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

Ti-Fe alloys are indispensable for crucial applications in aerospace, marine and energy industries. In this work, in an effort to understand the effect of rapid solidification on phase formation and microstructural evolution in Ti-Fe alloys, melting spinning of a typical Ti70.5Fe29.5 eutectic alloy at different cooling rates has been investigated. Experimental results show that the melt-spun ribbons exhibit a unique three-layered microstructure, consisting of a thin amorphous-nanocrystalline (Am-NC) hybrid layer at the chill-side, followed by a fully amorphous (Am) layer sandwiched in the middle, and a nanocrystalline (NC) layer at the free-side. This is in sharp contrast with conventional eutectic coupled microstructure as observed in slow-cooled eutectic alloys. More strikingly, it is found that, with increasing the wheel speed, the amorphous layer thickens and becomes more enriched in Fe, indicative of the effect of solute segregation on the glass-forming ability, rarely seen in the formation of bulk metallic glasses. It is also found that, an unexpected Ti4Fe2O phase exists in the nanocrystalline layer in additional to the β-Ti and B2-TiFe phases formed via a divorced eutectic growth mechanism. Analysis indicates that rapid solidification and moderate oxygen doping/contamination are essential in promoting the formation of the amorphous phase as well as the metastable Ti4Fe2O phase. This work helps to better understand 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. Keywords: Rapid solidification, Eutectic alloy, Melt-spinning, Glass formation, Amorphous, Metastable phase