can be correlated to the fully ductile yielding of a polycrystal by intragranular dislocation mechanisms, including dislocation climb and glide. The computed differential yield stresses, σ (in gigapascals), which empirically correspond to a strain rate of 10 −5 s −1, were well represented by an equation of the form σ = 9. 1 (±0. 3) ‐ 0. 23 (±0. 01) T2, where T is the absolute temperature (in degrees Kelvin), and the quoted variances are for 1 standard deviation. The olivine data therefore predict a high‐stress polycrystalline flow law that may be expressed as = 1. 3 × 10 12 exp ‐ (60×10 3) /T1 ‐ (σ/9. 1) 2 where is the strain rate in s −1. A similar functional dependence of strain rate on stress is indicated for Al 2 O 3 for temperatures below 900°C but is contraindicated for MgO and NaCl. Using a semiempirical method of dislocation rosette analysis, the critical resolved shear stress on the 110 001 slip system was estimated (to 20%) over the temperature range 20°C to 780°C as 1. 2 GPa and 0. 3 GPa, respectively. These data are useful in providing an upper bound to the yield stress in a region of stress and temperature space not easily accessible by other experimental methods.
Evans et al. (Mon,) studied this question.