The effect of silicon on the near-surface microstructural evolution duringhigh temperature heat treatment of a TiAl based alloy

The near surface microstructural evolution of a Ti-45Al-2Nb-0.4Mn alloy, he at treated at 1350 degreesC was investigated. The specimens were encapsulat ed in quartz capsules prior to the heat treatment and it was observed that during the heat treatment Si evaporated from the capsule wall and deposited on the surface of the specimen. It was also found that deposition of Si on the outer surface of the specimen caused Ti to diffuse outwards to form a titanium silicide layer at the surface, resulting in the formation of an al uminum-rich layer. However, no loss of any alloying element from the specim en was observed. It is, therefore, suggested that Si-rich coating could be appreciably effective in preventing the loss of aluminum during high temper ature exposure of TiAl based alloys. It was also observed that the aluminum -rich layer, which formed near the surface of the specimen during the heat treatment, consisted of single phase gamma and the interior bulk transforme d to massive gamma. The region between the aluminum-rich layer and the mass ively transformed bulk consisted primarily of lamellar structure with fine interlamellar spacing and small amounts of primary gamma -phase along the g rain boundaries. (C) 2001 Elsevier Science B.V. All rights reserved.