The effect of microstructure on fatigue crack propagation of α+β titanium alloys: In-situ observation of short fatigue crack growth

Fatigue crack initiation and early stage of crack growth for Ti–6Al–4V and Ti–4.5Al–3V–2Fe–2Mo were investigated on notched specimens. In-situ observation on fatigue cracking processes was made on a fatigue testing machine mounted in a scanning electron microscope. It was found that Ti–4.5Al–3V–2Fe–2Mo alloy with very fine α grains of ≈2 μm exhibited a much higher fatigue strength at 107 cycles than that of Ti–6Al–4V alloy. However, an opposite trend in fatigue life was obtained in the short life regime for the materials tested in the present study. The observed fatigue behavior in both the long and short life regimes is discussed in terms of metallurgical factors responsible for the fatigue behavior of the present materials.