Rapid Dendritic Growth Kinetics and Microstructure Modulation Approach of Anticorrosive Ti–Fe–Nb Alloy During Containerless Processing

Electromagnetic levitation was conducted to analyze the rapid growth kinetics, modulate the microstructure and investigate the anticorrosive properties of a Ti₇₀Fe_22.5Nb_7.5 alloy. Slow dendritic growth transformed into rapid dendritic growth when the undercooling reached 183 K, indicating that the crystal growth-driven mode changed from solutal control to thermal control. Correspondingly, the microstructure of the dendrites plus interdendritic lamellar eutectics transformed into total dendrites, and the eutectic reaction was completely inhibited. The much stronger texture and preferential growth direction of the (βTi) phase at high undercooling were also revealed. The microhardness of the alloy tended to increase with increasing undercooling as a result of the solution and precipitation strengthening. The phase transition of (βTi) → (ωTi) occurred in the Ti₇₀Fe_22.5Nb_7.5 alloy. The diffusely distributed (ωTi) in (βTi) became coarser, and local TiFe precipitated from (βTi) as the undercooling increased. The Fe solute content increased owing to the solute trapping effect and further influenced the lattice parameters. The excellent conformity between the lattices of the (βTi) and (ωTi) phases enhanced the probability of a metastable transformation. Electrochemical tests revealed that a more protective passive film with a larger passivation range and greater thickness formed as the undercooling increased, which was attributed to the micro–galvanic effect and a decrease in the grain boundary density.