Formation of lamellar eutectic structure and improved mechanical properties of directional solidified Al0.9CoCrNi2.1 high-entropy alloy

Abstract

In this study, Al_0.9CoCrNi_2.1 EHEA was prepared by directional solidification at various withdrawal rates from 6 to 120 μm/s. The lamellar eutectic structure is formed containing FCC (L1₂) and BCC (B2) at different withdrawal rates during directional solidification. The solid-liquid interface of the alloy transfers from nearly-planer to cellular eutectic interface and the lamellar spacing decreases from 7.17 μm to 1.65 μm as the withdrawal rate increases. The lamellar spacing is negatively correlated with withdrawal rates which was identified by the Jackson-Hunt model and its growth rate index (0.484) is closely related to the theoretical value (0.5). The tensile experiment demonstrates that the structure of Al_0.9CoCrNi_2.1 EHEA is finer and the ultimate tensile strength and elongation increase as the withdrawal rate increases. The elongation of Al_0.9CoCrNi_2.1 EHEA is two times higher than that of the as-cast alloy at the withdrawal rate of 120 μm/s. It is concluded that the formation of the L1₂ phase and Cr-rich precipitated phases in the directional solidified eutectic structure are associated with the enhancement of mechanical properties due to grain refinement at high withdrawal rates.