Microstructural degradation and interdiffusion behavior of NiAl and Ge-modified NiAl coatings deposited on Alloy 602 CA

摘要

Ge-modified and conventional nickel aluminide coatings were deposited on Alloy 602 CA via a single-step slurry manufacturing process and diffusion heat treated at 800 degrees C for 5 h. The effect of Ge-addition to slurry nickel aluminide coatings was investigated with respect to the microstructural degradation of coatings during isothermal exposure at different temperatures for 100 h. Addition of Ge resulted in the formation of Cr- and Ge-rich precipitates distributed within a beta-phase diffusion zone. The inter-diffusion zone (IDZ) consisted of (Ni,Cr)(3) Ge phase. On the other hand, the employment of pure Al powder led to the formation of Al-rich phases such as zeta and delta near the surface and a strong Al-gradient in the Ge-free beta-phase coating. While the extent of microstructural degradation was similar for both coatings after 100 h at 900 degrees C, exposure at higher temperatures resulted in Kirkendall void formation in the NiAl coating, whereas the initial (Ni,Cr,Fe) 3 Ge layer transformed into chromium carbides, which in-situ formed a dense interlayer below the aluminide and obstructed interdiffusion in the Ge-containing coating. At 1050 degrees C, the NiAl coating delaminated due to the severe Kirkendall porosity, which resulted in the oxidation of the interface and nitridation of the alloy and at 1200 degrees C it led to the complete dissolution of the NiAl coating. The Ge-containing equivalent maintained its integrity and protective behavior in both cases.