PNNL

Abstract

Addition of Al to face-centered cubic (FCC) based high entropy alloys (HEAs) or complex concentrated alloys (CCAs), containing the 3d transition elements Co, Cr, Fe, and Ni, results in an ordering tendency forming the L12 (or gamma prime) phase, as well as a lattice distortion, due to the relatively larger atomic radius of Al. This lattice distortion leads to the formation of BCC/B2 phases for higher Al contents. Guided by CALPHAD based solution thermodynamic modeling, the complex interplay between these competing tendencies has been tuned in a relatively simple Al0.3CoFeNi CCA, to investigate different regions of multi-phase stability. A novel complex eutectoid-like nano-lamellar (FCC+L12) / (BCC+B2) microstructure has been discovered in this CCA on annealing at 600°C, presumably resulting from this complex interplay. This nano-lamellar microstructure exhibits a tensile yield strength of 1074 MPa 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 (~900 MPa) with appreciable tensile ductility (>20%), via annealing at 700°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.