The influence of Si alloying on the crept microstructure and property of a TiAl alloy prepared by powder metallurgy

The addition of 0.3 at.% Si element into the alloy Ti–47Al–2Cr–1Nb–0.8Ta–0.2W–0.15B prepared by powder metallurgy results in a detrimental effect on creep resistance. To understand the mechanism of Si element action, an overall investigation has been made by combining transition electron microscopy (TEM), high-resolution electron microscopy (HREM), energy dispersive spectroscopy (EDS) and fast Fourier transform (FFT). It was found that Si atoms mainly exist at the α2/γ interfaces, or form Ti5Si3 precipitates and M2B type precipitates. The formation of Ti5Si3 precipitate is beneficial for creep resistance by preventing dislocation motion and improving phase stability. However, due to the short time of hot extrusion and heat treatment, most of the Si atoms remain dissolved in the matrix. Si solute promotes the formation of vacancy, and leads to microstructure instability, such as the dissolution of the α2 laths and the recrystallization of the γ phase. Si solute can also promote dislocation climbing. Cooperation of these effects leads to a reduction of creep resistance. Moreover, a novel mechanism of the α2→γ phase transformation has been found in the present work.