Anisotropic co-deformation behavior of nanolamellar structures in additively manufactured eutectic high entropy alloys

Laser-based directed energy deposition (DED) technique provides new opportunities for fabricating eutectic high entropy alloys (EHEAs) with directional nanolamellar structures that exhibit superior mechanical properties. The mechanisms of co-deformation, strengthening, fracture of the two constituent phases, and their resulting anisotropic mechanical properties, have not yet been systematically studied. Here we employ in situ synchrotron-based high-energy X-ray diffraction and X-ray tomography to study a DED-fabricated nanolamellar AlCoCrFeNi_2.1 that is comprised of face-centered cubic (fcc) and ordered body-centered cubic (B2) phases. The EHEA samples that are loaded along three orientations present obvious mechanical anisotropy: (i) Samples HEA0, loaded along the lamellar direction, exhibit both the highest tensile strength and ductility; (ii) Samples HEA45, loaded along a 45° angle with the lamellar direction, exhibit the lowest strength and medial ductility; (iii) Samples HEA90, perpendicular to the lamellar direction, exhibit the medial strength and lowest ductility. We find that such mechanical anisotropy is associated with the sequence of work hardening in the B2 and fcc phases, as well as martensitic transformation in the B2 phase. For HEA0 samples, the sequential events of martensitic transformation of the B2 phase and work hardening of the fcc phase prevent from the initial fracture of the samples. In contrast, for the HEA90 samples, the simultaneous events of work hardening of both the B2 and fcc phases promote the formation and propagation of cracks along the phase boundaries, facilitating the fracture of the samples. This study demonstrates that the DED-produced EHEAs exhibit the optimal strength-ductility synergy along the lamellar direction and provides a fundamental understanding of the co-deformation behavior of dual phases in directional nanolamellar structures.