Formation of the creep-induced β2 phase and its influence on deformation in a fully-lamellar TiAl alloy

Alloy Ti–46.5–2Cr–3Nb–0.2W with a fully lamellar microstructure was crept under 1073 K, 270 MPa, Some ordered β2 phase precipitates with the B2 structure formed at the primary creep stage, the orientation relationships are <111̄>β2//<101̄>γ//<112̄0>α2, {110}β2 //{111}γ//{0001}α2. EDS analysis shows that β2 phase is enriched in Cr and W. β2 precipitates always nucleate at γ/α2 interfaces and near some interface defects. The possible reasons are as follows: (a) at γ/α2 interfaces there are some areas where the arrangement of atoms is the same as the B2 structure; (b) W (β phase stabilizer) segregates at γ/α2 interfaces; (c) with the help of the applied stress, the alloying atoms are dragged by interface dislocations and deposit near interface defects. There are two types of interface defects; accordingly, the β2 phase precipitates grow in two different modes. The growth of β2 phase precipitates is controlled by the mechanism of alloying atom-diffusion and limited in α2 laths. During creep progress, the initial lamellae transform into the fine lamellae by the α2→γ phase transformation and γ→γt twinning. The α2→γ phase transformation is a driving force for the growth of β2 phase. The β2 precipitates are beneficial for improving creep resistance by preventing dislocation motion and phase transformation, but their effect is limited in the range of the initial α2 laths.