Mechanical Behaviour and Fabric Development in Experimentally Deformed Magnesiowüstite (Mg,Fe)O as a Function of Fe-content

In a series of tests in a HT-HP deformation apparatus ('Paterson rig') we investigated the deformation behaviour of magnesiowüstite (Mg,Fe)O in torsion at 1400 K, 300 MPa confining pressure and a strain rate of 2 x 10^-3 s^-1. The Fe-content was varied from 10 to 50 at. % in steps of 10 at. %. The samples were deformed to shear strains (γ) up to 15 in order to achieve steady state microstructures and textures. The shear stress - shear strain curves showed little to no weakening after the initial yield and the strength of the samples decreased slightly with increasing Fe. Analysis with electron backscattering diffraction (EBSD) in the SEM revealed that a crystallographic preferred orientation (texture) developed in all the samples indicating that dislocation creep contributes significantly to the deformation. Deformation is accompanied in all samples by grain size reduction. At low Fe-content (low homologous temperature) the recrystallization mechanism is progressive subgrain rotation; with higher Fe content (and increasing homologous temperatures) grain boundary mobility increases resulting in a larger recrystallized grain size at comparable shear strains. The development of the crystallographic preferred orientation is characterized by the transition from a deformation texture (γ = 1-6) to a recrystallization texture at γ > 6. The recrystallization texture remains constant to the highest strains. The deformation texture at lower strains is consistent with dislocation glide predominant on the \{111\}<110> and \{100\}<110> slip systems, whereas the recrystallization texture cannot be modelled by dislocation glide alone. This texture evolution was observed in all the samples except those with 50 at. % Fe, where a deformation texture develops only at high shear strains and a recrystallization texture was not found. Preliminary results suggest that the slow formation of a crystallographic preferred orientation in these samples is possibly due to increased grain boundary mobility and grain growth.