Fractography and Microstructural Analysis of Fatigue Crack Propagation in Beta-Annealed Ti6Al4V Alloy after Fatigue Test

Titanium alloy Ti6Al4V was oxidation annealed in the beta-phase region (1050 °C/3 hours + WQ) in a furnace under a non-protective atmosphere. The above treatment caused the formation of an alpha-case layer on the surface. The above layer, because of its high hardness and strength, has a significant effect on the surface properties of the alloy. However, undesirable effects include the formation of cracks in this layer and a change in the mechanism of initiation and propagation of fatigue cracks. Based on the above findings, it is also very complicated to predict the fatigue life of Ti alloys processed in this way because of the presence of cracks in the alpha-case layer and the varying thickness of this layer. From the fractography and microstructural point of view, the initiation of fatigue cracks in the heat-treated alloy is realized by transcrystalline cleavage and the formation of pronounced fracture facets as a consequence of surface cracks in the alpha-case layer. Fatigue crack propagation (in the alpha-case layer region) is realized along the interface of the alpha-phase needles and the beta-phase primer grain without the significant presence of so-called fatigue striations.