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 mum grain size containing 30 vol.% of 0.33 mum 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-AI and other dispersion-strengthened aluminum materials. Two separate creep regimes were observed in DSC-Mg, at low stresses (sigma < 30 MPa), both the apparent stress exponent (n(app) <approximate to> 2) and the apparent activation energy (Q(app) approximate to 48 kJ mol(-1)) are low, while at high stresses (sigma > 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. (C) 2001 Elsevier Science B.V. All rights reserved.