Exceptional strength-ductility synergy in hyper-eutectic Al19Fe20Co20Ni37.5Mo3Ta0.5 high entropy alloy with heterogeneous structure

Abstract

Eutectic high entropy alloys (EHEAs) characterized by dual-phase lamellar FCC/B2 structures represent a promising class of materials combining the advantages of both high entropy alloys (HEAs) and conventional eutectic alloys. Nevertheless, the mechanical properties of EHEAs have yet to fully meet the desired performance criteria. In particular, achieving both high strength and substantial plasticity remains a serious challenge, as these two properties are typically difficult to reconcile in most metallic materials. Herein, a hyper-eutectic Al₁₉Fe₂₀Co₂₀Ni_37.5Mo₃Ta_0.5 HEA comprising the face centered cubic (FCC) and ordered body centered cubic (B2) phases was fabricated with outstanding mechanical properties. Thermo-mechanical processing (TMP) followed by annealing at 900 ºC and 1000 ºC induced formation of ordered and topologically close-packed (TCP) nano-precipitates in this dual-phase HEA. The annealing at 900 ºC, and 1000 ºC led to the heterogeneous microstructures, including dominant FCC and B2 phases sharing Kurdjumov-Sachs (K-S) orientation relationship, along with the different precipitation behavior in both HEAs. After annealing at 900 ºC, the formation of coherent L1₂ nano-precipitates in the FCC matrix, and nano-sized µ phase particles within the B2 matrix contributed to the remarkable strength (1609 MPa) and ductility (12 %). After annealing at 1000 ºC, the strength was decreased to 1452 MPa while ductility was increased to 19 %. Moreover, the L1₂ and µ phase nano-precipitates dissolved within the matrix phases, and B2 nano-precipitates formed within the FCC matrix, exhibiting Nishiyama-Wassermann (N-W) orientation relationship. The considerable mechanical properties of both HEAs are attributed to the synergistic reinforcement provided by the interface and the presence of multiple nano-precipitates within the dual-phase matrix, which further activated the multiple strengthening mechanisms. We believe that these findings will offer valuable guidance for the design of precipitation strengthened HEAs with exceptional mechanical properties.