Abstract
In this paper, in order to improve the performance of Fe–Si3N4 composite synthesized by flash combustion, the detailed nitridation mechanism and formation process were discussed. In the process of high temperature nitriding, ξ phase rapidly melted to form Fe-Si melt and the cycle of rapid surface nitriding → rupture of nitridation shells → new melt exposing and nitriding occurred continually on the surface of Fe-Si melt. Since ions have different activities on the surface and internal, Fe ions near the surface migrated inwards and Si ions inside moved out; meanwhile, the Fe-Si melt kept shrinking. The nitriding reaction of the Fe-Si melt finished till the overall activity aSi approached 0, leaving the atom ratio of [Fe] to [Si] at 3:1. During the falling of the formed Si3N4 and Fe3Si melt in the N2 flow, the surface of Si3N4 was oxidized to form a SiO2 film. The nitridation product fell into the product pool, loosely stacking and adhering together by the SiO2 film. α-Si3N4 dissolved and precipitated to form β-Si3N4 crystals, and the β-Si3N4 crystals kept growing to form radioactive elongated crystals. As the temperature decreased, the Fe3Si melt cooled down; the Si-N-O melt, α-Si3N4 and the roots of elongated β-Si3N4 crystals formed the dense areas.
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