An investigation of the effects of microstructure on the fatigue and fracture behavior of α₂ + β forged Ti-24Al-11Nbforged Ti-24Al-11Nb

The results of a recent study of the effects of Widmanstätten and basket weave microstructures on the fracture toughness and fatigue crack growth behavior of Ti-24Al-11 Nb are reported Intrinsic and extrinsic toughening components due to crack blunting and bridging by the β phase, crack deflection, and microcracking are computed from existing crack-tip shielding models. Predictions of fracture toughness and fatigue thresholds obtained by the superposition of extrinsic toughening components are compared with measured values obtained from compression precraked single edge notch (SEN) bend specimens. The results indicate that the continuity of the β matrix between the α₂ laths is important for the effectiveness of crack-tip blunting mechanisms. Widmanstätten microstructures obtained by annealing solely in the α₂ + β phase field are shown to promote crack deflection and unstable room-temperature fatigue crack growth rates. i.e., crack growth rates that increase after further thermal exposure in the α₂ + β and field. Basket weave microstructures produced by two-stage annealing (TSA) in the β and α₂ + β phase fields are shown to promote crack bifurcation and deflection and significant improvements in fatigue crack growth resistance when the β anneal is followed by a furnacecool. The article highlights the significant role of microcracking in the fatigue propagation mechanism. The micromechanisms of fatigue and fracture are also discussed for the microstructures examined.