Pseudobinary Eutectic Growth and Structural Evolution Control of Quinary Ti₂₇Ni₂₅Co₂₀Nb₁₄V₁₄ High-Entropy Alloy Through Containerless Processing

Abstract

The rapid eutectic growth kinetics of undercooled liquid Ti₂₇Ni₂₅Co₂₀V₁₄Nb₁₄ high-entropy alloy (HEA) was investigated by electromagnetic levitation (EML) and drop tube (DT) techniques. The maximum liquid undercoolings attained 207 K (0.14 T_L) at levitated state and 385 K (0.25 T_L) under free fall condition, respectively. During EML experiments, a microstructural transition from regular lamellar eutectic to anomalous eutectic was achieved by controlling the liquid undercooling above 62 K. An increase in volume fraction of anomalous eutectic and significant grain refinement were modulated by further elevating the liquid undercooling. The maximum growth velocity of lamellar eutectic was measured as 1.47 mm s⁻¹, whereas that of anomalous eutectic reached 35.64 mm s⁻¹ at the largest undercooling of 207 K. Under microgravity condition, the microstructure transformed twice by adjusting alloy droplet diameters. Two critical diameters were determined, and the corresponding undercooling thresholds were calculated as 73 K and 251 K, respectively. When the droplet diameter decreased to 705 μm, the “lamellar eutectic–anomalous eutectic” microstructure transition occurred, which was similar to that under EML condition. Notably, a coarse structure consisting of primary hcp-NbV grains and intergranular anomalous eutectic was formed in tiny alloy droplets with less than 95 mm diameter, which is attributed to the independent nucleation of hcp-NbV phase. Furthermore, the crystalline orientation relationship between B2-TiNi and hcp-NbV phases was determined as $\left\langle {11\overline{2} \overline{3} } \right\rangle_{{{\text{hcp}}}}$//$\left\langle {111} \right\rangle_{{{\text{B}}2}}$ and $\left\{ {1\overline{1} 00} \right\}_{{{\text{hcp}}}}$//$\left\{ {110} \right\}_{{{\text{B2}}}}$ in lamellar eutectic by TEM analysis. The alloy microhardness was remarkably enhanced after containerless rapid solidification, which resulted mainly from the crystalline grain refinement.