The precipitation evolution and mechanism of micro-sized NbC in the melt of Fe-25wt.%Cr-3.5wt.%C-2.0wt.%Nb alloy

The in-suit precipitation evolution of micro-sized NbC in Fe-25 wt.% Cr-3.5 wt.% C-2.0 wt.% Nb alloy was studied by the HTCSLM, SEM and TEM observations in this paper. The morphology and size of NbC changed in four stages: pyramidal and triangular about 20 mu m-* multi-pyramidal about 20 mu m-* octahedral about 3 mu m-* worm-like about 300 nm. The three-dimensional morphology of initially precipitated NbC is determined as a pyramid. And the morphology of other micro-sized NbC was formed by the aggregation of pyramidal sub nucleus. The final worm-like NbC comes from the influence of surrounding austenite during eutectic reaction. The precipitation mechanism was clarified by the modified thermodynamic calculations, considering the crystal morphology. This study is a further understanding of the precipitation evolution and mechanism of the micro sized NbC in the melt of HHCCI.