Microstructural Evolution and Oxidation Behavior of Alloy Ni-13Mo-13Cr-9W-3Fe-3Ti-2Al During Isothermal Exposure at 900 °C

The microstructure evolution and oxidation behavior of alloy Ni-13Mo-13Cr-9W-3Fe-3Ti-2Al (alloy K4208), a newly developed Ni-Cr-W-Mo-based wear-resistant superalloy, were investigated by isothermal aging at 900 °C in air. Thermodynamic calculations, microstructure observations, electron probe microanalysis (EPMA), and precise microchemical analysis were used to study the microstructure evolution and oxidation mechanism of alloy K4208. The results showed that alloy K4208 was mainly composed of a P-like phase, M₆C (and M₁₂C) carbide, γ′ phase, μ phase, and γ matrix. During the 900 °C aging, the γ′ phase coarsened into irregular shapes, the M₆C (and M₁₂C) carbide decomposed, and the μ phase dissolved into the matrix, providing abundant Cr, Mo, and W for the growth of the P-like phase. Hence, the relationship between the different phases was established by element redistribution. In addition, the oxidation of alloy K4208 was a diffusion-controlled process, leading to the formation of a stable oxide layer composed of three parts: a thin layer of NiCr₂O₄ spinel located on the outermost side, a continuous Cr₂O₃ layer decorated with TiO₂ in the middle, and an internal Al₂O₃ layer.