The effects of colony microstructure on the fatigue crack growth behavior for Ti-6A1-2Zr-2Sn-3Mo-1Cr-2Nb titanium alloy

The fatigue crack growth (FCG) behavior for materials applied in aerospace field is very important, for the majority of incidents happened were due to fatigue fracture. Six double annealing processes were conducted to get different colony features for Ti-6A1-2Zr-2Sn-3Mo-1Cr-2Nb alloy. Then the effects of the colony microstructure on the FCG behavior of this alloy are studied. The results show that the FCG curves for Ti-6A1-2Zr-2Sn-3Mo-1Cr-2Nb alloy under four processes are distinguished by a clearly defined transition point. The values of δK_T (transitional stress intensity range) under these processes are almost the same, about 15MPa√m. The fatigue crack growth rates (FCGR) for six microstructures vary obviously at low δK-levels (δK≤δK_T) and then tend to approach each other at high δK-levels (δK≥δK_T). Above phenomena are all due to the difference of crack deflection effect caused by different colony microstructures. The α platelet thickness and colony size are two key parameters determining the crack deflection effect of colony microstructure. For microstructure with thin α platelets, the main crack cuts the α platelets easily and propagates in a flat way. In this situation, the colony size exerts no influence on crack propagation path. For microstructure with thick α platelets, the main crack tends to propagate along the colony boundaries or the α platelet spacings. In this situation, the colony size can greatly influence the crack propagation path. The bigger the colony size, the more the tortuous crack path.