Precipitation behavior and mechanical properties of intermetallic-strengthened high-entropy alloys

Abstract

High-entropy alloys (HEAs) have attracted extensive attention due to their vast composition space and unique mechanical properties, offering new opportunities for the development of advanced structural materials. Among various HEAs, face-centered cubic (FCC) HEAs have spanned a wide range of scientific interest due to their excellent toughness and ductility at both ambient and cryogenic temperatures. However, a single-phase FCC structure generally results in a low yield strength, which is insufficient for many practical applications. Precipitation strengthening has proved to be one of the most effective methods for improving the strength of FCC HEAs. Especially, introducing a coherent L1₂-ordered phase by incorporating Al and Ti into FCC HEAs has been demonstrated to enhance strength without significantly compromising ductility. Some other hard precipitates, such as Laves and σ precipitates, are strong barriers for dislocation motion, which can provide a high work hardening rate in the early stages of deformation, having the potential to avoid the formation of Luders bands. In view of the advantages and limitations of sheared and bypassed precipitates, it is interesting to design advanced HEAs with multiple types of precipitates. It is hoped to maximize the advantages of the different types of precipitates while minimizing their disadvantages, thereby achieving superior mechanical properties. However, precipitation in multicomponent HEAs is extremely complicated, which involves complex elemental partitioning and solute interactions. There was a lack of in-depth understanding on the precipitation and mechanical behaviors of HEAs. The purpose of this thesis is to quantitatively understand the precipitation behavior of HEAs and correlate their precipitate microstructure with bulk mechanical properties. First, the study explores the introduction of additional hard phases into continuous precipitation (CP) L1₂-strengthened HEAs to achieve dual precipitation, thereby further improving the strength-ductility trade-off of FCC-HEAs. Through the combined introduction of coherent L1₂ nanoparticles and incoherent Laves phases, dual precipitation allows the alloys to attain a yield strength exceeding 1400 MPa, an ultimate tensile strength above 1800 MPa, and a uniform elongation of 18%. This results in a remarkable balance between strength and ductility. This approach combines the advantages of shearing and bypass strengthening mechanisms, providing a significant enhancement in mechanical performance compared to single-precipitate systems. The research not only demonstrates the effectiveness of dual precipitation in overcoming the traditional strength-ductility challenge, but also offers a promising design strategy for developing high-performance structural materials with balanced and superior mechanical properties. Second, discontinuous precipitation (DP) has emerged as an effective strategy for designing alloys that combine high strength and excellent ductility, highlighting the importance of understanding how microstructural changes influence mechanical properties. In this study, we systematically investigated the precipitate evolution, recrystallization, and mechanical properties of a DP strengthened high-entropy alloy (HEA) under various aging conditions from 500 to 800 °C. The results indicate that at low aging temperatures, limited atomic mobility results in sluggish recrystallization, causing DP to preferentially initiate and propagate along grain boundaries, ultimately forming a fine nanorod microstructure. As the aging temperature increases, the kinetics of both recrystallization and DP are accelerated, resulting in the rapid formation of nanorod precipitates and the development of fully recrystallized ultrafine-grained structures. Mechanical tests demonstrate that by optimizing the grain structure and precipitate microstructure, an optimal combination of a yield strength exceeding 2000 MPa and a total elongation of approximately 18% can be achieved. Quantitative analysis reveals that precipitation strengthening and grain boundary strengthening are the primary factors contributing to the enhanced yield strength. These findings deepen our understanding of the structure–property relationships in DP-strengthened alloys and provide practical guidance for designing advanced alloys with tailored mechanical properties through controlled microstructural evolution. In addition, inspired by the experience of dual precipitation strengthening in CP systems, this research introduced σ phases into DP nanorods strengthened HEAs to further improve the strength–ductility balance. Microstructural analyses reveal that this alloy forms a hierarchical structure composed of FCC/L1₂ architectures and ultrafine spherical σ phases after aging at 600 °C. The introduction of the σ phase not only further enhances the yield strength (2080 MPa) and ultimate tensile strength (2300 MPa), but also improves the work hardening capability, enabling excellent strength–ductility synergy with a total elongation of about 17%. Furthermore, strengthening mechanism analysis indicates that the FCC/L1₂ nanorod structure and ultrafine grains are the primary contributors, while the σ phase provides additional work hardening and strength enhancement. In summary, this thesis elucidates the precipitation behavior and mechanical properties of L1₂-strengthened HEAs, demonstrating that synergistic strengthening mechanisms can achieve an outstanding balance of ultrahigh strength and good ductility. It provides comprehensive insight into the microstructural evolution of DP-HEAs, revealing the critical role of grain structure and precipitate morphology in tailoring mechanical properties. These findings not only deepen the understanding of the structure–property relationships in coherent nano-L1₂ strengthened HEAs, but also offer practical guidance for designing advanced structural materials with excellent mechanical performance through controlled microstructural evolution.