Phase stability of single phase Al_0.12CrNiFeCo high entropy alloy upon irradiation

Phase stability of NiCoFeCr and Al_0.12NiCoFeCr single phase high entropy alloys (HEAs) was studied under 3 MeV Ni²⁺ irradiation at 500 ^°C to a fluence of 1 × 10¹⁷/cm² reaching a peak dose of ~100 displacements per atom (dpa). Transmission electron microscopy (TEM) diffraction pattern and scanning transmission electron microscopy-energy dispersive X-Ray spectroscopy (STEM-EDS) were utilized to detect any second phases that formed in the irradiated regions of the specimens. While the single phase of NiCoFeCr alloy has remained stable under the irradiation, the phase in Al_0.12NiCoFeCr alloy decomposed into FCC matrix phase, and Ni₃Al type nanoprecipitates with L1₂ ordered structure. On the other hand, in Al_0.12NiCoFeCr alloy, the non-irradiated region of the TEM specimen (annealed for 10 h at 500 ^°C) and the as-cast alloy annealed for 490 h at 500 ^°C exhibited no such precipitates similar to those observed in the irradiated region. Based upon the thermodynamic stability of these alloys, we propose that radiation-enhanced diffusion under the irradiation accelerated the formation of the Ni₃Al type equilibrium phase. Our study clearly demonstrates that the phase stability of HEAs under irradiation should be considered as one of the deciding factors in the materials design and selection for nuclear applications.