A transmission electron microscopy study of polycrystalline SrCo0.8Fe0.2O3-delta creep in the diffusion-to-power law transition regime

This study is aimed at elucidating the nature of power-law creep, threshold stress and stress-induced diffusion in SrCo0.8Fe0.2O3-delta samples subjec ted to creep. A SrCo0.8Fe0.2O3-delta sample deformed in the transition from a diffusion to a power-law creep regime (900 degreesC, 50 MPa and 15% stra in) as well as an as-sintered sample were analysed by transmission electron microscopy and energy-dispersive X-ray spectroscopy. The dislocation struc ture has been found to be highly non-uniform with areas of high dislocation density confined only within a set of well-separated grains. The dislocati on structure and distribution indicate that this compound deforms by cooper ative grain-boundary sliding in the transition regime. In the grains that d o deform by dislocation movement, deformation is achieved by slip through t he screw [1 (1) over bar0][110] system. A planar map of the composition dis tribution of constituent cations revealed large fluctuations in cation comp osition over distances of the order of 100-200 nm, as well as a complementa ry distribution trend of cobalt and iron. This behaviour is found to be due to sequential stress-induced diffusion of cations, which is in accordance with the temperature dependence of the apparent activation energy. The conc entration profiles perpendicular to grain boundaries clearly reveal no pref erential segregation of cations at neither random nor coincident-site latti ce grain boundaries, which excludes dopant segregation as the mechanism res ponsible for the threshold stress-like behaviour at low stresses.