Phase separation with ordering in aged Fe-Ni-Mn medium entropy alloy

High- and medium-entropy alloys are currently receiving significant attention. Understanding short-range ordering and nano-scaled phase separation concerning elemental interaction is a key to controlling phase stability for designing of high entropy alloys. In this study, we aim to clarify concurrent phase separation and ordering during aging of Fe-35Ni-35Mn (at%) medium-entropy alloy using advanced characterization techniques such as spherical aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) and three-dimensional atom probe (3DAP). Obvious age hardening occurs after aging at 500 °C for 96 h due to the formation of L1₀ ordered phase combined with phase separation. By aging at 400 °C or shorter time, obvious precipitation of the ordered phase is not detected but the radial distribution functions obtained by 3DAP present that enrichment of Ni and Mn nearby the same kinds of the element is observed, and this tendency is stronger for Mn. This result suggests that phase separation is induced by Mn clustering in the early stage of aging. A statistical thermodynamic model of phase stability considering L1₀ ordered structure is developed. The analyses using the model suggest that phase separation is not possible in a disordered state at 500 °C, therefore, nucleation of the L1₀ phase should precede phase separation at this temperature. On the other hand, phase separation is possible even in a disordered state at 400 °C, and ordering further enhances phase separation.