Eutectic growth mechanism and microstructure of rapidly solidified Co23.2Fe13.7Ni24.6Cr10.9Sn27.6 high-entropy alloy: Effects of solute drag

Rapid solidification experiments on Co_23.2Fe_13.7Ni_24.6Cr_10.9Sn_27.6 eutectic high-entropy alloy (EHEA) were conducted by molten glass purification method and cyclic overheating techniques. Two solidification paths were identified. In the as-cast and low undercooling samples, the eutectic microstructure consisted of an FCC solid solution phase, M₃Sn₂ (M = Co, Ni) compounds with an HCP structure and BCC precipitates. The BCC precipitates disappeared at high undercoolings. A remarkably slow growth kinetics hindered by solute drag was discovered. The maximum growth velocity of the anomalous eutectic was 0.5797 m s⁻¹. Microstructural and quantitative elemental analyses indicate that the transition in eutectic growth mechanism is induced by the solute supersaturation with increasing undercooling. An orientation relationship (OR) of ${<\bar{1}01\bar{1}>}_{\text{hcp}}\Vert {<100>}_{\text{bcc}}$ and ${\{21\bar{3}1\}}_{\text{hcp}}\Vert {\left\{100\right\}}_{\text{bcc}}$ was determined between M₃Sn₂ (M = Co, Ni) compounds and BCC precipitates by transmission electron microscopy (TEM) analysis. Solute drag hinders the formation of the OR at the eutectic two phases, which is promoted by rapid solidification. Microhardness and uniaxial compression tests revealed that the increase in strength at low undercoolings was due to lamellar refinement. The anomalous eutectic structure exhibited low-stress brittle fracture, likely resulting from growth mechanism transition.