Evolution of Divorced Eutectic Structure in NiCoCrFeAl Eutectic High Entropy Alloy by Selective Laser Melting

In this study, a NiCoCrFeAl eutectic high entropy alloy (EHEA) was prepared by selective laser melting (SLM). The sample was a cellular eutectic structure composed of BCC/B2 cells and FCC cell boundaries. The results show that under the extremely high-temperature gradient and cooling rate of selective laser melting, the microstructure characteristics of cellular eutectic produced by strong non-equilibrium solidification will be greatly affected by scanning speed and laser power. The reasons for the formation of the cellular structure are as follows: The growth rate of B2 dendrites is much faster than that of eutectic at high undercooling, resulting in hypoeutectic structure, which evolves into divorced eutectic under certain conditions. When the laser scanning speed is high, the cooling rate of the molten pool is increased, and the primary arm spacing of dendrites is shortened; thus, the cell structure size is refined. However, at a lower scanning speed, the cyclic thermal loading time of the deposited materials is prolonged, which is beneficial to the diffusion and enrichment of elements and promotes the amplitude modulation decomposition process of the BCC phase in the cell. These results provide new insight into the formation and evolution behavior of the rapidly solidified structure of SLMed EHEAs.