Unraveling the formation of L1₂ nano-precipitates within the FCC-phase in AlCoCrFeNi_2.1 eutectic high entropy alloy

The present study investigates the formation mechanism of L1₂ nano-precipitates within the FCC phase during the solidification process of the AlCoCrFeNi_2.1 eutectic high entropy alloy. For this purpose, advanced characterization techniques, including high-energy synchrotron X-ray diffraction, transmission electron microscopy, and atom probe tomography were employed, along with CALPHAD calculations. The solidified microstructure of the AlCoCrFeNi_2.1 eutectic high entropy alloy consists of five phases: disordered FCC, disordered BCC, ordered B2 BCC, ordered L1₂ FCC, and sigma (σ). The dendritic regions of the disordered FCC phase were formed due to non-equilibrium solidification, while the L1₂ nano-precipitates were uniformly embedded within the disordered FCC matrix. These nano-precipitates were enriched with Al and Ni, while being depleted in Co, Cr, and Fe. These nano-precipitates might form around the critical temperature of 600 °C due to short-range diffusion of Al and Ni because of nucleation and growth mechanism during the solidification process.