PRE-YIELD CRACKING IN TiAl ALLOYS

摘要

Fully lamellar and duplex samples of Ti44Al8Nb1B with various surface-finish conditions have been tested in tension and the onset of cracking, which occurs during tensile tests, has been monitored using Acoustic Emission. It has been found that cracking occurs in fully lamellar samples at stress levels below the 0.2% proof stress (625MPa) in all the surface conditions examined. The surface condition has a significant effect on the stress at which cracking is first detected. Cracking occurs at a stress of 480MPa for the as-machined condition and 620MPa for electro-polished surfaces. Annealing of the as-machined sample at a temperature of 700℃ for 0.5h before testing results in cracking at a stress as low as 200MPa. However in the duplex microstructure, cracking does not occur below a stress of 700MPa (i.e. close to the yield stress for these samples) even in the annealed condition. The significance of these observations is discussed in terms of the microstructure of the various samples, the residual stress caused by machining and the nature of the surface of electropolished samples.