Yield strength and strain hardening of MgO up to 8 GPa measured in the deformation-DIA with monochromatic X-ray diffraction

Cylindrical polycrystalline MgO samples have been deformed with total axial strains up to 35% at pressures up to 8 GPa. Hysteresis loops between elastic lattice strain and total axial strain were established by repeatedly shortening and lengthening the samples at selected strain rates. The yield point was identified as the first deviation from linearity in the lattice strain versus total strain curves. MgO samples yielded at low total strains (< 1%) and the yield strength (ca. 0.5 GPa) was insensitive to pressure. Beyond the yield point, MgO showed a strong strain-hardening behavior, characterized by an initial rapid increase in flow stress, followed by a linear stage with a constant hardening parameter. At pressures below 5 GPa, lattice strains associated with reflections 200, 220, and 222 diverged with increasing total strain. Above 5 GPa, these lattice strains converged, suggesting a possible change in flow mechanism. (C) 2004 Elsevier B.V. All rights reserved.