The temperature variation of hardness of olivine and its implication for polycrystalline yield stress

Abstract

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 )/T][1 ‐ (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.