Two types of η phase synergistically improving the mechanical properties of LDED CoCrFeNiTi high-entropy alloy

CoCrFeNiTi, a precipitation-strengthened high-entropy alloy (HEA), exhibits remarkable mechanical properties. Among its crucial secondary phases, the eta phase holds significant importance. The CoCrFeNiTi HEA was fabricated using laser-directed energy deposition (LDED). The rapid non-equilibrium solidification process restricts the precipitation of the eta phase. Utilizing the distinctive microstructure characteristics of LDED, the aging treatment was designed to promote the precipitation of two types of eta phases. The eta phase is primarily formed through a discontinuous precipitation reaction and exhibits a typical Blackburn orientation relationship with the matrix FCC phase: [1 (1) over bar0](FCC) parallel to [2 (11) over bar0](eta) and (111)(FCC) parallel to (0001)(eta). At the L interface between the eta phase and the FCC phase, the crystal planes are well-aligned and orderly, resulting in a coherent structure. However, at the W interface, the crystal planes are disordered, leading to an incoherent structure. This interface is clearly visible and is accompanied by a large number of crystal defects, such as dislocations and stacking faults. This unique interface structure significantly influences the nucleation, growth, and evolution of the eta phase, further determining the overall microstructure and related properties of the HEA. The type 1 eta phase appears blocky with an average size of similar to 920 nm and contains a higher proportion of W interfaces and stacking faults (SFs). Conversely, the type 2 eta phase appears rodlike with an average size of similar to 375 nm and a high proportion of L interfaces. The nanohardness of type 1 eta phase is 13.7 GPa, exhibiting inconsistent deformation behavior compared to the FCC phase matrix, such as the pop-in phenomenon. On the other hand, the nanohardness of type 2 eta phase is 12.8 GPa, showing a better match with the matrix and playing a strong role in plastic equilibrium. By strategically designing the aging process and precisely controlling the morphology of the eta phase, it becomes possible to facilitate the simultaneous proper precipitation of both types of eta phase. This advancement contributes to enhancing the strength and promoting good plasticity of HEAs. This study offers an effective methodology for develop high-performance HEAs through microstructure and phase composition adjustments.