Precipitation of β particles in a fully lamellar Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si (at.%) alloy

The precipitation of beta (B2) particles at intermediate temperatures, between 760degreesC and 1050degreesC, is investigated in a fully lamellar TiAl-WMoSi alloy. The beta particles, having a thin-plate shape, usually precipitate on the alpha(2) side of alpha(2)/gamma interfaces at low aging temperatures in an uneven two-dimensional growth mode. While those formed at higher aging temperatures, growing extensively within the alpha(2) plate and into the adjacent gamma lamellae, have an ellipsoid shape. The growth of beta particles at low aging temperatures yields an activation energy of about 366 kJ/mol. It is suggested that at low aging temperatures the growth of beta particles proceeds via an alpha(2) --> beta precipitation process controlled by diffusion of W and Mo along the beta/alpha(2) and beta/gamma interfaces. (C) 2002 Kluwer Academic Publishers.