Simultaneous Strength-Ductility Enhancement of a Nano-Lamellar AlCoCrFeNi2.1 Eutectic High Entropy Alloy by Cryo-Rolling and Annealing

Nano-lamellar (Ll(2) + B2) AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) was processed by cryorolling and annealing. The EHEA developed a novel hierarchical microstructure featured by fine lamellar regions consisting of FCC lamellae filled with ultrafine FCC grains (average size similar to 200-250 nm) and B2 lamellae, and coarse non-lamellar regions consisting of ultrafine FCC (average size similar to 200-250 nm), few coarse recrystallized FCC grains and rather coarse unrecrystallized B2 phase (similar to 2.5 mu m). This complex and hierarchical microstructure originated from differences in strain-partitioning amongst the constituent phases, affecting the driving force for recrystallization. The hierarchical microstructure of the cryorolled and annealed material resulted in simultaneous enhancement in strength (Yield Strength/YS: 1437 +/- 26 MPa, Ultimate Tensile Strength/UTS: 1562 +/- 33 MPa) and ductility (elongation to failure/e(f) similar to 14 +/- 1%) as compared to the as-cast as well as cold-rolled and annealed materials. The present study for the first time demonstrated that cryo-deformation and annealing could be a novel microstructural design strategy for overcoming strength-ductility trade off in multiphase high entropy alloys.