A development mechanism of graded microstructures in iron-containing SiC fibers revealed by electron microscopy

Transmission electron microscopy is utilized to investigate the microstructure development of iron-containing SiC fibers as a function of their annealing temperatures. Compositional and structural gradients along the radial directions start to form within SiC fibers fabricated at temperatures above 1200 degrees C. The graded microstructures are associated with the outward diffusion of iron-rich particles along the radial direction. In-situ heating TEM experiments suggest that the iron-rich particles within SiC fibers are highly mobile at elevated temperatures. Therefore, we propose that the graded microstructures along the radial directions are the synergetic effects of iron catalyzation and outgassing, namely, iron-rich particles are progressively driven to the surface of SiC fibers and in the trajectory of iron-rich particles, the decomposition of the amorphous SiCxOy phase is accelerated, promoting the growth of SiC grains. The current study emphasizes the important role of the outgassing process to the microstructure evolution of SiC fibers. Our proposed mechanism can be extended to understand the microstructural evolution of SiC fibers that use metallic additives as sintering aids.