High temperature deformation of a 5 wt.% zirconia-spinel composite: influence of a threshold stress

Creep experiments performed on a 5 wt.% zirconia- MgAl₂O₄ spinel material, in the stress and temperature ranges 8–200 MPa and 1350–1410°C, have shown the importance of grain boundaries in deformation of this material. Deformation can be analysed as the result of two sequential contributions. At low stress, an increase in the apparent stress exponent and the occurrence of a threshold stress, whose value roughly varies inversely proportional to spinel grain size, were observed. At high stress, grain boundary diffusion is the most likely mechanism that controls the grain boundary sliding. These observations are consistent with previous experiments showing that sliding of spinel/spinel boundaries is more difficult than sliding of spinel/zirconia boundaries in the low stress range. The plastic flow is analysed by means of grain boundary dislocations whose density increases with stress. At low stress, when the density of boundary dislocations is low, creep rates are interface-controlled while at high stress, when the boundary dislocation density is large, rates are limited by the long-range diffusion process.