The influence of the alpha grain size on internal fatigue crack initiation in drawn Ti-6Al-4V wires

Abstract

In the very high cycle fatigue regime, the location of crack initiation in titanium alloys is known to shift from the surface towards the bulk of the material. This internal fatigue crack initiation results in faceted features on the fracture surface. These facets are in fact alpha grains that have been broken in a planar manner. Typically, a cluster of many facets is observed either just below the surface or deeper inside the bulk. In this study, uniaxial tension-tension fatigue tests are performed on Ti-6Al-4V wires which have been subjected to different heat treatments in order to vary the alpha grain size. Four different microstructures are obtained, with average alpha grain sizes of approximately 1, 2, 5 and 10 µm. The fatigue life is found to decrease with increasing grain size. Electrochemical polishing of the wires prior to fatigue testing is applied in order to promote internal crack initiation at higher stresses and consequently shorter testing durations. Four samples broke due to an internal crack: three samples with average alpha grain size 5 µm, which failed after 2.6 x 107, 5.7 x 107 and 9.6 x 107 cycles, and one sample with average alpha grain size 10 µm, which failed after only 7.6 x 106 cycles. The threshold stress intensity factor range, which is calculated from the size of the facet-containing area, is between 5 and 6 MPa.m1/2 for all four samples. Fractographic examination of the facets reveals that they are not smooth, but show roughness at the nanoscale. This roughness has a linear appearance for nearly all facets, except for one anomalous facet in the sample with the largest grain size, which shows a fan-shaped pattern. From electron backscatter diffraction measurements on cross-sections of the fracture surfaces obtained by focused ion beam milling, it is also found that nearly all the facets coincide with a prismatic plane, and the linear markings are parallel to the prismatic slip direction. Only the anomalous facet has a near-basal orientation. These observations suggest the possibility that facets are formed by either a slip-based mechanism or a cleavage-based mechanism, and that the alpha grain size is one of the parameters that controls which mechanism occurs.