Achieving outstanding strength−ductility matching in dual-phase high-entropy alloys via modulation of BCC phase

Abstract

The impact of Mo on the microstructure, phase constitution, and tensile properties of Al1.25CoCrFeNi3−xMox (x=0.05, 0.1, 0.2, 0.3, and 0.5) high-entropy alloys (HEAs) was explored systematically through phase diagram simulation and experimental validation. The findings indicate that Mo addition transforms the microstructure from eutectic to hypereutectic and eventually to dendritic. Mo promotes the nucleation of the body-centered cubic phase by reducing the nucleation barrier and altering the valence electron concentration. As Mo content increases, yield strength rises, while the tensile strength and plasticity increase first and then decrease. Notably, the Al1.25CoCrFeNi2.8Mo0.2 HEA achieves an impressive tensile strength of 1234.80 MPa and a fracture strain of 19.33%. Key strengthening mechanisms include solid solution strengthening, grain boundary strengthening, and heterogeneous interface strengthening.

1. Introduction

High-entropy alloys (HEAs), also termed multi-principal element alloys, exhibit exceptional properties, including ultra-high strength [1], excellent plasticity [2], high wear resistance [3], corrosion resistance [4,5] and extreme temperature stability [6]. These characteristics make HEAs promising candidates for applications under severe working conditions. Among the various types, face-centered cubic (FCC) HEAs [7], known for their excellent plasticity, and body-centered cubic (BCC) HEAs [8], recognized for their high strength, are currently the focus of extensive research. However, the relatively low strength of the FCC HEAs and the limited plasticity of the BCC HEAs hinder their further development. To address the “strength−ductility trade-off” challenge, researchers have developed FCC/BCC eutectic HEAs (EHEAs) that leverage the advantages of FCC and BCC HEAs.

The composition of FCC/BCC EHEAs is mainly Al−Co−Cr−Fe−Ni, such as AlCoCrFeNi2.1 [9,10], Al19Fe20Co20Ni41 [11] and FeCoNi2Al0.9 [12] EHEAs. The introduction of alloying elements such as Ta, W and Mo can significantly improve the mechanical properties of EHEA by realizing the strengthening mechanism of solution strengthening and second phase strengthening [13,14]. For instance, KIM et al [15] synthesized AlCoCrFeNi2.1Ta0.76 HEA by incorporating Ta. This modification led to a significant change in the microstructure, resulting in the transformation from FCC/BCC eutectic structure to primary BCC + eutectic FCC/Laves. WU et al [16,17] demonstrated that the addition of small amounts of W did not alter the eutectic structure. Ni30Co30Cr10Fe10Al18W2 HEA maintained a dual-phase lamellar structure composed of FCC and BCC phases. This unique microstructure enabled the alloy to exhibit an exceptional combination of strength and plasticity. Mo was also added to EHEA to prepare Co30Cr10Fe10Al18Ni32−xMox HEAs by YANG et al [18], indicating that Mo as a solid solution strengthening element promoted the growth of BCC phase. Co30Cr10Fe10Al18Ni30Mo2 achieved an impressive tensile strength of 1250 MPa, along with a satisfactory elongation of 14%. PENG et al [19] developed Al19Fe20−xCo20−xNi41Mo2x HEAs and investigated the microstructure evolution. The addition of Mo promoted the transformation of eutectic structure into hypoeutectic structure. ZHU et al [20] added Mo to the AlCoCrFeNi HEA with BCC structure to prepare AlCoCrFeNiMox HEAs. When the Mo content (x) exceeded 0.1, the eutectic structure was formed. The increase of Mo content led to a significant increase in the compressive yield strength from 1051 to 2757 MPa, but the compressive ...