Hierarchical microstructures in Al-Cr-Fe-Ni multi-principal element alloys via simple change of Cr/Ni ratio for enhanced mechanical properties

The multi-principal element alloys (MPEAs) provide a broader compositional and structural design space for the development of metallic structural materials. In the Al-Cr-Fe-Ni system, subtle changes of composition can induce complex hierarchical microstructures, giving the opportunity to achieve excellent strength-ductility synergy. Here in this work, a series of Al₁₄Fe₂₀Cr_xNi_66-x (x = 6, 9, 12, 15 at.%) MPEAs were designed and prepared by combining the Cr/Ni ratio modulation and thermomechanical treatment. The increasing Cr/Ni ratio triggers a transition from single FCC phase to FCC/B2 dual phase, and the B2 phase pins the FCC grain boundaries to refine grain size. Meanwhile, the increased Cr/Ni ratio alters the volume fraction and size of L1₂ and BCC precipitates on the nanoscale. The obtained multiscale hierarchical microstructure significantly enhances the room-temperature strength, raising the tensile strength from ∼905 MPa to ∼1233 MPa, with a slight reduction in elongation (∼32.0 %). The variation in Cr/Ni leads to the formation of a multi-scale heterogeneous structure, which induces the hetero-deformation induced strengthening effect, significantly enhancing the alloy's work hardening ability and ultimately achieving an excellent strength-ductility balance. This study systematically explores the composition-microstructure-mechanical property relationships, offering a pathway to engineer multiscale hierarchical microstructure in the Al-Cr-Fe-Ni system.