The effects of lamellar features on the fracture toughness of Ti-17 titanium alloy

The effects of lamellar features on the fracture toughness of Ti-17 titanium alloy are studied in the present paper. Three cooling methods were used to prepare different lamellar features of Ti-17 titanium alloy after β forging. Then the same solid solution plus aging treatment were conducted to get the final microstructures. The results show that the microstructure with long and thick needle-like α platelets gets higher fracture toughness as well as strength than the microstructure with short rod-like α platelets. This seemingly “abnormal” phenomenon can be explained based on the theory that the fracture toughness is attributed to two major contributions, namely the crack path tortuosity (extrinsic part) and material plastic deformation along the crack path (intrinsic part). The respective contribution of the plasticity and crack path tortuosity to the fracture toughness of Ti-17 alloy are quantitatively evaluated based on the existent models proposed by previous researchers. The results show that the intrinsic contributions for the three microstructures with different lamellar features do not show a big difference. However, their extrinsic contributions are dramatically different. The microstructure which contains the longest and thickest α platelets gets the most rugged crack propagation path and moderate plasticity among the three microstructures, which results in the highest fracture toughness. Moreover, due to the nature of the near-β Ti-17 alloy, the long and thick α platelets in microstructure also get high aspect ratios, which results in high interfacial strengthening effect. Thus for Ti-17 alloy studied in the present work, the long and thick α platelets in microstructure can realize a good combination of fracture toughness and strength.