Hierarchical Eutectoid Nano-lamellar Decomposition in an Al0.3CoFeNi Complex Concentrated Alloy

This paper reports a novel eutectoid nano-lamellar (FCC+L1(2))/(BCC+B2) microstructure that has been discovered in a relatively simple Al0.3CoFeNi high entropy alloy (HEA) or complex concentrated alloy (CCA). This novel eutectoid nano-lamellar microstructure presumably results from the complex interplay between Al-mediated lattice distortion (due to its larger atomic radius) in a face-centered cubic (FCC) CoFeNi solid solution, and a chemical ordering tendency leading to precipitation of ordered phases such as L1(2) and B2. This eutectoid microstructure is a result of solid-state decomposition of the FCC matrix and therefore distinct from the commonly reported eutectic microstructure in HEAs which results from solidification. This novel nano-lamellar microstructure exhibits a tensile yield strength of 1074MPa with a reasonable ductility of 8%. The same alloy can be tuned to form a more damage-tolerant FCC+B2 microstructure, retaining high tensile yield stress (similar to 900MPa) with appreciable tensile ductility (>20%), via annealing at 700 degrees C. Such tunability of microstructures with dramatically different mechanical properties can be effectively engineered in the same CCA, by exploiting the complex interplay between ordering tendencies and lattice distortion.