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

Abstract

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 °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 β-phase diffusion zone. The inter-diffusion zone (IDZ) consisted of (Ni,Cr)3Ge phase. On the other hand, the employment of pure Al powder led to the formation of Al-rich phases such as ζ and δ near the surface and a strong Al-gradient in the Ge-free β-phase coating. While the extent of microstructural degradation was similar for both coatings after 100 h at 900 °C, exposure at higher temperatures resulted in Kirkendall void formation in the NiAl coating, whereas the initial (Ni,Cr,Fe)3Ge 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 °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 °C it led to the complete dissolution of the NiAl coating. The Ge-containing equivalent maintained its integrity and protective behavior in both cases.