Creep-induced phase transformations in a Ti-Al alloy

The alloy Ti-46.5 at.% Al-2 at.% Cr-3 at.% Nb-0.2 at.% W with a fully lamellar microstructure was subjected to creep deformation at 1073 K and 270 MPa. After creep deformation, fine lamellae consisting of gamma and alpha (2) laths and beta precipitates were observed, which do not exist in the undeformed material, and these observations are explained by alpha (2) --> gamma and alpha (2) --> beta -phase transformations. To understand the mechanisms of phase transformations, an overall investigation has been made by combining transmission electron microscopy, high- resolution electron microscopy, energy- dispersive spectroscopy and the fast Fourier transform technique. It was found that the two kinds of phase transformation have a close relationship but proceed by different mechanisms. The alpha (2) --> gamma -phase transformation is a kind of stress-induced phase transformation, while the alpha (2) --> beta -phase transformation is due to the segregation of the elements Cr and W.