Creep of Polycrystalline MgO and MgO‐Fe ₂ O ₃ Solid Solutions at High Temperatures

Polycrystalline MgO and MgO‐Fe ₂ O ₃ solid solutions (0.10 to 8.08 wt% Fe ₂ O ₃ ) were fabricated to almost theoretical density by vacuum hot‐pressing. Specimens were creep‐tested in air under four‐point dead‐load conditions between 1000° and 1400°C at stresses between 50 and 550 kg/cm ² . Steady‐state creep was never achieved in the experiments, which sometimes lasted more than 50 h. The strain rate vs time ( t ) data were described by an equation of the form = c ₁ /(t+C ₂ ) ^p , which is consistent with the assumptions that creep occurs at least in part by a “viscous” mechanism and that grain growth occurs simultaneously. Doping MgO with Fe ₂ O ₃ enhanced the viscous contributions to creep and inhibited the nonviscous ones. Creep rates in these specimens increased with increasing Fe ₂ O ₃ additions. The occurrence of simultaneous grain growth during the high‐temperature creep of magnesiowustite (i.e. MgO‐Fe ₂ O ₃ solid solutions) was used in establishing the strain rate vs grain size dependence. The results of this study are consistent with a transition between grain boundary and lattice diffusion mechanisms as the grain size increases (4 to 44 μan). The creep of polycrystalline MgO is a mixed process in that viscous and nonviscous (dislocation) contributions are present.