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The complex interplay between competing phase stabilities of FCC, L1 2, BCC, and B2 phases in the Al 0.25 CoFeNi (7Al-31Co-31Fe-31Ni in at.%) high entropy alloy (HEA) leads to non-classical phase transformation pathways and resultant novel microstructures. Specifically, the competition between the homogenous precipitation of L1 2 and heterogenous precipitation of BCC/B2 can be studied at a temperature of 500 C in the Al 0.25 CoFeNi alloy. Upon isothermally annealing the single FCC phase microstructure of this HEA at 500 C up to 50 h, the transformation initiates with the formation of a transient ordered L1 2 phase with minor Ni–Al enrichment, which is far-from equilibrium, as revealed by atom probe tomography, and can be considered non-classical nucleation. The near equilibrium L1 2 phase eventually replaces the transient L1 2 during continued annealing at the same temperature. However, the …