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

Abstract

Nano-lamellar (L1₂ + B2) AlCoCrFeNi_2.1 eutectic high entropy alloy (EHEA) was processed by cryo-rolling 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 ~200-250 nm) and B2 lamellae, and coarse non-lamellar regions consisting of ultrafine FCC (average size ~200-250 nm), few coarse recrystallized FCC grains and rather coarse unrecrystallized B2 phase (~2.5 µm). 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 cryo-rolled 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 ~ 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.

Figure 1

(a) TEM micrograph of the starting EHEA. The SADPs in (b) and (c) correspond to the L1₂ (marked by green circle in (a)) and B2 (indicated by red circle in (a)), respectively. The zone axes (Z.A.) of the L1₂ and B2 phases are [011] and [001], respectively. (d) Shows the microstructure in the 90% cryo-rolled condition (viewed along ND). The SADPs in (e) and (f) correspond to the nanocrystalline disordered FCC (indicated by the green circle in (d)) L1₂ (marked by green circle in (d)) and B2 (indicated by red circle in (d)), respectively. The ZA of the SADP of the B2 is [001].

Figure 2

Nano-indentation (a) topographic and (b) hardness maps of the as-cast EHEA.

Figure 3

(a) EBSD phase map of 90% cryo-rolled EHEA after annealing at 800 °C for 1 h. The region enclosed by the ellipse in (a) shows a typical lamellar region while the arrows indicate coarse B2 regions. (b) TEM micrograph (observed along ND) shows B2 subgrains in the coarse non-lamellar region of the cryo-rolled and annealed EHEA separated by LABs (marked by arrows) along with their misorientation angles. (c) Shows the microduplex structure of the 90% cold-rolled EHEA after annealing at 800 °C for 1 h.

Figure 4

Nano-indentation topographic images of cryo-rolled and annealed specimen obtained from the (a) coarse non-lamellar and (b) lamellar regions; (c) and (d) are the corresponding hardness maps.

Figure 5

Engineering stress-strain plots of the EHEA in various heat-treated conditions. The dimensions of the tensile specimen are shown inset (all dimensions in mm).

Figure 6

Strength-elongation plot of selected HEAs. The closed and open symbols correspond to YS and UTS, respectively. σ_y and σ_TS are YS and UTS, respectively.