Phase Selection and Microhardness of Directionally Solidified AlCoCrFeNi2.1Eutectic High-Entropy Alloy

In the present work, the solidification behaviors and microhardness of directionally solidified AlCoCrFeNi2.1eutectic high-entropy alloy (EHEA) obtained at different growth velocities are investigated. The microstructure of the as-cast AlCoCrFeNi2.1EHEA is composed of bulky dendrites (NiAl phase) and lamellar eutectic structures, indicating that the actual composition of the alloy slightly deviates from the eutectic point. However, it is interesting to observe that the full lamellar structure of this alloy is obtained through directional solidification. In order to explain this phenomenon, the maximum interface temperature criterion and the interface response function (IRF) theory are applied to calculate the velocity range of the transition from the primary phase to the eutectic, which is 1.2–2 × 104μm/s. Furthermore, microhardness is one of the important parameters to measure the mechanical properties of materials. Therefore, the microhardness test is performed, and the test result indicates that the microhardness (HV) increased with increasing growth velocity (V) or decreased with increasing lamellar spacing (λ). The dependences of λand HV on Vare determined by using a linear regression analysis. The relationships between the λ, Vand HV are given as: λ=11.62V-0.48, HV = 305.5V 0.02and HV = 328.1λ−0.04, respectively. The microhardness of the AlCoCrFeNi2.1EHEA increases from 312.38 HV to 329.54 HV with the increase in growth velocity (5–200 μm/s). Thus, directional solidification is an effective method to improve the mechanical properties of alloys.