Opposite effects of Al and Ti on the precipitation behaviors of D0₂₂-L1₂ dual superlattices in high entropy alloys

Aluminum (Al) and titanium (Ti) are widely used for L1₂ precipitation, whereas niobium (Nb) promotes D0₂₂ precipitates formation. However, to regulate L1₂-D0₂₂ dual superlattices, Al and Ti are typically co-regulated as a compositional module in conjunction with Nb additions, which neglects their individual effects on precipitation behaviors. Our study systematically investigates the distinct effects of Al and Ti on L1₂-D0₂₂ dual superlattices formation through controlled individual additions in a Ni_2.1CoCrFeNb_0.2 high entropy alloy (HEA). At 0.5 at.% Al, L1₂ superlattice nucleates at D0₂₂/matrix interfaces, forming D0₂₂-L1₂ siamese-twin precipitates, while 1 at.% Al produces exclusive L1₂ precipitates. Conversely, Ti (1–2 at.%) yields isolated L1₂ precipitates within matrix with dominant D0₂₂ precipitates. These results demonstrate that Al drives D0₂₂ to L1₂ transformation whereas Ti maintains D0₂₂ stability while allowing discrete L1₂ formation. The HEA strengthened by siamese-twin precipitates achieves a balance strength and elongation, offering a design strategy for dual-superlattice strengthened HEAs.