The significance of acoustic emission during stressing of TiAl-based alloys. Part I: Detection of cracking during loading up in tension

Samples of fully lamellar Ti44Al8Nb1B and Ti44Al4Nb4Hf0.2Si have been tested in tension at room temperature while they were interfaced to an acoustic emission system. The generation of acoustic signals, at stresses well below the 0.2% yield stress and at higher stresses, has been shown to be due to the formation of cracks within individual lamellar colonies. Data obtained in the micro-strain region in loading-unloading measurements confirmed that there is significant plastic flow at stresses 200 MPa below the 0.2% proof stress of 625 MPa, and it is inferred that this flow gives rise to the cracking. The mechanism whereby such cracking occurs and the possible influence of these cracks on subsequent fatigue behaviour are discussed.