Deformation twinning in a refractory high entropy alloy with B2-ordered crystal structure

Abstract

A B2-ordered Al₁₆Nb₁₀Ta₅Ti₃₃V₅Zr₃₁ refractory high entropy alloy (RHEA) was previously found to exhibit good plasticity in compression from 20 °C to 1000 °C, partially attributed to the extensive deformation twinning occurring within this temperature range. This study observed that deformation twins form almost immediately after yielding, with their density increasing as plastic strain accumulates. Microstructural and crystallographic analyses show that the B2-ordered crystal structure is retained in these twins, which form through the rotation of domains within the parent grain (matrix) about 〈110〉 or 〈112〉 axes to discrete angles that match coincidence-site lattice (CSL) relationships. Conventional shear twin formation is energetically unfavorable in this alloy under the studied conditions because this mechanism requires extensive atomic shuffling to maintain the B2 order. Twins with rotation angles of 10.10°, 13.44°, 14.65°, 16.10°, 17.86°, 20.05°, 22.84°, 26.53°, 31.59°, and 38.94° were identified and analyzed. Twins with larger rotation angles are typically observed at higher plastic strain levels. A rotational twin formation mechanism, consistent with the experimental observations, is proposed and discussed.