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

The 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, i.e., Al15Ni25Fe40Cr20 (0Ti), Al14Ni24Ti2Fe40Cr20 (2Ti), and Al12Ni20Ti8Fe40Cr20 (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 Ni2AlTi L21 Heusler compound due to Ti substitution in the B2 lattice. The ellipsoidal and plate-like morphology in the 0Ti alloy and the maze-like structures in 2Ti and 8Ti alloys were likely associated with eutectic solidification and phase separation or spinodal decomposition due to the increasing Ti content. All three alloys underwent 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. The FCC phase nucleated at the 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 L21 formation. However, the HT-XRD thermal cycling led to the reduction in 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.