Phase separation of AlCoCrFeNi2.1 eutectic high-entropy alloy during directional solidification and their effect on tensile properties

AlCoCrFeNi2.1 nominal eutectic high entropy alloy (EHEA) was prepared by directional solidification at different withdrawal rates (v = 5 - 100 mu m/s). The directionally solidified samples exhibit superior mechanical properties. When the withdrawal rate is 5 mu m/s, the solidified microstructure consists of primary CoCrFe FCC phase and eutectic CoCrFe/AlNi lamellar phase, and the fracture strength is 696 MPa. As the withdrawal rate increase, the size of primary CoCrFe FCC phase and eutectic lamellar spacing decreased obviously. When the withdrawal rate is 100 mu m/s, the solidified microstructure is nearly full eutectic lamellar phase, and the fracture strength is 1012 MPa. Meanwhile, the elongation of the sample prepared at 5 mu m/s and 100 mu m/s is 33.3% and 14.5%, respectively. In order to reveal the relationship between phase formation and mechanical properties of EHEA, phase diagram of AlCoCrFeNi2.1 is simplified as AlNi-CoCrFe binary phase diagram. The results indicate that directional solidification is a suitable method to balance the strength and ductility of AlCoCrFeNi2.1 EHEA. When the solidified phase is nearly full eutectic soft/hard lamellar phase, the strain field shows wavy distributed during the tensile test.