Single Crystal Growth and Chemical Disorder Trapping of Refractory MoNbReTaW High-Entropy Alloy Solidified Under Electrostatic Levitation State

Abstract

The disorder trapping induced by supercooling is innovated to promote the formation of chemically disordered solid solution phase in high-entropy alloy (HEA) when the ordered phase is more inclined to form. We design a new MoNbReTaW HEA, consisting of the top five refractory metals with the highest melting points, whose liquidus temperature attains 2788 K. It is featured with a very large negative enthalpy of mixing and a strong mismatch of atomic size. As a result of sluggish atomic diffusion under a sufficient supercooling up to 500 K and consequent disorder trapping at a crystal growth velocity of 15 to 25 m/s, the original single solid solution phase with a conspicuous local chemical ordering transforms into a chemically disordered state. Owing to the decrease of atomic size mismatch, not only the lattice is turned out to be well-aligned but also the dislocation density is reduced. By the supercooling-based rapid solidification approach under electrostatic levitation condition, we successfully achieved the growth of spherical single crystals with 2 to 3.5 mm in diameter and interior misorientation less than 2 angle degrees. The notable suppression of microsegregation helps to enhance the microhardness to 6.8 GPa at the maximum supercooling of 538 K. Disorder trapping contributes to the increase of nanohardness up to 11.2 GPa. The liquid supercooling is capable of strengthening the mechanical property by optimizing the local chemical ordering.

Graphic Abstract