Synergistic refinement of divorced eutectic Fe₂Ti and precipitated Ti₂Ni phases in highly undercooled titanium alloy

The rapid solidification mechanism and corrosion resistance of ternary Ti_42.3Ni_36.1Fe_21.6 alloy were investigated using electromagnetic levitation technique. The microstructure solidified under near equilibrium condition consisted of the primary Ti(Ni, Fe) phase, Ti(Ni, Fe)+Fe₂Ti divorced eutectic, Ti(Ni, Fe)+Ni₃Ti peri-eutectic and two precipitates of Ti₂Ni and Ni₄Ti₃ phases. As liquid undercooling rose, the Ti(Ni, Fe) dendrite growth velocity increased following the power function and reached 185 mm/s at the maximum undercooling of 230 K. The crystal orientation of Ti(Ni, Fe) dendrites tended to be consistent under rapid solidification, and the average misorientation between the neighboring grains decreased to 2.5°. The rapid growth of the primary Ti(Ni, Fe) phase significantly suppressed the interdendritic phase transitions that occurred in the remaining liquid. The solidified microstructure was only composed of Ti(Ni, Fe), Fe₂Ti and Ti₂Ni phases once the undercooling exceeded 136 K. Meanwhile, the divorced eutectic Fe₂Ti phase transformed from continuous strips to broken flakes, and the precipitated Ti₂Ni phase was dispersed in the Ti(Ni, Fe) matrix as smaller particle. Eventually, the refined and homogenous microstructure induced by high undercooling promoted the formation of continuous as well as dense passivation film, and enhanced the corrosion resistance of the Ti_42.3Ni_36.1Fe_21.6 alloy in substitute seawater.