Improving the mechanical performance of FeCoNi compositionally complex alloys through Al and V additions: Optimizing phase stability and microstructure

FCC-structured compositionally complex alloys (CCAs) are recognized for their excellent ductility at room temperature, but their relatively low strength limits their structural applications. This study addresses the strength-ductility trade-off by investigating the effects of Al and V contents on the phase stability, microstructure, and mechanical properties of (FeCoNi)₇₅V_25−xAl_x (x = 0–25 at.%) CCAs. The Al and V additions promote a phase transition from the ordered L1₂ to the disordered FCC phase upon heating, forming a multiscale hierarchical dual-phase structure that significantly enhances mechanical performance. Among them, the annealed (FeCoNi)₇₅V₁₆Al₉ alloy shows an impressive ultimate tensile strength of approximately 1504 MPa and a tensile elongation exceeding 15 %. Experimental observations indicate that the FCC phase is reinforced by L1₂ nanoprecipitates within grains and dot-like or needle-like L2₁ precipitates at grain boundaries. Additionally, the BCC islands are strengthened by L2₁ nanoprecipitates and toughened by dot-like or lath-like FCC/L1₂ phase. These combined strengthening mechanisms synergistically endow this CCA with its remarkably high yield strength. Moreover, the multiscale heterogeneous distribution of grains and phases, along with the diverse precipitate structures, enables the alloy to maintain high strain-hardening rates during plastic deformation, thus achieving an optimal balance of strength and ductility. These findings thus offer valuable insights for designing CCAs with improved mechanical properties.