Effects of temperature and Al-concentration on formation mechanism of an aluminide coating applied on superalloy IN738LC through a single step low activity gas diffusion process

Effects of temperature and Al-concentration on formation mechanism of aluminide coating applied on a nickel base superalloy IN738LC via low activity gas phase aluminizing process was studied in this research. In addition, coating microstructures were investigated using optical and scanning electron microscopes; EDS and X-ray diffraction (XRD) techniques. The results showed by increasing the temperature from 850 °C to 1050 °C, formation mechanism of the coating changed. At 850 °C coating formed by inward diffusion of Al, and at 1050 °C it was initially formed by inward diffusion of Al, followed by outward diffusion of Ni. The coating layer in the sample coated at 1050 °C using low activity powder consisted of 4 wt% Al and 2 wt% activator, grown outward, while the coating obtained under other conditions grown inward. By increasing the amount of the Al from 2 wt% to 4 wt% and the activator from 1 wt% to 2 wt% in low activity powder mixture, the concentration of diffused Al in the top NiAl layer decreased sufficiently; so that the top layers gradually became rich in Ni from its interface with interdiffusion zone (IDZ). This process finally can lead to annihilation of NiAl phase rich in Al and form a layer of NiAl rich in Ni on the surface layer of the sample.