The significance of acoustic emission during stressing of TiAl-based alloys. Part II: Influence of cracks induced by pre-stressing on the fatigue life

Abstract Fully lamellar samples of Ti44Al8Nb1B have been tested in tension–tension fatigue, after being pre-stressed, in order to investigate the effect on fatigue life of the cracks which are introduced by pre-stressing. It has been found that pre-stressing samples at 0.95 of the 0.2% proof stress leads to early failure when they are fatigue-tested at stress levels of about 0.65 of the 0.2% proof stress. Samples tested in fatigue at this same stress level, but which have not been pre-stressed, can show significantly longer fatigue lives, although the fatigue lives show a very large scatter. Tinting experiments, carried out after pre-stressing, have shown conclusively that the cracks initiated by pre-stressing act as failure initiation sites during the subsequent fatigue tests. Even in the absence of pre-stressing, samples of Ti44Al8Nb1B fail in fatigue at stress levels of about 0.65 of the 0.2% proof stress in contrast to the fatigue limit of low strength TiAl alloys of about 0.9 of their 0.2% proof stress. The significance of these observations on the use of grain-refined higher strength TiAl alloys under fatigue conditions is discussed.