Mechanical size effect of eutectic high entropy alloy: Effect of lamellar orientation

The effect of lamellar orientation on the deformation behavior of eutectic high entropy alloy at the micrometer scale, and the roles of two rarely explored laminate orientations (i.e., the lamellar orientation at ∼ 0° and 45° angles with the loading direction) in regulating size-dependent plasticity were investigated using in-situ micropillar compression tests. The alloy, CoCrFeNiTa_0.395, consists of alternating layers of Laves and FCC phases. It was found that the yield stress of the 0° pillars scaled inversely with the pillar diameters, in which the underlying deformation mode was observed to transform from pillar kinking or buckling to shear banding as the diameter decreased. In the case of the 45° pillars with diameters ranging from 0.4 to 3 μm, there exists a ‘weakest’ diameter of ∼ 1 μm, at which both constraint effect and dislocation starvation are ineffective. Irrespective of the lamellar orientations, the strain hardening rate decreased with decreasing pillar diameter due to the diminishing dislocation accumulation that originated from the softening nature of large shear bands in the 0° pillars, and the enhanced probability of dislocation annihilation at the increased free surfaces in the 45° pillars. The findings expand and deepen the understanding of the mechanical size effect in small-scale crystalline materials and, in so doing, provide a critical dimension for the development of high-performing materials used for nano- or microelectromechanical systems.