Cyclic deformation response and micromechanisms of Ti alloy Ti–5Al–5V–5Mo–3Cr–0.5Fe

Cyclic deformation response of Ti-5553 alloy with β–α bimodal structure is systematically studied through total strain controlled fatigue tests. Results on the relationship between mechanical response and microstructure evolution are presented in this report. It was found that cyclic hardening/softening behavior of the alloy depended strongly on the applied strain amplitude. At low strain levels, the material showed moderate hardening behavior at the early stage of cycling and then behaved elastically; whereas at high strain amplitudes, the alloy showed softening behavior from the beginning to the end of cycling. In the intermediate strain range, moderate hardening at the beginning followed by softening was detected. Transmission electron microscopy investigation revealed that such special macroscopic responses were due to the microstructure heterogeneity in the material. The activation and participation in the cyclic deformation of different constituents, namely the soft primary αp phase, the higher strength transformed β phase and the fine embedded αs precipitates in the β matrix played different roles at different strain levels and at different stages of cycling. Thus, the aforementioned specific cyclic hardening/softening behavior was introduced. Specifically, dislocation activities including activation of multiple slip systems and annihilation of pre-existing dislocations in the primary αp phase were found to play a very important role in the overall cyclic deformation behavior.