Fabric evolution during high shear strain deformation of magnesiowüstite (Mg0.8Fe0.2O)

Abstract

High‐strain deformation experiments have been performed on aggregates of magnesiowüstite to investigate textural evolution and associated changes in mechanical behavior. The experiments were performed using a torsional configuration in a gas‐medium deformation apparatus at a confining pressure of 300 MPa and temperatures of 1300 and 1400 K. Samples were deformed at constant angular velocities equivalent to shear strain rates in the range 10−4 to 10−3 s−1 and an oxygen fugacity fixed by the iron jacket. In all cases, little change in the flow stress of the samples was observed after the initial yield, even to shear strains as large as γ = 15.5. Significant changes, however, were observed in the microstructures and textures of the samples as analyzed with the scanning electron microscope. Initial grain elongation and shearing were followed at higher shear strains by grain refinement due to subgrain rotation recrystallization. The initial deformation texture, compatible with dislocation glide on all three probable slip systems ({111}, {110}, {100} in the 〈110〉 direction), is transformed into a recrystallization texture. Investigations of the dislocation microstructure in the transmission electron microscope confirm the assignment of slip systems. The change in crystallographic preferred orientation due to recrystallization is not large enough to cause a significant weakening of the sample, which is in accordance with the mechanical data and with subgrain rotation as the recrystallization mechanism. The formation of subgrains at these relatively low homologous temperatures (<0.5Tm) is possibly promoted by cross slip between the different glide planes.