Abstract:Tensile testes were carried out at ambient temperature on Ti-600 alloy before and after being exposed at 600 °C for 100 h. Fracture morphologies were investigated and the fracture mechanism was also analyzed. The results indicate that the strength of the sample without oxidizing layers increases by 3% or so, the elongation decreases by about 20% for the alloy exposed at 600 °C for 100 h. While for the sample with oxidizing layers, the strength decreases a little bit, and the elongation decreases by 45% or so, which indicates that the plasticity decreases abruptly during the exposure process. For the solutioned plus aged samples, cracks originate from the center of the fracture, and dimple typed fracture can be found in the fractographies. After being exposed at 600 °C for 100 h, for the samples without oxidizing layers, dimples and cleavage facets can both be seen in the fractographies, which displays the feature of hybrid fracture. While for the samples with oxidizing layers, only cleavage facets can be observed in the fractographies, and fractures propagate along the interfaces of lamellar a phases. These results are caused by the precipitation and the existence of the brittle oxidizing layers. The cracks would be formed in the oxidizing layers first and then propagate into the matrix during the tensile tests. Oxygen permeation in the surface layer is one of the important reasons for the decrease of plasticity.