Fatigue crack initiation in Ti-6Al-4V has been investigated in high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes at stress ratio R=0.1 using the replication technique. In all four tested α/β microstructures, the crack was initiated by fracture of equiaxed alpha grain. Fractured alpha grains are seen on the fracture surface as flat facets with features characteristics of cleavage fracture. In the regime of low stress amplitudes and in the absence of reverse loading, cleavage fracture contributes to crack initiation and early stages of crack growth in Ti-6Al-4V. This mechanism is discussed in relation to the anomalous mean stress fatigue behavior exhibited by this alloy.

Investigation of fatigue crack initiation in Ti-6Al-4V during tensile-tensile fatigue

The fractography of structure-sensitive and structure-insensitive vacuum fatigue in fine grained Ti--6Al--4V was investigated using both scanning electron microscopy and transmission electron microscopy of replicas. It is shown that structure-sensitive fatigue of fine grained Ti--6Al--4V in vacuum takes place principally by basal or near-basal cleavage. It is likely that the cleavage is essentially noncyclic, and mechanically rather than environmentally induced. The structure-sensitive to structure-insensitive transition correspond with a cyclic plastic zone size intermediate between the minor and major dimensions of the elongated alpha grains. Cyclic crack growth was detached for structure-insensitive fatigue, as reported by others. (auth)

R.J.H. Wanhill

Papers

Vacuum fatigue fracture of Ti--6Al-4V