A Study of Fatigue Crack Initiation in a Two Phase Β-metastable Titanium Alloy

α/β titanium alloys are choice materials for aeronautical and automotive industries due to their high performance to density ratio. For several applications, fatigue resistance is one of the critical design parameter. Particularly, initiation of fatigue crack plays a key role. Furthermore, hardly anything can be found in the literature on the fatigue crack initiation in multiphase microstructures. This work deals with the characterisation of the high cycle fatigue crack initiation in the TIMETAL LCB α/β titanium alloy. Microstructure was characterised by SEM, XRD, EBSD and TEM. Fatigue tests were carried out on a microstructure presenting a finely grained and equiaxed α/β morphology. Observations of the crack initiation were conducted at the scale of the microstructure thanks to SEM. Orientation Image Microscopy (OIM) allowed to correlate microstructural and micromechanical aspects. Introduction Titanium alloys are more and more used for structural applications. Automotive and aerospace industries are certainly the main application fields. These alloys are in fact choice materials due to their high performance to density ratio. Particular attention has been raised recently on metastable beta titanium alloys, i.e. alloys that can remain β at room temperature by quenching. Thanks to an appropriate thermomechanical treatment, the suitable two-phase α/β microstructure can then be generated. These alloys present good mechanical and forming properties, excellent corrosion resistance and good cold deformability. Despite these advantages, the use of titanium alloys was often slowed down owing to their cost with respect to other materials. To overcome this difficulty, new processing routes are developed. This is the case for the new TIMET Ti LCB (for low cost beta) that presents a lower cost compared to other Ti alloys by using an inexpensive Fe-Mo master alloy widely used in the steel industry. New applications then emerged, particularly in the automotive industry (suspension springs) (Schauerte [1]). The resistance to crack initiation under cyclic loading is one of the critical design parameter for several applications in the automotive and aerospace industries. The way the first defects appear must be studied at the scale of the microstructure in order to be understood. It is well documented that fatigue initiation is strongly influenced by several characteristics of the microstructure like grain size, grain orientation and misorientation of the grains (Jin and Mall [2]). Numerous publications recently dealt with the general behaviour of small fatigue cracks in several materials (Rodopoulos and de los Rios [3]). Particularly, the initiation of fatigue cracks in the Ti LCB was characterised but in the case of a fully β microstructure (Hu et al. [4], Krupp et al. [5], Floer et al. [6]). However, hardly