Phase evolution of surface-modified Incoloy 825 superalloy using pack aluminization

The objective of the study is to investigate the phase evolution of the aluminized surface of iron-concentrated nickel-based superalloys. Aluminization is carried out via pack cementation at constant temperature of 900 °C at three processing times of 1, 2.25 and 4 h. Backscattered images and X-ray diffractions are used for phase identification. Small incident angle X-ray diffraction applied on the surface and subjected to repetitive polishing elucidate a formation sequence of iron aluminides. The overall thickness of the aluminized layers is proportional to the processing time squared. The samples are composed of 4 zones, identified by elemental concentration profiles. Only iron aluminides are observed which is in agreement with thermodynamic- and atomic diffusion. The results conclude that aluminization on alloys is similar to oxidation and that nickel aluminide forms with great difficulty on the surface of an aluminized Incoloy 825 alloy.