This article presents the high temperature tensile and creep behaviors of a novel high entropy alloy (HEA). The microstructure of this HEA resembles that of advanced superalloys with a high entropy FCC matrix and L12 ordered precipitates, so it is also named as “high entropy superalloy (HESA)”. The tensile yield strengths of HESA surpass those of the reported HEAs from room temperature to elevated temperatures; furthermore, its creep resistance at 982 °C can be compared to those of some Ni-based superalloys. 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. Positive misfit between FCC matrix and precipitate has yielded parallel raft microstructure during creep at 982 °C, and the creep curves of HESA were dominated by tertiary creep behavior. 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.

Abstract:The present work investigates the high temperature oxidation and corrosion behaviour of high entropy superalloys (HESA). A high content of various solutes in HESA leads to formation of complex oxides, however the Cr and Al activities of HESA are sufficient to promote protective chromia or alumina formation on the surface. By comparing the oxidation and corrosion resistances of a Ni-based superalloy-CM247LC, Al 2 O 3 -forming HESA can possess comparable oxidation resistance at 1100˝C, and Cr 2 O 3 -forming HESA can exhibit superior resistance against hot corrosion at 900˝C. This work has demonstrated the potential of HESA to maintain surface stability in oxidizing and corrosive environments.

The microstructure and high temperature hardness of two face-centered cubic high entropy Ni-based alloys with L1 2 g 0 precipitates have been studied. Both alloys exhibit higher mixing entropy and with the advantages in lower density and lower cost of raw materials than conventional Ni-based superalloys. Their g 0 solvus are above 1 150 C, and the g-g 0 microstructure can be thermodynamically stable after isothermal ageing from 700 to 1 100 C for at least 500 h. By XRD peak deconvolution, positive lattice misfits between g and g 0 have been shown till elevated temperatures. The results from nano-indentation test indicate that their highly alloyed g 0 phase have rendered more significant strengthening, and the underlying mechanism can be attributed to the higher anti-phase boundary energy. Therefore, with minor refractory additions, the bulk hardness of present alloys can surpass that of commercial superalloy from room to high temperature.

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