In Situ Observation of the Competition Between Metastable and Stable Phases in Solidification of Undercooled Fe-17at. pctB Alloy Melt

High-speed video (HSV) technique was used to investigate the solidification behavior of undercooled Fe-17at. pctB alloy melt. Competitive growth between Fe/Fe₃B (metastable eutectic) and Fe/Fe₂B (stable eutectic) was captured, which is correlated with the critical nucleation undercooling \( \Delta T_{n}^{*} \)[=104 K (104 °C)] and the critical growth undercooling \( \Delta T_{\text{g}}^{*} \)[=151 K~183 K (151 °C~183 °C)]. For the initial undercooling, \( \Delta T < \Delta T_{n}^{*} \), only the stable eutectic reaction (L → Fe/Fe₂B) occurs during solidification, whereas, for \( \Delta T > \Delta T_{n}^{*} \), the metastable eutectic reaction (L → Fe/Fe₃B) occurs firstly, followed by the stable eutectic reaction. Nevertheless, \( \Delta T > \Delta T_{n}^{*} \) does not guarantee that the metastable phase (Fe₃B) appears finally. Only if \( \Delta T > \Delta T_{\text{g}}^{*} \), Fe₃B phase can be found in the room microstructure, as the growth velocity of metastable eutectic (Fe/Fe₃B) is larger than stable eutectic (Fe/Fe₂B) from HSV results; otherwise, the final structure consists of Fe₂B and α-Fe. Accordingly, not only the competitive nucleation, but the competitive growth also determines the final microstructure of Fe-17at. pctB alloy.