State and effect of oxygen on high entropy alloys prepared by powder metallurgy

Abstract

To reveal the state and effect of oxygen on high entropy alloys (HEAs) prepared by powder metallurgy, a VNbMoTaW refractory HEA was prepared by mechanical alloying and vacuum hot-pressing sintering. During mechanical alloying, with increasing ball milling time, the elemental distribution of the VNbMoTaW powders gradually became homogeneous, and the particle size decreased, while ~0.2 atom% O was introduced. After sintering at 1700 °C and 40 MPa for 2 h, a Ta-rich oxide with a grain size of approximately 15 nm was formed, leading to brittle fracture of the sintered VNbMoTaW HEA. When the sintering temperature was increased to 1900 °C, the grain size of the oxides decreased to ~8 nm and was dispersed on the BCC solid solution matrix. The yield strength of the VNbMoTaW HEA at room temperature and 1600 °C reached 2800 MPa and 350 MPa, respectively, due to Orowan strengthening caused by dispersed nano-sized oxides; additionally, the plasticity deformations at room temperature were ~2%. Although oxygen combined with the constituent metals to form oxides, dispersed nanosized oxides could improve the mechanical properties of HEAs prepared by powder metallurgy. The feasibility of powder metallurgy for HEAs was proven by this work.