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

The near surface microstructural evolution of a Ti–45Al–2Nb–0.4Mn alloy, heat treated at 1350°C was investigated. The specimens were encapsulated 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 aluminum-rich layer. However, no loss of any alloying element from the specimen was observed. It is, therefore, suggested that Si-rich coating could be appreciably effective in preventing the loss of aluminum during high temperature 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 γ and the interior bulk transformed to massive γ. The region between the aluminum-rich layer and the massively transformed bulk consisted primarily of lamellar structure with fine interlamellar spacing and small amounts of primary γ-phase along the grain boundaries.