An investigation of the small fatigue crack growth behavior in Ti-6.5Al-2Zr-1Mo-1V alloy

In this paper, we describe the experimental generation of a small fatigue crack in an SENT (single-edge notched tension) specimen of Ti-6.5Al-2Zr-1Mo-1V alloy. The growth behavior of the crack was analyzed using optical microscopy, scanning electron microscopy, transmission electron microscopy and other methodologies. We determined that when the crack length was approximately equal to the size of the plastic deformation zone at the crack tip, a knee point appeared in the da/dN-DK curve. At the knee point, the crack fracture mode transitioned from a crystallographic-faceted fracture to a striation-type fracture. Before the knee point, the change in the crack growth mode caused a zigzag crack growth path. The phase boundary and crack closure resulted in an irregular FCGR (fatigue crack growth rate). After the knee point, the small crack propagated forward by sliding while retaining traces of slip bands at the crack tip. The directions of the slips in the plastic deformation zone could be affected by the crystal orientations. Therefore, the crack growth path transitioned from a zigzag to a linear path due to the similar crystal orientation of the parallel a lamellae. In addition, the crack driving force far exceeded the resistance of the microstructures, which resulted in a significant acceleration of the FCGR. (C) 2016 Elsevier B. V. All rights reserved.