The complex interplay between competing phase stabilities of FCC, L12, BCC, and B2 phases in the Al0.25CoFeNi (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 homogenous precipitation of L12 and heterogenous precipitation of BCC/B2 can be studied at a temperature of 500 o C in Al0.25CoFeNi alloy. Upon isothermally annealing the single FCC phase microstructure of this HEA at 500 o C up to 50 hours, the transformation initiates with the formation of a transient ordered L12 phase with minor Ni-Al enrichment, which is far-from equilibrium, as revealed by atom probe tomography, and can be considered as non-classical nucleation. The near equilibrium L12 phase eventually replaces the transient L12 during continued annealing at the same temperature. However, the resultant FCC+L12 microstructure is metastable because the true equilibrium for the Al0.25CoFeNi alloy at 500°C is a mixture of L12+B2 phases, as revealed by solution thermodynamics modeling. The higher nucleation barrier for the BCCbased ordered B2 phase coupled with the slower kinetics at 500 o C, leads to the homogeneous precipitation of L12, while the B2 phase appears to sluggishly grow from grain boundaries acting as heterogeneous nucleation sites.< Leer menos