Abnormal near-threshold fatigue crack propagation of Ti alloys: role of the microstructure

This study deals with a perplexing phenomenon recently reported on several titanium alloys which were found not to exhibit a threshold in ambient air at room temperature or moderate temperature when tests are run at high mean stress levels and very low stress intensity factor ranges. In such conditions, these materials revealed a stationary regime with a constant growth rate. A comparative study of the condition for the occurrence of this phenomenon has been conducted on three Ti alloys, Ti6246, Ti17 and Ti6242 and other alloys reported in the literature. A special attention is focused on the role of the microstructure on the critical mean stress level. The sensitivity of a titanium alloy to this abnormal behaviour is proposed to be evaluated in term of the ratio of the critical stress intensity factor level to initiate this phenomenon against the fracture toughness of the same material. The volume fraction and the morphology of the α phase appear as a critical microstructure parameters.