Microcavity Formation in Alumina Using Ti Templates II: Mechanism and Kinetics

In part I of this work, it was found that titanium (Ti) wire encapsulated within mechanically milled alumina powder and sintered at 1350°C forms potentially useful microcavities due to the consolidation of Kirkendall porosity. Here a series of samples sintered at 1350°C in the range 0–24 h has shown the remarkable way in which these cavities form. The cavity has already started in samples quenched from the top of the heating ramp (0 min at 1350°C). It is surrounded by a diffusion zone ∼300 μm in diameter, which does not change size throughout the firing process although the contents change markedly. The diffusion zone microstructure is initially complex with phase sequence TiO₂/Al₂O₃/TiO₂+Al₂O₃/Al₂TiO₅. Microstructure evolution may be summarized as outward growth of the cavity accompanied by inward growth of the Al₂TiO₅ resulting in a ∼190-μm-diameter cavity surrounded by a 50-μm-thick layer of Al₂TiO₅. The formation of the cavity and surrounding microstructure is discussed although some features, such as the nucleation of Al₂TiO₅ in the part of the diffusion zone furthest from the Ti source and the ring of Al₂O₃, which persists in between Ti-rich parts of the diffusion zone are still poorly understood.