Titanium-enhanced thermal stability in AlNiFeCr medium-entropy alloys: An in-situ high-temperature XRD analysis

Rising demands for efficient, high‑temperature systems such as gas turbine engines and nuclear reactors continue to push the development of advanced structural materials. AlNiCrFe-based medium-entropy alloys (MEAs) are promising candidates because their heterogenous disordered-ordered BCC (A2-B2) structures offer superior thermal and mechanical performance. Understanding how Ti alters these alloys is essential for establishing key composition-structure-property relationships. This work investigates three alloys, Al 15 Ni 25 Fe 40 Cr 20 (0Ti), Al 14 Ni 24 Ti 2 Fe 40 Cr 20 (2Ti), and Al 12 Ni 20 Ti 8 Fe 40 Cr 20 (8Ti) alloys using in‑situ high‑temperature X‑ray diffraction (HT‑XRD). In the as‑cast state, the 0Ti and 2Ti alloys exhibited FeCr‑rich A2 and AlNi‑rich B2 phases, while the 8Ti alloy contained uniquely the Ni 2 AlTi L2 1 Heusler compound due to Ti substitution in the B2 lattice. The ellipsoidal and plate-like morphology in the 0Ti alloy and maze-like structures in the 2Ti and 8Ti alloys were likely associated with eutectic solidification and phase separation or spinodal decomposition due to increasing Ti content. All three alloys underwent a BCC-to-FCC transformation during heating, with FCC nucleation temperatures rising from ~699°C to ~872°C as Ti content increased. This shift reflected enhanced lattice distortion and sluggish diffusion associated with larger Ti atoms. FCC phase nucleated at grain boundaries and grew as Widmanstätten-like laths and plates. The <110> coefficient of thermal expansion (CTE) decreased significantly from ~9.0×10 -6 K -1 (0Ti) to ~5.9×10 -6 K -1 (8Ti) as Ti strengthened bonding and increased lattice distortion. Microhardness increased with Ti addition due to grain refinement and L2 1 formation. However, the HT-XRD thermal cycling led to reduced microhardness mainly due to grain coarsening, while the formation of FCC phase provided an additional contribution towards softening. These findings shed light on the atomistic mechanisms and provide guidance for developing advanced MEAs for high-temperature structural applications.