The influence of Fe on grain boundary mobility of alumina

Grain growth in polycrystalline materials is governed by grain boundary velocity, which depends on GB energy and mobility. Using alumina (α-Al 2 O 3 ) as a model material, we investigated the influence of Fe dopants that exhibit two valence states, Fe 3+ and Fe 2+ , on grain boundary mobility. Fe-doped Al 2 O 3 with Fe contents (0.18∼2.93 wt.%) below the solubility limit (8.9 wt.%) at 1600°C were annealed in air or Ar. For those annealed in air, suppressed grain growth due to solute-drag was quantified by a reduced grain boundary mobility with increasing Fe content, and by measuring the Fe excess at grain boundaries. In contrast, annealing in Ar resulted in increased grain boundary mobility and anisotropic grain growth. The influence of oxygen partial pressure in the annealing atmosphere on the ratio of Fe 3+ and Fe 2+ and associated oxygen vacancies was hypothesized to be the reason. X-ray photoelectron spectroscopy and electron energy loss spectroscopy were conducted to measure and characterize the existence and distribution of Fe 3+ and Fe 2+ . Decreasing the oxygen partial pressure led to an increasing concentration of Fe 2+ versus Fe 3+ and increased anisotropic grain growth. Based on white line ratio analysis of the Fe L 2,3 edges, it is concluded that the anisotropy is attributed to selective adsorption of Fe 3+ /Fe 2+ on specific grain boundaries that changed their mobility and led to elongated grains. Fast-moving grain boundaries were segregated with more Fe 2+ than the slow-moving counterparts. This study demonstrates modifying grain size and morphology via tailoring dopant valence state and oxygen vacancy concentration.