Hierarchical phase evolution in a lamellar Al0.7CoCrFeNi high entropy alloy involving competing metastable and stable phases

Guided by solution thermodynamic modeling coupled with detailed experimental characterization, the present study establishes that alternating FCC and BCC lamellar microstructure in Al0.7CoCrFeNi high entropy alloy, is a result of non-equilibrium partitionless solidification from liquid to single B2 phase, followed solid-state decomposition. Widmanstätten lamellae form allotriomorphic precipitates at grain boundaries, leading microstructure, divided into two distinct sub-systems. Isothermal annealing further drives these individual sub-systems towards equilibrium via precipitation ordered intermetallic phases. The transformation initiates formation metastable L12 shorter times, which are eventually replaced phases, forming composite B2+BCC laths, on long term annealing. These results exemplify interesting pathways lead hierarchical microstructures within HEAs, fact as processed conditions alloys often far-from equilibrium.