Design and characterization of eutectic refractory high entropy alloys

Eutectic alloys are natural in-situ composites with near-equilibrium structures, impressive strength-ductility trade-off, and perfect castability. In the current study, novel eutectic Al-Cr-Nb-Ti-Zr refractory high entropy alloys (RHEAs) were designed using a CALPHAD (CALculation of PHAse Diagrams) approach. A starting Nb30Ti40Zr30 (at.%) alloy was based on a single-phase bcc solid solution. Alloying with Cr and Al induced the formation of hypoeutectic (Cr20Nb30Ti40Zr10, Al15Cr20Nb15Ti40Zr10, Al23Cr20Nb15Ti32Zr10), eutectic (Al28Cr20Nb15Ti27Zr10), and hypereutectic (Al33Cr20Nb15Ti22Zr10) structures due to the Laves phase precipitation. Al additions also resulted in the B2 ordering of the bcc phase and changing the Laves phase polytype from C15 (fcc) to C14 (hcp). A transfer from the single-phase to eutectic microstructure, connected with the Laves phase amount's growth, increased the alloys' compressive strength and the brittle-to-ductile transition temperature pronouncedly. Quantitative relationships between the Laves phase's volume fraction and the alloys' yield strength at different temperatures were established. Detailed analysis of eutectic alloy after plastic deformation at 800 degrees C demonstrated the stability of a lamellar B2/C14 Laves phase microstructure due to saving an initial orientation relationship (OR) of (011)(B2)parallel to(10 (1) over bar3)(C14), [1 (1) over bar1](B2)parallel to[3 (3) over bar0 (1) over bar] C14 ensuring a small lattice mismatch. The precipitation of a new phase with a D0(19) structure having the (011)(B2)parallel to(10 (1) over bar3)C-14 parallel to(0 (2) over bar 21)(D019) OR was also revealed. The obtained findings demonstrate new approaches to design RHEAs with (nearly) eutectic structures and a promising balance of mechanical characteristics.

Automated summary

Eutectic alloys with near-equilibrium structures, impressive strength-ductility trade-off, and perfect castability were designed using a CALPHAD approach. Alloying with Cr and Al induced the formation of hypoeutectic, eutectic, and hypereutectic structures due to Laves phase precipitation, improving the mechanical characteristics of the alloys. The stable B2/C14 Laves phase microstructure after plastic deformation at high temperatures and the precipitation of a new D0(19) structured phase were observed, indicating new approaches for designing RHEAs with promising mechanical properties.