Transition of solid-liquid interface and tensile properties of CoCrFeNi high-entropy alloys during directional solidification

In order to understand effect of solidification velocity on microstructure evolution and mechanical properties of high entropy alloys (HEAs), CoCrFeNi HEA samples were directionally solidified at different solidification velocities (5–50 μm/s). The results show that all samples were directionally solidified, but the solid-liquid interface and columnar grains are different. Microstructure of them is only composed of FCC solid solution phase. Morphology of solid-liquid interface transforms from cellular, to cellular-dendritic, and then to dendritic with increasing of solidification rate. The criterion of solidification rate for cellular/dendrite transition is calculated as about 26.1 μm/s based on experimental results. The width of columnar grains decreases from 200 μm to 100 μm with solidification rate increasing from 5 μm/s to 50μm/s. Meanwhile, secondary dendrite spacing decreases from 200 μm to 30 μm for dendritic solidified samples (v = 10–50 μm/s). Tensile testing show that the yield strength improves from 233 MPa to 383 MPa, and the ultimate tensile strength improves from 431 MPa to 596 MPa with increasing of solidification rate. Meanwhile, the elongation of all samples is more than 34.3%, but rupture morphology analysis shows that the rupture changes from quasi-cleavage fracture to ductility fracture. The improvement of tensile properties is resulted from grain boundary strengthening of the columnar grain and second dendrite.