Hierarchical eutectic structure and superior mechanical property in low cobalt content AlCo0.2CrFeNi2.1 alloy by laser metal deposition

Abstract

The fabrication of AlCo 0.2 CrFeNi 2.1 high entropy alloy via laser metal deposition (LMD) was motivated and implemented by the demand for overcoming the strength-ductility trade-off of the as-cast alloys due to the Co concentration. The microstructure features of AlCo 0.2 CrFeNi 2.1 alloy were characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and electron back-scattered diffraction. The mechanical properties were evaluated by a tensile test at room temperature. The synergy of reduced Co content and LMD process led to the formation of hierarchical eutectic microstructures consisting of columnar grains, eutectic colonies, alternately arranged fcc(L1 2 )+B2 lamellae, and coherent Cr-rich nanoprecipitates in B2. The growth direction of those eutectic structures was found to be mainly parallel to the building direction (BD). The fcc(L1 2 )/B2 eutectics agreed well with the Kurdjumov–Sachs orientation relationship of {110} B2 //{111}L1 2 , and< 111 > B2 //< 110 >L1 2 . The LMD fabricated AlCo 0.2 CrFeNi 2.1 exhibited anisotropic mechanical properties when stretched in BD and transverse direction (TD). An excellent combination of the ultimate tensile strength (1246 MPa) and ductility (17.1%) was achieved in the BD, better than the as-cast counterpart. The elevated mechanical properties could be attributed to the high cooling rate solidification induced microstructural refinement. The same interlamellar spacing in BD and TD embraced a nearly equal ability to block dislocation movement and gave rise to the similar yield tensile strength. The differences in grain/colony size induced boundary strengthening in BD and TD were evaluated to account for the remarkable changes in ultimate tensile strength as well as ductility. • High entropy alloy AlCo 0.2 CrFeNi 2.1 was made by laser metal deposition(LMD). • Excellent combination of strength and ductility was achieved with less Co content. • LMD overrode chemical composition to affect strength-ductility trade-off. • Refined microstructure by LMD caused better property than casting. • Interlamellar spacing contributed to yield strength, grain/colony size affected ultimate strength.