Formation of non-equilibrium metastable phases in rapidly solidified Ti–Fe eutectic alloy

Ti–Fe alloys are indispensable for crucial applications in the aerospace, marine, and energy industries. To understand the effect of rapid solidification on phase formation and microstructural evolution in Ti–Fe alloys, melt spinning of a typical Ti_70.5Fe_29.5 eutectic alloy at different cooling rates was investigated in this study. The experimental results show that the melt-spun ribbons exhibit unique three-layered microstructure consisting of thin amorphous–nanocrystalline (Am–NC) hybrid layer on the chilled side and NC layer on the free side, which sandwich a fully Am middle layer. This microstructure is distinctly different from conventional eutectic-coupled microstructures observed in slow-cooled eutectic alloys. In particular, increasing the wheel speed resulted in a thicker Am layer and Fe enrichment, indicating the effect of solute segregation on the glass-forming ability, which is rarely seen in the formation of bulk metallic glasses. In addition, an unexpected Ti₄Fe₂O phase is observed in the NC layer in addition to β-Ti and B2-TiFe phases formed via a divorced eutectic growth mechanism. The analysis indicated that rapid solidification and moderate oxygen doping/contamination are essential for promoting the formation of amorphous and metastable Ti₄Fe₂O phases. This study contributes to a better understanding of the phase-selection mechanism and microstructural evolution in Ti–Fe alloys under far-from-equilibrium conditions, providing useful implications for the fabrication of Ti–Fe-based alloys using rapid-solidification techniques.