Effect of Microstructure on Fatigue Crack Growth Behavior in Ti-6Al-4V Alloy

The effect of microstructure on fatigue crack growth behavior under constant stress amplitude was investigated using CT specimens of two kinds of Ti-6Al-4V alloy. Both of them were under the same annealing condition and consisted of α+β phases, but their microstructures showed obvious distinction because of the different reduction of plate in the β and α+β rolling processes. One of them was composed of spherical α phase surrounded by thin β phase (material A) and the other possessed longitudinally stretched α and β phases (material B). For both materials, rough crack surfaces were formed in the low Kmax region and roughness-induced crack closure was added to plasticity-induced one while relatively flat crack surfaces were built in the high Kmax region where plasticity-induced crack closure was dominant. However, the crack surface morphologies of material A and B were quite distinct in the low Kmax region due to their microstructures, which had a pronounced effect on crack growth and crack closure behavior. The effect of crack surface morphology was qualitatively evaluated by the optical observation of crack surface profiles and quantitatively by the measurement of the effective stress intensity factor ΔKeff.