Influence of long-term thermal exposure on the tensile properties of a high-temperature titanium alloy Ti-55

Stability of Ti-55 after a long-term thermal exposure was studied on samples taken from 25 mm rolled bars. In order to evaluate the variation of mechanical properties with service time, the tensile properties of this alloy after exposure at 500, 550 and 600°C for times up to 3000 h were investigated. The samples were exposed under two conditions: before and after machining the specimens. It was found that both yield strength and fracture strength increase after exposure. However, both the tensile elongation and reduction in area decrease with exposure time at all three exposure temperature. For the exposure of bars, even after 500 h at 600°C both the tensile elongation and reduction in area remain unchanged with exposure time. Examination of the microstructure by optical and electron microscopy shows that the factors that influence stability are: surface oxidation, decomposition of the residual beta phase and precipitation of the alpha-2 Ti3X phase.