Phase field investigation on the alumina nanocrystalline growth controlled by mullite-alumina phase transition in alumina-mullite fibers

The overgrowth of alumina nanocrystalline is the most important cause for the degradation of alumina-mullite fibers at high temperatures. A profound understanding on the kinetics and mechanisms of alumina nanocrystalline growth controlled by the mullite-alumina phase transition is of great significance to the development and service of alumina-mullite fibers. Here, by using the phase field simulations coupling to materials thermodynamic data, we systematically investigated the growth of alumina nanocrystalline and the mullite-alumina phase transition in alumina-mullite fiber at different temperatures and concentrations of the mullite matrix. The growth rates and exponents of alumina grains at different conditions are quantified and agree well with the experimental results of Nextel 720 fibers. We found that the growth of polycrystalline alumina grains undergoes two stages: rapid growth in the very early stage and then grains coarsening. The mullite-alumina phase transition dominates the rapid growth of alumina grains and the grain coarsening proceeds mainly through grain annexation and dissolution-precipitation. Our results clarify the influence of mullite-alumina phase transition on alumina grain growth from mullite matrix, and quantify the mechanisms and kinetics of alumina grain growth from mullite matrix in the alumina-mullite system.