Low Cycle Fatigue Life Threshold for Titanium Aluminides

Although there exists a good understanding of dislocation glide mechanisms during fatigue of titanium aluminides,([1]) fatigue life prediction remains a crucial issue because of the low amount of macroscopic plastic strain and the existence of microstructural inhomogeneities. The concept proposed here aims at a threshold value, allowing a reasonable low cycle fatigue life for isothermal and thermomechanical fatigue conditions. It can be stated that under isothermal testing conditions a maximum strain amplitude of Delta epsilon/2 = 0.35% (corresponding to a stress amplitude of about Delta sigma/2 = 500 MPa) for temperatures up to 750 degrees C is feasible for TNB-V2 in order to reliably reach a sufficient LCF life. Concerning satisfactory thermomechanical fatigue life the proposed concept suggests that the acting combination of mean stress and strain amplitude needs to be located below the Goodman line. It can be concluded that with increase in maximum cyclic temperature and temperature range, the tolerable strain amplitude decreases. Because of the well-documented unfavorable combination of tensile mean stresses and environmental attack during OP-TMF, an acceptable strain amplitude for TMF conditions is significantly lower than under isothermal conditions.