The topologically close-packed (TCP) μ phase is usually known as an undesirable precipitation in highly alloyed Ni-base superalloys and steels. However, the ultrastrong μ phase with micron/nano-scale distribution plays a key role in driving the emergence of self-sharpening in our recently developed WMoFeNi high-entropy alloy (HEA). Herein, a detailed study is carried out to understand the substructure and atomic occupation of the μ phase by scanning electron microscope (SEM) and

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... cted transmission electron microscope (ACTEM). The Fe/Ni and W/Mo element pairs are equivalent in the μ-phase structure. Moreover, the elements in μ phase exhibit a near-equiatomic ratio, and the μ phase can grow during annealing at 1150 °C. (0001)μ and (11¯02)μ. Twins are the main substructures of the μ phase, and their atomic configurations and twinning mechanisms are investigated. The geometrical structural analysis of μ phase possesses a great significance for the design of self-sharpening HEAs.