Ultrastrong nanocrystalline FeCoNiCr high entropy alloy with outstanding thermal stability

FeCoNiCr-based high-entropy alloys (HEAs) exhibit excellent irradiation resistance and outstanding mechanical performance, particularly at cryogenic temperatures, rendering them strong candidates for manufacturing high-performance parts in extreme environments. Nanocrystalline (NC) FeCoNiCr-based HEAs exhibit greater strength compared to their coarse-grained (CG) equivalents. Yet, their thermal stability is often subpar. The reason is that the nanograins experience considerable coarsening upon heating to a temperature between 0.4 and 0.6 of their melting point (T_m). In this work, we report an NC FeCoNiCr HEA, containing 1 at% lanthanum (NC FeCoNiCr-La HEA), synthesized via mechanical alloying (MA) and high temperature & high pressure (HTHP) sintering methods, with an average grain diameter of 59 nm and an exceptionally high hardness of 715 HV, displaying prominent thermal stability up to 1000 °C (0.74T_m). The thermal stability is explained by the segregation of elemental lanthanum and La-O-rich nanoprecipitates (NPs) at the grain boundaries (GBs).