Phase Equilibria of the Ni-Al-Cr-Y Quaternary System and the Ni-Al-Y Ternary System at 900 °C

Abstract

The phase relationships of the Ni-Al-Cr-Y quaternary system can provide theoretical guidance for the design of MCrAlY (M = Ni and/or Co, Fe) overlay coatings on the surface of superalloys. The 900 °C isothermal section of the Ni-Al-Cr-Y quaternary system with Ni fixed at 55 at.% and the Ni-Al-Y ternary system were studied experimentally by using equilibrated alloys, with x-ray diffraction and scanning electron microscopy with energy dispersive spectroscopy. Thirty three-phase regions were confirmed, and six three-phase regions were determined in the isothermal section of the Y-poor side in Ni-Al-Y ternary system at 900 °C. Aluminium could replace Ni in the line compounds Ni₅Y, Ni₃Y and Ni₂Y; similarly, Ni could replace Al in the line compounds Al₂Y, AlY and Al₂Y₃. The experimental results showed that there are twelve ternary compounds stable at 900 °C, and the τ₁₃-Ni₆Al₁₆Y₃ phase and τ₁₄-NiAl₃Y_2.25 phase are the new ternary compounds found in this work with composition ranges from about 23.5-24.9 at.% Ni, 63.1-64.8 at.% Al, 11.2-12.1 at.% Y and 16.3-17.0 at.% Ni, 47.2-48.2 at.% Al, 35.5-36.8 at.% Y, respectively. In the isothermal section of the Ni-Al-Cr-Y quaternary system at 900 °C with 55 at.% Ni, six four-phase regions were experimentally determined, and one four-phase region was inferred. The results indicated that (Cr) could equilibrate with all phases in the quaternary system. In the present work, no new quaternary compounds were found.