Advanced TiAl Based Alloys: From Polycrystals to Polysynthetic Twinned Single Crystals

TiAl alloys stand out for low density, high specific strength, and excellent creep resistance, making them promising for high-temperature aerospace applications. However, traditional TiAl alloys suffer from poor room-temperature ductility and low service temperature that limit their critical applications in aerospace structures. To address these issues, research has focused on improving the mechanical properties of TiAl alloys through alloying and microstructural design. After decades of effort, the evolution of TiAl alloys has progressed from polycrystalline TiAl to high-performance polysynthetic twinned (PST) TiAl single crystals. The well-aligned PST TiAl single crystals enriched with Nb enable an excellent combination of strength and ductility, significantly outperforming polycrystalline TiAl alloys. This review summarizes recent progress on TiAl alloys, particularly focusing on newly developed PST single crystals. First, the development history of TiAl alloys is overviewed; then their crystal structures, phase diagrams, and typical microstructures are systematically discussed, along with the design strategies based on alloying elements. Additionally, recent advances in TiAl columnar crystals, which are between polycrystals and single crystals, are reviewed. Subsequently, the mechanical anisotropy, preparation methods, and superior mechanical properties of the PST single crystals are analyzed in detail. The final remark highlights the future development and application prospects of TiAl alloys.