Creep of magnesium strengthened with high volume fractions of yttria dispersoids

Creep experiments were performed on dispersion-strengthened-cast magnesium (DSC-Mg), consisting of unalloyed magnesium with 1 μm grain size containing 30 vol.% of 0.33 μm yttria particles. Strain rates were measured for temperatures between 573 and 723 K at compressive stresses between 7 and 125 MPa. DSC-Mg exhibits outstanding creep strength as compared with other magnesium materials, but is less creep resistant than comparable DSC-Al and other dispersion-strengthened aluminum materials. Two separate creep regimes were observed in DSC-Mg, at low stresses (σ < 30 MPa), both the apparent stress exponent (n_app ≈ 2) and the apparent activation energy (Q_app ≈ 48 kJ mol^-1) are low, while at high stresses (σ > 34 MPa), these parameters are much higher (n_app = 9 - 15 and Q_app = 230-325 kJ mol^-1) and increase, respectively, with increasing temperature and stress. The low-stress regime can be explained by an existing model of grain-boundary sliding inhibited by dispersoids at grain-boundaries. The unexpectedly low activation energy (about half the activation energy of grain boundary diffusion in pure magnesium) is interpreted as interfacial diffusion at the Mg/Y₂O₃ interface. The high-stress regime can be described by dislocation creep with dispersion-strengthening from the interaction of the submicron particles with matrix dislocations. The origin of the threshold stress is discussed in the light of existing dislocation climb, detachment and pile-up models.