Non-equiatomic FeNiCoAl-based high entropy alloys with multiscale heterogeneous lamella structure for strength and ductility

A new method based on conventional thermomechanical processing is proposed for design of non-equiatomic FeNiCoAlTaB (NCATB) high entropy alloy (HEA) with multiscale heterogeneous lamella (HL) structures to achieve outstanding mechanical properties. Two kinds of microstructures are produced in this study: 1) specimens annealed at 1050 °C exhibit structure composed of fine grains and ultrafine grains, which exhibits a combination of high strength (UTS ~1050 MPa) and good ductility (elongation ~23%); and 2) specimens annealed at 1200 °C which exhibit structure composed of fine grains and coarse grains, which demonstrates excellent synergy of good strength (UTS ~890 MPa) and exceptional ductility (elongation ~43%). It has been shown that shear bands formed in the cold rolling step are responsible for the fine-grain area in both specimens. Additionally, precipitation of NiAl (B2) particles also affects the formation of HL structures due to Zener pinning effect. The back-stress strengthening mechanism, responsible for high strength and ductility, is verified through combined “loading-unloading-reloading” (LUR) cyclic tensile tests and electron backscatter diffraction (EBSD) based geometrically necessary dislocation (GND) density analysis.