Microstructure and hot corrosion properties of an Al-Y coating on TiAl alloy

To improve the hot corrosion performance of TiAl alloys, an Al-Y coating was prepared by a pack cementation process. The effect of catalysts on the structure of the Al-Y coating and its thermal shock resistance was studied. The thermal corrosion performance of both the TiAl alloy and the coating in a mixed molten salt system of 25% NaCl+75% Na2SO4 (wt.%) was comparatively investigated. The results showed that the Al-Y coatings prepared with different catalysts had similar structures and good metallurgical bonding with the substrate, which was mainly composed of an Al-rich outer layer, a TiAl3 middle layer, and a TiAl2 inner layer. However, the coating prepared using NH4Cl as the catalyst was more uniform and denser than those formed using NaF and AlCl3 center dot 6H(2)O. The Al-Y coating improved the thermal shock resistance of the TiAl alloy under thermal shock at 1273 K. Hot corrosion tests showed that the lamellar alpha(2)-Ti3Al phase in the TiAl alloy first underwent selective corrosion by O and S atoms in the medium, followed by catastrophic corrosion. Moreover, the Al-Y coating formed a dense Al2O3 film in the early stage of hot corrosion, which effectively protected the permeable layer. Upon extending the hot corrosion time, the coating gradually cracks due to the internal and external diffusion of atoms and corrosion stress. The formed cracks served as diffusion channels for S and O atoms, and the TiAl3 phase in the coating continued to decompose, providing more Al atoms to the cracks. This eventually formed a dense layer of Al2O3 to compensate for the cracks, delaying the internal diffusion rate of S and O atoms, and significantly improving the thermal corrosion resistance of the TiAl alloy.