Effect of orthorhombic case on the creep rupture of Ti-22Al-25Nb (at%) orthorhombic alloy

Creep rupture tests and tensile tests of Ti-22Al-25Nb alloy with bimodal size lamellar O phase microstructure were carried out at 650 °C and 700 °C. The fracture surface analyses indicate that the tensile fracture originated from the center of the cylinder sample while the creep rupture started at the surface. The different fracture mechanism of creep rupture samples is ascribed to the orthorhombic cases (O cases). It is found that the sub-surfaces of the creep rupture samples were enriched with the element oxygen. The interstitial oxygen, an alpha stabilizer for titanium alloys, promoted the transformation from B2 phase to O phase. Thus the sub-surfaces contained more acicular O phases than the substrates. The sub-surfaces with more O phase component are named as O cases. The dissolved interstitial oxygen and the added O phase component increased the strength but reduced the ductility and the toughness in the O cases. It was the incompatibility of the creep strains between the O cases and the substrates that made the creep rupture to originate at the surfaces.