Fractographic Observations of the Stress Corrosion Cracking of Titanium Alloys in Methanolic Environments

The stress corrosion cracking of titanium of three different impurity levels and of Ti-5Al-2.5Sn alloy containing both low and high amounts of Fe has been examined by scanning electron microscopy. Two types of testing were employed: U-bends and dynamic tensile straining. Increasing levels of impurity resulted in a change in the mode of cracking in titanium from intergranular separation to transgranular cleavage. Both alloys exhibited transgranular cleavage. Introducing small amounts of the hydride phase in the most impure titanium resulted in (1) a larger amount of cleavage in subsequent stress corrosion tests, and (2) air fractures that were similar to stress corrosion fractures. Pre-exposure of unstressed specimens to the environment followed by fracture in air also resulted in small regions of similar fractures. Anodic dissolution superimposed on specimens during the pre-exposure period eliminated this region provided that the rate of dissolution was similar to the rate of H diffusion. A detailed discussion of the results interprets intergranular dissolution as arising mainly from impurity segregation and transgranular cleavage as being due to a form of H embrittlement.