Effect of microstructure on small fatigue crack initiation and propagation characteristics of Ti-6Al-7Nb alloy

The effect of microstructure on fatigue strength, small fatigue crack initiation and propagation characteristics in biomedical Ti-6Al-7Nb and Ti-6Al-4V ELI alloys were investigated. In the Widmanstatten a structure, the tensile strength of Ti-6Al-7Nb is slightly lower than that of Ti-6Al-4V ELI. In equiaxed a structure, the tensile strength of Ti-6Al-7Nb is apparently lower than that of Ti-6Al-4V ELI. The elongation of both alloys with same structure is nearly the same in each microstructure. The fatigue strength of Ti-6Al-7Nb alloy nearly equals that of Ti-6Al-4V ELI alloy with the Widmanstatten alpha structure. While the fatigue strength of Ti-6Al-7Nb alloy is lower than that of Ti-6Al-4V ELI in equiaxed alpha structure, in particular, in the high-cycle fatigue region. The fatigue crack initiation in each titanium alloy with equiaxed alpha (volume fraction of primary alpha : 50%) and Widmanstatten alpha structures mainly occurs at the primary alpha grain boundary while in the colony and prior beta grain which are about 45 degree inclined to the stress axis direction in Widmanstatten alpha structure. The fatigue life increases when the small fatigue crack propagation resistance increases with the volume fraction of primary alpha. When the cooling rate after solutionizing is increased, the fatigue strength of Ti-6Al-7Nb, approaches that of Ti-6Al-4V ELI.