Aluminium and oxygen tracer diffusion in polycrystalline alpha-Al2O3

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Alpha-Al2O3 is an important refractory material which has numerous technical applications: as an in situ growing self-healing oxide scale, as a massive material and as reinforcement fibres in composites. This means that polycrystalline microstructures are of prime importance in the technological use of alpha-Al2O3. It is therefore extremely disadvantageous that to date only very few data on directly measured oxygen grain boundary diffusivities exist and that there is no data at all (!) available on aluminium grain boundary diffusion. This lack of data considerably hinders quantitative modelling of diffusion controlled processes in polycrystalline alpha-Al2O3. Therefore, for modelling diffusion controlled processes (creep, sintering, alpha-alumina scale growth on aluminium bearing Fe or Ni base alloys) it is generally conjectured that, in analogy to bulk transport, oxygen grain boundary diffusion is much slower than aluminium grain boundary diffusion. In order to check this conjecture quantitatively, it is absolutely necessary to perform direct tracer diffusion experiments. For the first time ever (!) experimental data on the 26Al grain boundary tracer diffusion in polycrystalline alpha-Al2O3 will be determined in this project. This will allow to quantify the ratio Db-Al/Db-oxy of grain boundary diffusivities which is a key parameter in theoretical studies.Yttrium doping has, from the technological point of view, a very positive effect as it slows down both the creep rate of polycrystalline alpha-Al2O3 and the growth rate of alpha-Al2O3 scales on aluminium bearing Fe or Ni base alloys. In order to demonstrate that yttrium doping affects oxygen grain boundary diffusion two other groups performed 18O tracer diffusion studies whose results are, however, controversial. In order to fully understand the technologically important effect of yttrium doping, it is necessary to supply data for the aluminium grain boundary tracer diffusion in yttrium doped alpha-alumina. Therefore, in this project the aluminum grain boundary diffusion in yttrium doped alpha-alumina will be measured and the controversial oxygen grain boundary diffusion data will be experimentally checked.