Duplex dendrite growth kinetics and grain structure orientation features of containerlessly manufactured Fe–Cr–Ni alloys

The formation mechanism of the metastable δ phase in two undercooled liquid Fe–Cr–Ni alloys with different Cr/Ni ratios was investigated in situ by the electromagnetic levitation method. As for the Fe–13%Cr–5%Ni alloy, the δ phase dendrite grew primarily from alloy melt at all levels of undercooling, and the solid/liquid interface migration rate increased monotonously as the undercooling rose, leading to significant refinement of lath martensite with enhanced texture in the microstructure. In particular, large amounts of metastable δ phase were preserved among the martensitic laths to ambient temperature at high undercooling. Competitive nucleation between the γ phase and metastable δ phase was observed in real time in the Fe–14.6%Ni–4.4%Cr alloy. The growth velocity of the primary phase dropped suddenly from 10.4 to 3.9 m s−1 once the undercooling exceeded the critical value of 110 K, indicating that the metastable δ phase grew preferentially from alloy melt. Subsequently, the γ phase nucleated in the δ phase with multi-point feature, while the duration time of the metastable δ phase decreased from 0.55 to 0.15 ms as the undercooling rose. The rapidly solidified Fe–14.6%Ni–4.4%Cr alloy was composed of lath martensite completely. As undercooling increased, the fraction of high-angle grain boundaries declined from 39.9% to 13.9%, and the microstructure transformed from coarse columnar grain to equiaxed grain with more consistent orientation.