Effect of SiO2 Addition in Pack Aluminizing Mixture on Phase Evolution of Si-Modified Intermetallic Layers on IN800HT Alloys

Abstract

Effect of SiO₂ addition on aluminides formation in a single-step high-activity pack aluminization on Incoloy 800HT is studied. SiO₂ powder was added into pack aluminizing mixture at different theoretical ratios of the reduced Si and the remained Al, i.e. Si/(Si+Al), in range of 0-37.5 at.%. Aluminization was carried out at 1,000°C for 4 h. Microstructures, chemical compositions and phase evolution of the coatings were examined using scanning electron microscope (SEM), energy dispersive spectroscope (EDS) and X-ray diffractometer (XRD). For the undoped condition, the coatings showed three layers: (i) an outer layer as a mixture of various Al-rich intermetallics, (ii) a hyperstoichiometric β-(Fe,Ni)Al middle layer and (iii) an interdiffusion zone; it implies the formation is governed by Al inward diffusion. Although aluminides of the Si-doped specimens still showed three-layers structure, low Al content intermetallic prevailed in the outer layer while a thickness significantly decreased with higher SiO₂ content. Silicon was found either as the dissolved Si in aluminide coatings and Si segregation at the interdiffusion layer or as separated domains in β-(Fe,Ni)Al layer with restricted concentration. The results emphasize a possibility of Al-Si co-deposited single-step aluminization using SiO₂ as silicon source along with a reduction in the thermodynamic diffusing Al activity.