Influence of microstructure on fatigue crack growth behavior of Ti-6Al-3Nb-2Zr-1Mo alloy: Bimodal vs. lamellar structures

The influence of microstructure (bimodal and lamellar structure) on the fatigue crack growth (FCG) rate of Ti-6Al-3Nb-2Zr-1Mo alloy was investigated using compact-tension (CT) specimens. The da/dN curves showed that the FCG rate of lamellar structure was slower than that of bimodal structure, especially in the early stage of crack growth. The FCG paths in the early stage were observed by optical microscopy and scan electric microscopy. The results revealed that the FCG paths of lamellar structure were more tortuous than that of bimodal structure. The frequency of crack deflection and bifurcation of the lamellar structure is significantly higher than that of bimodal structure. The dominant mode of crack propagation for bimodal structure is crossing beta trans and alpha p, and for lamellar structure is crossing alpha colonies. Fatigue crack growth mechanisms at the early stage have been studied by observation of crystalline orientation near the crack propagation path at the crack tip, in combination with the slip trace analyzing. The results show that the fatigue crack growth directions in alpha p or beta trans for bimodal structure and alpha colony for lamellar structure are mostly consistent with that of prismatic or basal plane trace. This indicated that the cracks propagation modes of crossing alpha p or beta trans in bimodal structure and crossing alpha colonies in lamellar structure are resulting from the slip band cracking of the microstructure at crack tip, as the evidence of {10 (1) over bar0}11 (2) over bar0 prismatic dislocation lines were observed in alpha p or beta trans for bimodal structure and alpha colony for lamellar structure by transmission electron microscope (TEM) near the fracture surface.