Unraveling the formation of L12 nano-precipitates within the FCC-phase in AlCoCrFeNi2.1 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 AlCoCrFeNi2.1 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 AlCoCrFeNi2.1 eutectic high entropy alloy consists of five phases: disordered FCC, disordered BCC, ordered B2 BCC, ordered L12 FCC, and sigma (sigma). 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 degrees C due to short-range diffusion of Al and Ni because of nucleation and growth mechanism during the solidification process.