Characterization of mechanical anisotropy in titanium alloys

A significant limitation in the use of LEFM to predict crack propagation behaviour is the assumption that mechanical behaviour is isotropic. This assumption has important implications for materials with an essentially HCP crystallographic structure, such as titanium alloys, in which the number of slip systems is limited. This view is reinforced here by showing that slight variations in the crystallographic orientation of a near α titanium alloy can markedly change fatigue crack propagation characteristics. The paper concentrates on load controlled fatigue data generated on thin section plate fatigue specimens. Interactions between the fatigue crack and microstructural features were studied and later related to crystallographic orientation through the EBSD technique. Features that affect crack propagation include colony/prior β grain boundaries and secondary cracking. Furthermore, it is shown that many fatigue crack propagation characteristics are a direct consequence of crystallographic orientation. In particular, the orientation of the alpha colony within which the crack initiates is the dominant factor in determining overall fatigue life.