Comparative study of crack growth behaviors of fully-lamellar and bi-lamellar Ti-6Al-3Nb-2Zr-1Mo alloy

The fatigue crack growth (FCG) behaviors of fully-lamellar and bi-lamellar Ti-6Al-3Nb-2Zr-1Mo alloy were investigated comparably. The FCG rate of bi-lamellar structure is lower than that of fully-lamellar structure, especially at the low stress intensity factor range (Delta K < 40 MPa m(1/2)). The fatigue crack growth path, deformed sub-structure and surface damage morphology were observed by SEM and TEM. For the fully-lamellar structure, because the same orientation of alpha lamellae and Burgers relationship between alpha and beta lamellas in the alpha colony, the fatigue cracks often grow along a straight line in the alpha colony by cutting the beta lamellas. For the bi-lamellar structure, because the resistance of secondary alpha (alpha(s)) precipitations in the beta lamellas to dislocation slips, the frequency of crack propagation along the alpha/beta interface increases. The fatigue cracks often grow along a zigzag in the alpha colony, which results in the fatigue crack growth path is more circuitous than that of the fully-lamellar structure in alpha colonies, and this is the main reason that the FCG rate of bi-lamellar structure is lower than that of fully-lamellar structure. This research has an important engineering significance to improve the low cycle fatigue (LCF) performance of the coarse lamellar structured Ti alloys. (C) 2019 Elsevier B.V. All rights reserved.