[SCG55-06] Rheology of bcc-iron at high-pressure and -temperature
Keywords:inner core, bcc-iron, rheology
Uniaxial deformation experiments were carried out using a deformation-DIA apparatus SPEED-MkII-D installed on a beamline BL04B1 at SPring-8. Using polycrystalline iron aggregate as a starting material, deformation experiments were carried out at pressure of 2.0-6.0 GPa, temperature of 523-823 K, and strain rate of 0.8–5.7×10–5 s–1. Stress and strain during deformation were determined based on two-dimensional X-ray diffraction and X-ray radiography, respectively, using monochromatized synchrotron X-ray with energy of 60 keV.
A preliminary analysis of the derived data suggests that power-law dislocation creep with stress exponent of ~5 and activation volume of 2-3 cm3/mol is dominant. Extrapolation of the data to ambient pressure is generally consistent with the well-established ambient pressure rheology of this material (Frost and Ashby, 1982). When compared with rheology of hcp-iron at same deformation condition, stress value of bcc-iron is lower by ~1 order of magnitude. This means that viscosity of bcc-iron is lower by ~5 orders of magnitude than that of hcp-iron because stress exponent for the both iron polymorphs is considered to be ~5. If the inner core mostly consists of bcc-iron, the viscosity in the inner core is estimated to be significantly lower than the previous estimates where hcp-iron was assumed to be the dominant inner constituent.