An electron microscope study of precipitation in MgO single crystals

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Abstract
An electron microscope study was made of precipitation in vanadium and titanium-doped MgO single crystals. In both crystals cubic spinel precipitates form in parallel orientation with the cubic matrix lattice in the temperature range 1100°C-1500°c. Precipitation of the vanadium spinel, Mg2VO4 is accompanied by the nucleation and growth of dislocation loops composed of edge dislocation dipoles with their axes parallel to the principal axes of the cubic MgO matrix lattice. It is concluded that dislocation loop nucleation occurs by the condensation of vacancies previously existing in the form of clusters with solute ions in the as-grown crystals. Similar loops grow from existing edge dislocations constituting sub-grain boundaries and those formed by prismatic punching from unidentified impurity particles. The equilibrium structure at 1200°c is that of 0·5 μ diameter spinel particles decorating irregular dislocation networks. No similar dislocation configurations are developed on precipitation of the titanium spinel. It is tentatively suggested that in the latter case, which involves an increase in oxygen ion lattice spacing on precipitation, precipitate nuclei form from solute ion-vacancy clusters which are too small for dislocation loop nucleation.

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