Microstructures and room temperature tensile properties of as-cast and directionally solidified AlCoCrFeNi2.1 eutectic high-entropy alloy

The microstructures and room temperature tensile properties of as-cast and directionally solidified AlCoCrFeNi2.1 eutectic high-entropy alloy were investigated using OM, SEM, EDS, TEM and tensile tests. The microstructure of as-cast alloy, including the well-aligned lamellar structures, the radial emanating lamellar structure and mesh-like structure, consists of NiAl-rich (B2) and CoCrFeNi-rich (L12) phases. From the longitudinal section of directionally solidified alloy, when the withdrawal rate is 6 μm/s, the solid-liquid interface morphology is planar, and the well-aligned lamellar structure is observed. The three-dimensional morphologies of well-aligned lamellar structures are well characterized by deep etching. With increasing the withdrawal rate to 15 μm/s, the solid-liquid interface morphology change from planar to cellular, and the microstructure changes from planar eutectic to cellular eutectic, even the dendritic eutectic at local region occurs at 60 and 120 μm/s. From the transverse section, the well-aligned lamellar structures occur in the cell interior, and the interlamellar spacing decreases gradually with increasing the withdrawal rates. The TEM result indicates that NiAl-rich phase has an orientation relationship with CoCrFeNi-rich phase of [1¯11] NiAl-rich//[1¯22]CoCrFeNi-rich and (211)NiAl-rich//(022¯)CoCrFeNi-rich. The nano-precipitates occur in NiAl-rich (B2) and CoCrFeNi-rich (L12) phases. The directionally solidified alloy possesses a good combination of strength and ductility at 60 μm/s. In particular, using the directional solidification technology is a good way to improve the ductility of the alloy (∼37%). The strengthening and plasticizing mechanisms are discussed by analyzing the fracture and side surfaces. •Planar, cellular even dendritic lamellar eutectic are observed at different rates.•The 3D morphologies of lamellar structures are well characterized by deep etching.•NiAl-rich phase has an orientation relationship with CoCrFeNi-rich phase.•The alloy possesses a good combination of strength and ductility at 60 μm/s.•Directional solidification is a good way to improve the ductility of the alloy.