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 Widmanstätten α structure, the tensile strength of Ti-6Al-7Nb is slightly lower than that of Ti-6Al-4V ELI. In equiaxed α 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 Widmanstätten α structure. While the fatigue strength of Ti-6Al-7Nb alloy is lower than that of Ti-6Al-4V ELI in equiaxed α structure, in particular, in the high-cycle fatigue region. The fatigue crack initiation in each titanium alloy with equiaxed α (volume fraction of primary α : 50%) and Widmanstätten α structures mainly occurs at the primary α grain boundary while in the colony and prior β grain which are about 45 degree inclined to the stress axis direction in Widmanstätten α structure. The fatigue life increases when the small fatigue crack propagation resistance increases with the volume fraction of primary α. When the cooling rate after solutionizing is increased, the fatigue strength of Ti-6Al-7Nb approaches that of Ti-6Al-4V ELI.