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

Abstract

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 Ti70.5Fe29.5 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.

1. Introduction

Ti–Fe alloys are of technological importance for various advanced engineering applications because of their distinctive combination of mechanical robustness and functional versatility. In aerospace applications, for instance, their high specific strength enables lightweight designs for components, such as fuselage frames, enhancing fuel efficiency and payload capacity without compromising structural integrity [1–3]. In marine environments, these alloys are widely used in ship hulls, offshore drilling components, and subsea pipelines because of their unparalleled seawater corrosion resistance arising from the stable, self-repairing titanium oxide passive layers that resist chloride-induced pitting and stress corrosion cracking [4–5]. For hydrogen storage, Ti–Fe alloys provide intermetallic phases (e.g., TiFe, TiFe2) to reversibly absorb hydrogen at near-ambient conditions, offering the high volumetric capacity and cyclability critical for fuel-cell vehicles and grid-scale energy systems [6–8].

These diverse properties are intrinsically linked to the alloy microstructures, which can be tailored using advanced processing techniques. Among these, rapid solidification, a method employing ultrahigh cooling rates (105–108 K/s), is of particular significance. By kinetically “freezing” atomic arrangements, rapid solidification suppresses equilibrium phase formation and promotes nonequilibrium metastable phases, such as amorphous structures, supersaturated solid solutions, and nanoscale intermetallics (e.g., TiFe2). Furthermore, this method refines microstructural features to sub-micron or nanoscale grain sizes, which are unattainable via conventional slow cooling. This dual effect of phase control and grain refinement enhances the mechanical properties and functional behaviors of the final alloy [9–13]. Thus, understanding the mechanisms governing nonequilibrium phase formation during rapid solidification is pivotal for designing Ti–Fe alloys with tailored microstructures and superior performance in aerospace, marine, energy, and biomedical applications [14–16].

Melt spinning is a widely used rapid solidification technique that involves the ejection of molten alloys onto a high-speed rotating Cu wheel to achieve ultrafast cooling rates (typically 104–106 K/s). Over the past few decades, researchers have studied the phase transformation and microstructural evolution of Ti–Fe alloys using melt-spinning [17–26]. Lee et al. [17] reported a melt-spun Ti73Fe27 alloy with a microstructure consisting of supersaturated β-Ti and B2-TiFe phases, characterized by equiaxed nanocrystals. Krishnamurthy et al. [18] studied three Ti-rich alloys with 3wt%, 16wt%, and 22wt% Fe. The microstructures ...