Microstructure evolution and elemental diffusion behavior near the interface of Cr₂AlC and single crystal superalloy DD5 at elevated temperatures

As one of the promising MAX phase materials for high-temperature applications, Cr₂AlC is considered as a potential substitution bond coat material in thermal barrier coating systems. In this paper, the microstructure evolution and elemental diffusion behavior near the interface of the diffusion couple composed of Cr₂AlC and single crystal superalloy DD5 were investigated at 1100 °C, 1150 °C, and 1200 °C. Elemental interdiffusion between Cr₂AlC and DD5 occurs significantly, resulting in the formation of a thick layer of Kirkendall holes after 20 h heat treatment at 1100 °C and higher temperatures. The outward diffusion of Ni into Cr₂AlC and the inward diffusion of Al into DD5 alloy causes the formation of β-NiAl matrix embedded with dispersed Cr₇C₃ phase. Simultaneously, the precipitation of σ-TCP phase and degradation of the γ/γ′ matrix occurs in the alloy. Additionally, TaC, M₂C (where M = Ta, W, Cr), and M₂₃C₆ (M = Cr, Re, W) compounds are formed near the interface along with the dissolution of σ-TCP phases. It is further found that Al in Cr₂AlC exhibits the highest average effective diffusion coefficient among the four dominant diffusing elements. It also displays the lowest diffusion activation energy which is due to its relatively weak Cr[sbnd]Al and Al[sbnd]Al bonds.