Influence of Iron Solute on the Structure of Small-Angle [001] Twist Boundaries in Magnesium Oxide

The structure of small-angle 001] twist boundaries in pure and Fe-doped MgO was examined by electron imaging techniques in order to study the influence of solute on grain-boundary structure. In addition, the effect of the oxidation state of the Fe on the boundary structure was also examined. The 001] twist boundaries were produced in the form of bicrystals, by the hot-pressing of cleaved single crystals of pure MgO and MgO containing Fe. In pure MgO the 001] twist boundaries produced with small plastic strains are planar and contain a square array of screw dislocations with Burgers vectors of the type a /2<110>. In the presence of solute the original interface becomes wavy and dissociates into subboundaries. Distorted hexagonal dislocation networks as well as square arrays of screw dislocations are often observed in Fe-doped MgO. These observations are evidence that 001] twist boundaries in MgO whose production was accompanied by small amounts of plastic deformation undergo a structural transformation induced by the presence of Fe solute. The influence of the Fe oxidation state on boundary structure is less well understood. It appears that when the 001] twist boundaries are produced using crystals containing a substantial amount of Fe³⁺ ions, the resulting boundary structure is more likely to contain distorted hexagonal dislocation networks than when the boundary is produced from crystals containing predominantly Fe²⁺ ions.