Complex precipitation behavior in a Co-free high entropy alloy during aging

High entropy alloys (HEAs) demonstrate high strength, thermal stability, and irradiation resistance, making them desirable for applications in nuclear reactors and other harsh environments. Many existing HEAs contain cobalt (Co), which makes them unsuitable for nuclear applications due to the long-term activation of Co. A previously studied Co-free alloy, (Fe_0.3Ni_0.3Mn_0.3Cr_0.1)₈₈Ti₄Al₈, exhibits high strength but compromised ductility due to its network of brittle precipitates upon aging. This work investigates the formation mechanism and evolution of the precipitates, including L1₂ (Ni₃Ti type, ordered face-centered cubic structure), B2 (NiAl type, ordered body-centered cubic structure), and Chi (FeCr-rich ordered α-Mn structure), in this alloy at 650 °C, the aging temperature that yields the alloy's peak strength after 120 hours. Using ex-situ scanning electron microscopy, transmission electron microscopy, and atom probe tomography, the precipitates were characterized in terms of their composition, morphology, and crystal structure at various aging times. In addition, synchrotron-based, in-situ X-ray diffraction was used to probe the evolution of precipitates in the bulk during key phases of in-situ aging. The L1₂ precipitates exhibit complex growth and coarsening behavior – their coarsening starts after just 24 hours, and competition between the L1₂ and B2 phases arises after 72 hours leading to a decrease in L1₂ volume fraction. Conversely, B2 and Chi precipitates exhibit delayed nucleation and growth, and do not form in observable quantities until after 24 hours. Once formed, however, these precipitates quickly dominate the microstructure, with >50 % volume fraction after 120 hours, leading to the previously observed mechanical properties. This study provides a foundational understanding of the evolution of multi-precipitate structures in HEAs and guides future alloy development and optimization by tailoring the precipitates.