Synergistic control of microstructures and properties in eutectic high-entropy alloys via directional solidification and strong magnetic field

The AlCoCrFeNi2.1 eutectic high-entropy alloy (Ni2.1 EHEA), as an exemplary representative of the high-entropy alloy family, has garnered significant research attention owing to its exceptional comprehensive properties. In this study, we investigated the influence of various growth velocities on the microstructure, lamellar spacing, and mechanical properties of the Ni2.1 EHEA. We observed that at lower growth velocities, the structure consisted of an alternating face-centered-cubic (FCC) phase and B2 phase lamellae aligned in a single direction, with the lamellae orientation parallel to the direction of the heat flow. The yield strength increased with the growth rate, while the ultimate tensile strength decreased with increasing the growth velocity. Ductility remained relatively consistent, and a double yield phenomenon was observed in the elastic-plastic deformation region. Under conditions of high growth velocities, the microstructure transitioned from a single-directional full lamellar structure to a multi-stage lamellar arrangement. The most favorable comprehensive mechanical properties were achieved at a growth rate of 200 μm/s, resulting in a yield strength of 450 MPa, an ultimate tensile strength of 1092 MPa, and a remarkable ductility of ∼32% in the directionally solidified samples—double that of the arc-melted sample. The evolution law of directional solidification structures under the coupling effect of different magnetic fields and different growth rates was studied. The interaction of the thermoelectric-magnetic force and thermoelectric-magnetic convection and the potential mechanism of microstructure evolution under the effect of magnetic field were deeply analyzed. The results reveal that at a growth rate of 2 μm/s, the spacing between eutectic layers decreases as the magnetic induction intensity increases, leading to the transformation of some regular layers into spherical layers. Similarly, at a growth velocity of 10 μm/s, the eutectic structure exhibited a Columnar-to-equiaxed transition (CET). However, as the growth rate further increases, the limited exposure time to the magnetic field prevented significant structural changes.