Fe-Mn-Al-C high-entropy steels with superior mechanical properties at 4.2 K

Abstract

There exist urgent demands to develop structural materials with superior mechanical properties at 4.2 K. Some high-entropy steels (HESs) show potentials as cryogenic materials, but their deformation behaviors and mechanical properties at 4.2 K have been rarely investigated. Moreover, the aging-induced embrittlement of HESs also severely restricts their applications. In this work, the Fe-Mn-Al-C HESs with different Al contents (0, 4, 10 or 15 at.%) and grain sizes were fabricated, and their deformation behaviors and mechanical properties at 4.2 K were systematically studied. With increasing the Al content, (Fe,Mn)23C6 carbides are effectively inhibited in 10%Al HES after aging at 923 K for 10 days. Therefore, the premature fracture of 10%Al HES is avoided, leading to the excellent combination of high strength (∼1.5 GPa) and high fracture toughness (255 MPa·m1/2) at 4.2 K. The deformation mechanisms shift from the extensive twinning and deformation bands in 0Al HES to the lesser twinning, Taylor lattices and deformation bands in 10%Al HES, which contribute to the high strength and ductility of the 10%Al HES. Furthermore, the 10%Al HES with larger grains displays a much higher fracture toughness at 4.2 K, and this inverse size effect on the cryogenic toughness was elaborately revealed.