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Therefore, the development for precipitation strengthened HEAs is only at the incipient stage, and more research efforts will be required.
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Furthermore, there were phase instability associated with the formation of L2 1 Ni 2AlTi, B2, and Cu-rich FCC 5, 11, 12. Nevertheless, the γ′ solvus temperatures of the reported γ′-bearing HEAs still remained relatively low comparing to those of superalloys 9. The physical metallurgy of superalloys 9 have been employed to design HEAs by utilization of significant volume fractions of stable and coherent L1 2 γ′ to provide high temperature strength, e.g., Al 10Co 25Cr 8Fe 15Ni 36Ti 6 HEA was designed to contain 46 vol% of γ′ precipitates 10, and its tensile strength could surpass those of Inconel 617 and Alloy 800 H. As a result, precipitation strengthening should be considered in developing HEAs for high temperature applications. Several HEAs, such as CoCrFeNiNb x 7 and CoCrFeNiMo x 8 demonstrated that a moderate increase in tensile strength can be achieved by various precipitates. However, the strength of single phase type HEAs were reported to be insufficient at elevated temperatures 6. Recent studies have shown attractive mechanical properties of HEAs at cryogenic temperature 4 and room temperature 5 with excellent combinations of high strength and toughness, indicating their potential as the structural materials. The concept of high entropy alloys (HEAs) has allowed the exploration of large composition space of alloys, and is currently one of the most intriguing research topics in the field of materials science 1, 2, 3.
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To the best of authors’ knowledge, this article is the first to present the elevated temperature tensile creep study on full scale specimens of a high entropy alloy, and the potential of HESA for high temperature structural application is discussed. Positive misfit between FCC matrix and precipitate has yielded parallel raft microstructure during creep at 982 ☌, and the creep curves of HESA were dominated by tertiary creep behavior. Analysis on experimental results indicate that HESA could be strengthened by the low stacking-fault energy of the matrix, high anti-phase boundary energy of the strengthening precipitate, and thermally stable microstructure. The tensile yield strengths of HESA surpass those of the reported HEAs from room temperature to elevated temperatures furthermore, its creep resistance at 982 ☌ can be compared to those of some Ni-based superalloys. The microstructure of this HEA resembles that of advanced superalloys with a high entropy FCC matrix and L1 2 ordered precipitates, so it is also named as “high entropy superalloy (HESA)”. This article presents the high temperature tensile and creep behaviors of a novel high entropy alloy (HEA).