Cyclic Deformation Response And Fatigue Behavior Of Metastable-β Ti-5Al-5V-5Mo-3Cr-0.5Fe Alloy

The improved hardenability and reduced sensitivity to forming variables of Ti-5Al-5V-5Mo-3Cr-0.5Fe (Ti-5553) alloy has made it a candidate to replace Ti-10V-2Fe-3Al and Ti-6Al-4V for landing gear manufacturing in Boeing-787 and Airbus-A380. Systematic work on the fatigue and cyclic deformation behavior of Ti-5553 is meaningful for the damage tolerance designing. In the present study, the cyclic deformation response and the corresponding micromechanical mechanism is thoroughly investigated. In general, it includes two major parts. The first part is about the cyclic deformation response and fatigue damage mechanism of the BCC Ti-5553 withβ-annealed treatment, for fundamental understanding this new material for future applications. The second part systematically investigates the cyclic hardening/softening response and corresponding dislocation configuration of the Ti-5553 alloy with bimodal microstructure.The main contribution of this dissertation can be discussed as follows.1. Mechanical deformation response of theβ-annealed metastable Ti-5Al-5V-5Mo-3Cr-0.5Fe alloy under the condition of pure compressive fatigue stress has been initiated to investigate. This BCC Ti-5553 material demonstrates the initial cyclic softening followed by saturation mechanism irrespective of the employed compressive peak stress level. Dislocation movement upon cycling has been initially studied for better understanding the cyclic deformation behavior of theβ-annealed metastable Ti-5Al-5V-5Mo-3Cr-0.5Fe alloy. TEM investigation reveals dislocation annihilation and detwinning process in the total strain cycling specimens. Such activities, together with the intersection of coherent omega precipitates by moving dislocations, are considered to be responsible for the initial softening; whereas the dislocation dipole flip-flop mechanism is presumably responsible for the cyclic saturation behavior.2. Attempt has been made for the first time to explain the strain localized planar slip behavior by considering the stacking fault energy (SFE) as well as the free-electron-to-atom (e/a) ratio. The progressive observation of surface morphology evolution reveals typical planar slip behavior and early formation of strain localization-induced fatigue microcracks. Using the new criterion: relationship between the SFE and e/a, with consideration of the shearing process of nano-scaled omega precipitates, as well as the as-received microstructure, to analyze the strain localization phenomenon and planar-slip mode in the material.3. Cyclic deformation response of Ti-5553 alloy withβ-αbimodal structure is systematically investigated through total strain controlled fatigue tests. The cyclic hardening/softening behavior was found to be depended strongly on the applied strain amplitude. 4. Results on the relationship between mechanical response and microstructure evolution are presented for the first time. Quantitative TEM investigation revealed that special macroscopic responses in different loading conditions were introduced by the microstructure heterogeneity in the material. The activation and participation in the cyclic deformation of different constituents, namely the soft primaryαphase, the higher-strength transformedβphase and the fine embedded secondaryαprecipitates, play different roles at different strain levels and at different stages of cycling.5. Cyclic deformation response of Ti-5553 alloy withβ-αbimodal structure is systematically investigated through total strain controlled fatigue tests. TEM investigation revealed that special macroscopic responses in different loading conditions were introduced by the microstructure heterogeneity in the material. The ductility in the weaker primary alpha phase was responsible for the earliest slip mode, and naturally for the initial hardening upon low or moderate strain cycling. Dislocation annihilation and microstructure simplification in the transformed beta matrix appear to be reason for the softening behavior at the high stain levels.