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 ␣ 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 ␤ 0 along lamellar interfaces, with the ␤ 0 size increasing with aging time. The ␤ 0 precipitates nucleate and grow in the ␣ 2 lamellae. Concurrently, with the formation of ␤ 0 , the ␣ 2 decomposes into discontinuous lamellae. Aging to precipitate ␤ 0 along lamellar interfaces increases the 760°C tensile strength (with a slight reduction of ductility) and reduces the instantaneous creep strain, since ␤ 0 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 ␤ 0 along interfaces substantially reduces the primary creep strain, primarily due to the influence of ␤ 0 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.

Creep behaviour of a cast TiAl-based alloy for industrial applications

“…In addition, discontinuous a 2 -lamellae contain fine needlelike particles with average width of 25 nm and length of 200 nm, which were identified as B2-phase [13,15]. As reported by Seo et al [16,17], fine B2 precipitates formed at the a 2 /g interfaces before creep testing improve the creep resistance of the alloy by impeding interface dislocation mobility, reducing the generation of g matrix dislocations from lamellae and preventing dislocations from passing through lamellar interfaces. In the case of the ABB-2 alloy, the strengthening effect of the B2 precipitates is enhanced by co-precipitation of fine Ti 5 Si 3 particles along the lamellar interfaces with a similar effect on the dislocation mobility.…”

In situ neutron diffraction studies on the elevated-temperature deformation behavior of a TiAl–W alloy

“…1,2 Various microstructures can be obtained through heat treatment, in particular, precipitation-strengthened ␥-TiAl alloys exhibit good elevated-temperature strength and modulus. 3 For many hightemperature structural applications, the allowable total deformation is only about 1%. Since a significant portion of this can occur during primary creep, there is a need for better understanding of this phenomenon.…”

“…The grain size of the ␥-TiAl was about 150 m. Details of the processing and microstructures of the Ti-48Al-2W alloy used in this study can be found in the literature. 3 Cylindrical tensile specimens, with a nominal gauge diameter of 6.5 mm, were machined for two series of in situ neutron-diffraction measurements using the SMARTS instrument at Los Alamos Neutron Science Center. 5 During the quasistatic tension test, the applied stress was increased incrementally with steps of 20 up to 260 MPa (macroscopic 0.2% yield point) and then with steps of 10 to a maximum load of 435 MPa.…”

Isothermal oxidation behaviour of beta gamma powder metallurgy TiAl–4Nb–3Mn alloys

“…It is noticeable that the lamellar structure no longer exists in the diffusion zone as shown in Fig. 10a and b. Seo et al 22 reported that during the aging of PM Ti-48Al-2W alloy at 950uC, the a 2 lamellar became discontinuous as the aging time increased. In this study, the discontinuous a 2 lamellae were also observed in the area below the diffusion zone as shown in Fig.…”