Formation of the creep-induced beta(2) 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 u nder 1073 K, 270 MPa, Some ordered beta (2) phase precipitates with the B2 structure formed at the primary creep stage, the orientation relationships are < 111 > B-2// < 101 > gamma// < 1120 > alpha (2), {110}beta (2) //{111} gamma//{0001}alpha (2). EDS analysis shows that beta (2) phase is enriched in Cr and W. beta (2) precipitates always nucleate at gamma/alpha (2) inter faces and near some interface defects. The possible reasons are as follows. (a) at gamma/alpha (2) interfaces there are some areas where the arrangeme nt of atoms is the same as the B-2, structure: (b) W (beta phase stabilizer ) segregates at gamma/alpha (2) interfaces; (c) with the help of the applie d stress. the alloying atoms are dragged by interface dislocations and depo sit near interface defects. There art: two types of interface defects, acco rdingly, the beta (2) phase precipitates grow in two different modes. The g rowth of beta (2) phase precipitates is controlled by the mechanism of allo ying atom-diffusion and limited in alpha (2) laths. During creep progress, the initial lamellae transform into the fine lamellae by the alpha2 --> gam ma phase transformation and gamma --> gamma (t) twinning. The alpha (2)--> gamma phase transformation is a driving force for the growth of beta (2) ph ase. The beta (2) precipitates are beneficial for improving creep resistanc e by preventing dislocation motion and phase transformation, but their effe ct is limited in the range of the initial alpha (2) laths. (C) 2001 Elsevie r Science Ltd. All rights reserved.