Investigation of fatigue crack initiation facets in Ti-6Al-4V using focused ion beam milling and electron backscatter diffraction

In the very high cycle fatigue regime, internal crack initiation can occur in Ti-6Al-4V because of the formation of facets, which are grains that have fractured in a transcrystalline and planar manner. Because this crack initiation phase occupies most of the fatigue life, it is essential to understand which mechanisms lead to facet formation. Fatigue tests have been performed on drawn and heat-treated Ti-6Al-4V wires, and the facets at internal crack initiation sites have been analysed in detail in terms of their appearance, their spatial orientation and their crystallographic orientation. The facets were not smooth, but showed surface markings at the nanoscale. In nearly all cases, these markings followed a linear pattern. One anomalous facet, in a sample with the largest grain size, contained a fan-shaped pattern. The facets were at relatively steep angles, mostly between 50 degrees and 70 degrees. Cross-sections of the fracture surfaces have been made by focused ion beam milling and were used to measure the crystallographic orientation of facets by electron backscatter diffraction. Most facet planes coincided with a prismatic lattice plane, and the linear markings were parallel to the prismatic slip direction, which is a strong indication that prismatic slip and slip band formation led to crack initiation. However, the anomalous facet had a near-basal orientation, which points to a possible cleavage mechanism. The cross-sections also exposed secondary cracks, which had formed on prismatic lattice planes, and in some cases early stage facet formation and short crack growth phenomena. The latter observations show that facets can extend through more than one grain, and that there is crack coalescence between facets. The fact that drawn wires have a specific crystallographic texture has led to a different facet formation behaviour compared to what has been suggested in the literature. Lay Description Ti-6Al-4V is a titanium alloy that is often used for applications in which the material is subjected to periodic loading and unloading, for example, due to vibrations. As a result of these cyclic loading conditions, metal fatigue can occur, which means that the material breaks after a certain amount of loading cycles. If the load levels are small and the number of loading cycles to failure is very high, fatigue cracks can initiate internally in Ti-6Al-4V. After failure, the fracture surfaces reveal that facets, which are short planar cracks through individual grains, are present at these internal crack initiation sites. The purpose of this paper is to understand how these facets are formed. This has been investigated by performing fatigue tests on drawn Ti-6Al-4V wires with two different microstructures. The appearance, spatial orientation and crystallographic orientation of the facets have been analysed. The crystallographic orientation was measured by sectioning through fracture surfaces and analysing these sections with advanced electron microscopy techniques. By studying the facet properties, it has been found that the internal crack initiation mechanisms in these wires are different compared to the mechanisms in forged or rolled Ti-6Al-4V, which is most often used for fatigue testing. This can be attributed to the different crystallographic texture, which is the distribution of crystallographic grain orientations in the material.