MECHANISMS OF FATIGUE-CRACK GROWTH IN TI-48AL AT AMBIENT AND ELEVATED-TEMPERATURE

Crack-tip deformation in Ti-48Al at room-temperature occurs by a combination of slip and deformation-induced twinning. Crack-tip deformation at 700 °C occurs pmdominantly by conventional reversed slip mechanisms. The slip components ofdeformation are partially reversible, but the twimring components of deformation are irreversible. Hence, the levels of kinematic irreversibility are greater at room-temperature due to deformation-induced twnming. The moderate levels of twin toughening (& - 1.66) are not sufticient to reverse the trends in the fatigue crack growth rate data, which are dominated by crack-tip deformation phenomena. The fatigue crack growth rates at room-temperature are due to the balance between the degree of kinematic irreversibility and twin toughening promoted by detbrmation-induced twin. Slower fatigue crack growth rates and higher fatigue thnxholds observed at 700 °C are attributed to the combined effects of oxide-induced crack closure and lower levels of kinematic irreversibility at 700 °C. © 1995.