Experimental Determination of Isothermal Sections in the Ni–Al–Cr–Ru Quaternary System: Implications for Ni-Based Superalloys and High-Entropy Alloys

The phase equilibria of the Ni–Al–Cr–Ru quaternary system were systematically investigated using the equilibrated alloy method combined with scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). This study focuses on three key isothermal sections within the system: 55 at.% Al at 1423 K, 55 at.% Ni at 1173 K, and 60 at.% Ni at 1423 K. In the 55 at.% Al section at 1423 K, a four-phase equilibrium region comprising Bcc(Cr), β-(Ni,Ru)Al, Al₈Cr₅, and Al₂Ru, along with three three-phase regions, was identified. Complete mutual solubility between the NiAl and AlRu phases was achieved with approximately 10 at.% Cr. In the 55 at.% Ni section at 1173 K, two four-phase and seven three-phase equilibrium regions were observed. The addition of Cr was found to promote the emergence of the Fcc(Ni) + β-(Ni,Ru)Al + Ni₃Al three-phase region and the Fcc(Ni) + β-(Ni,Ru)Al two-phase region. Critically, Cr addition enabled complete solubility between the β₁ (NiAl) and β₂ (AlRu) phases even at 1173 K. For the 60 at.% Ni section at 1423 K, while no four-phase equilibrium was found, two three-phase regions—(Ni,Ru)Al + Hcp(Ru) + Fcc(Ni) and (Ni,Ru)Al + Ni₃Al + Fcc(Ni)—were confirmed. Notably, the (Ni,Ru)Al + Fcc(Ni) two-phase region exhibited a wide compositional range. This work provides essential experimental phase diagram data and insights for the design of Ni–Al–Cr–Ru-X high-entropy alloys and next-generation Ni-based superalloys.