Microstructure and texture evolution during growth of magnesio-aluminate spinel at corundum-periclase interfaces under uniaxial load: The effect of stress concentration on reaction progress
Abstract
Reaction rims of magnesio-aluminate spinel were grown at the contacts between peridase and corundum at temperatures of 1250 degrees C to 1350 degrees C and under uniaxial load of 0.026 and 0.26 kN per 9 mm(2) of initial contact area. Single crystals of periclase with [100] and of corundum with [0001] perpendicular to the polished reaction interface as well as polycrystalline corundum were used as starting materials.
Immediate application of the load before heating resulted in deformation twinning and fracturing of corundum introducing stress concentration and lateral variations in the quality of physical contact at the reaction interface. The tight contacts are characterized by enhanced reaction progress which together with the positive volume change of the reaction and limits on plasticity of the studied phases led to the opening of void spaces along the reaction interface and large lateral variations in rim thickness occur.
Spinel shows strong topotactic relations to the reactant phases including full topotaxy between spinel and peridase, partial topotaxy with (111)(spi)parallel to(0001)(cor) and {101}(spi)parallel to{10-10}(cor) and (111)(spi) parallel to (0001)(cor) between spinel and corundum. Oriented nucleation and selective growth were the main mechanism of texture formation.
Stress concentrations and tight physical contacts across the reaction interface may enhance nucleation of topotactic grains. The respective spinel-periclase and spinel-corundum reaction interfaces are mostly semi-coherent with sets of line dislocations accounting for the lattice misfit.
The systematic occurrence of porosity along the semi-coherent and its absence along the incoherent sections of the spinel-periclase interface reflect the different capacities of the (semi)-coherent and incoherent interface sections for annihilating the vacancies that were emitted from the advancing spinel-periclase reaction interface.