Characterization of as-cast microstructural heterogeneities and damage mechanisms in eutectic AlCoCrFeNi2.1 high entropy alloy

Highlights: • Ballistically impacted AlCoCrFeNi2.1 eutectic high entropy alloy was examined. • Damage mechanisms were linked to the initial microstructure using EBSD and TEM. • Damage mechanisms varied from lower to higher impact velocities. • Initial microstructure determined damage at lower velocities. • Adiabatic shear bands governed the damage at higher velocities. -- Abstract: We have correlated as-cast microstructural features of AlCoCrFeNi_2.1 with adiabatic shear localization (ASL) and crack formation, due to dynamic deformation from impact with a spherical projectile. The as-cast microstructure contained extensive FCC/B2 Kurdjumov-Sachs interfaces, and the BCC-ordered B2 domains had plate-like lamellae and clusters of irregularly-shaped morphologies. The primary FCC and B2 phases also contained FCC-ordered L1₂ and Cr-rich precipitates, respectively. ASL propagated across all FCC/B2 interfaces, and the varying B2 morphologies did not hinder its formation. ASL propagation involved subgrain formation and nanoscale-twinning inside FCC, while the B2 domains experienced extensive dislocation plasticity, even though the impact velocities and B2 morphologies influenced crack formation discernibly. At lower impact velocities the lamellar B2 regions were prone to crack propagation along the FCC/B2 interfaces. In comparison, at higher impact velocities, the cracks traced the locus of ASL paths; i.e., cracks propagated through lamellar and irregularly-shaped B2 domains alike. Deformation-induced steps formed at the FCC/B2 interfaces may have facilitated such trans-domain morphology-agnostic crack propagation. Finally, L1₂ precipitates minimally impacted any damage mechanisms, while Cr-rich precipitates deflected crack paths inside the B2 domains. Thus, efforts to engineer eutectic high entropy alloy microstructures should strengthen FCC/B2 interfaces in the lamellar regions while retaining the strengthening Cr-rich precipitates.