Primary creep behavior of Ti-48Al-2W as a function of stress and lamellar morphology

Abstract

The fully lamellar microstructure of powder metallurgy Ti-48Al-2W after cooling from the α region to 1280 °C, followed by air cooling and aging at 950 °C for up to 96 hours, is presented. Aging times as short as 5 hours result in acicular-shaped precipitates of W-rich β0 along lamellar interfaces, with the β0 size increasing with aging time. The β0 precipitates nucleate and grow in the α2 lamellae. Concurrently, with the formation of β0, the α2 decomposes into discontinuous lamellae. Aging to precipitate β0 along lamellar interfaces increases the 760 °C tensile strength (with a slight reduction of ductility) and reduces the instantaneous creep strain, since β0 precipitates at lamellar interfaces hinder interface dislocation mobility. The deformed microstructures from interrupted creep tests at 140 to 276 MPa at 760 °C indicate that the precipitation of β0 along interfaces substantially reduces the primary creep strain, primarily due to the influence of β0 on interface dislocation emission and motion. These results are discussed in terms of the influence of lamellar morphology on the instantaneous creep strain and primary creep transient, and the possible creep mechanisms are highlighted.