High‐Temperature Creep and Kinetic Demixing in (Co,Mg)O

The creep behavior of fine‐grained (Co 0.5 Mg 0.5 )O and (Co 0.25 Mg 0.75 )O has been characterized as part of an investigation of kinetic demixing in solid‐solution oxides due to a nonhydrostatic stress. (i) For low stresses and small grain sizes, the dominant deformation mechanism for both compositions is diffusional creep limited by the transport of oxygen along grain boundaries. The oxygen grain‐boundary diffusivity, D o b is independent of oxygen partial pressure. The values of ω D o b , where ω is the grain‐boundary width, that have been determined from the steady‐state diffusional creep rates are given by ω D o b =4.7×10 −8 exp[‐230 (kJ/mol)/ RT ] (cm 3 /s) for (Co 0.5 Mg 0.5 )O in the range 950° to 1200°C and ω D o b =7.4 × 10 −8 exp[‐263 (kJ/mol)/ RT ] (cm 3 /s) for (Co 0.25 Mg 0.75 )O in the range 1100° to 1250°C. Since oxygen diffusion controls the rate of diffusional creep, kinetic demixing is not observed in deformed samples of either composition. (ii) For high stresses and large grain sizes, the dominant deformation mechanism in both cases is dislocation‐climb‐controlled creep, where the rate of dislocation climb is controlled by oxygen lattice diffusion. Based on the positive dependence of creep rate on oxygen partial pressure, it is concluded that oxygen diffuses through the lattice by an interstitial mechanism.