Microstructural study of pre-yielding and pre-yield cracking in TiAl-based alloys

Samples of Ti44Al8Nb1B (at.%), Ti46Al8Nb (700 ppm oxygen) and of the alloy K5 (Ti-46Al-2Cr-3Nb-0.2W-0.15B-0.4C (800 ppm oxygen) which have been tested in tension at stresses below their macroscopic yield stresses, have been examined using transmission electron microscopy. In lamellar samples it has been shown that dislocation multiplication takes place at stresses from about 400 MPa, well below their 0.2% proof stress. In samples with a duplex microstructure no dislocation activity is observed until the applied stress exceeds the macroscopic yield stress. Deformation twinning is initially observed in lamellar samples at stresses just below the 0.2% proof stress. No acoustic emission events are observed corresponding to the twinning seen in the fully lamellar samples. These observations are discussed in terms of the different pre-yielding behaviour of lamellar and duplex samples and in terms of acoustic emission signals, pre-yield cracking and pre-yield twinning. It is concluded that pre-yield cracking is caused by slip in gamma grains in near fully lamellar samples and by slip in lamellae longer than about 70 μm in fully lamellar samples. It is further concluded that all pre-yield acoustic emission is caused by cracking.