Mechanistic insights into alloying-induced microstructural evolution and stability of TiAl alloys

Abstract

The drive for higher service temperatures, together with advances in additive manufacturing (AM), motivates efforts to improve the high-temperature microstructural stability of TiAl alloys. This study investigates the mechanisms by which Hf and Zr additions affect the formation and stability of lamellar structures at 1100 °C, as well as their impact on the mechanical properties of a Ti-48Al-2Cr-based alloy (at. %), using a 2Nb-containing alloy as a reference. Hf alloying alone refined the lamellar structure and enhanced compressive strength at 700 °C for as-homogenized TiAl alloy. However, it also intensified Cr segregation, which facilitated the formation of coarsened γ phase via the B2 phase during annealing at 1100 °C. Zr alloying produced the finest lamellar colonies and spacing in the as-homogenized alloy, yet it promoted an undesirable network-like γ structure prone to cracking under load. Notably, the combined Hf+Zr addition mitigated these adverse effects, yielding an optimal balance of hardness, high-temperature strength, and structural stability. The exceptional thermal stability of the Hf- and Zr-co-alloyed TiAl alloy is attributed to a higher equilibrium α-phase fraction at 1100 °C, a reduced driving force for phase transformation due to lattice distortion, and decreased diffusion coefficients and α₂/γ interfacial energy. Although the materials were produced by casting and controlled heat treatments, the 1100 °C anneal reproduces the thermal environment encountered in AM builds. Accordingly, these mechanistic insights and alloying guidelines identified here are directly informative for tailoring TiAl alloys to withstand AM-related thermal histories.