Design and characterization of eutectic refractory high entropy alloys

Abstract

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 °C demonstrated the stability of a lamellar B2/C14 Laves phase microstructure due to saving an initial orientation relationship (OR) of (011)B2||(101¯3)C14, [11¯1]B2||[33¯01¯]C14 ensuring a small lattice mismatch. The precipitation of a new phase with a D019 structure having the (011)B2||(101¯3)C14||(02¯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.