The legacy effect of steam-assisted pyrolysis: Composition, structure, and properties of alumina-mullite fibers

The performance of sol-gel-derived fibers is critically governed by the pyrolysis process. This study investigates the structure-directing effect of water vapor during the pyrolysis stage on the phase evolution, microstructural development, and mechanical properties of alumina-mullite fibers. The results reveal a strong non-monotonic dependence of microstructural evolution on water vapor flow rate. At 0.72 mL/h, a moderate degree of hydroxylation effectively modifies the amorphous aluminosilicate network. Through a significant legacy effect, this early-stage chemical modification promotes uniform crystallization of γ-Al 2 O 3 nanocrystals at 800 °C and subsequent homogeneous reaction sintering. Consequently, the fibers develop a dense, fine-grained microstructure at 1400 °C with a peak tensile strength of 1.59 GPa. Insufficient hydroxylation triggers premature mullitization and coarsening, whereas excess moisture fragments the network and promotes abnormal grain growth. Finally, this study proposes a cross-scale regulatory model that links the early-stage chemical modification via hydroxylation to microstructural inheritance.