Influence of Microstructure and Environment on the Fatigue Crack Growth Fracture Topography of Ti-6Al-2Sn-4Zr-2Mo-0.1Si

An examination of fatigue fracture surfaces as influenced by microstructure (β processed versus α + β processed) and environment (air versus vacuum at 298 and 811 K) for a high-temperature creep-resistant near-α titanium alloy, Ti-6Al-2Sn-4Zr-2Mo-0.1Si, has been undertaken utilizing scanning electron microscopy. Results show that fracture topography is very sensitive to microstructural and environmental causes. Beta processing leads to a more irregular fracture surface than α + β processing. At low growth rates (10⁻⁶ mm/cycle) in air, shear mode growth predominates, resulting in large transgranular facets, whereas at growth rates of 10⁻⁵ in./cycle, striated and dimpled fracture features were present. At elevated temperatures, where oxidation effects were quite pronounced, particularly at low growth rates, the fracture surface was smoother than at room temperature, but faceting persisted to higher growth rates. In vacuum, however, the degree of faceting and striation formation was markedly reduced. Finally, the observed fractographic features influenced by microstructure and environment are discussed in terms of the mechanism of crack growth.