铸态高Nb-TiAl合金高温蠕变变形及断裂机制

In this paper, the microstructure of as-cast Ti-46Al-6Nb-2.5V-0.2B₄C alloy was analyzed by using X-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). And the elevated temperature creep tests at 800-850 ℃ under 250-325 MPa were conducted to investigate creep deformation and rupture mechanisms of as-cast high Nb-containing TiAl alloy. The results show that the as-cast alloy exhibits fully lamellar structure. Besides, TiB precipitates inside lamellar colony or at grain boundary and C atoms exist in the matrix in the form of solid solution. According to creep data, the stress exponent (n) at 800 ℃ and the apparent activation energy (Q_C) under 300 MPa of Ti-46Al-6Nb-2.5V-0.2B₄C alloy are measured to be 5.71 and 313.316 kJ/mol, respectively. Combining microstructure observation by TEM, it is revealed that the creep deformation is controlled by dislocation climbing and mechanical twinning. And dynamic recovery and dynamic recrystallization induced by creep play a softening role. In addition, elements B and C play an important role in improving the creep resistance of high Nb containing TiAl alloys. Particularly, Ti₂AlC precipitates dynamically at the interface between TiB and matrix during creep process. Moreover, TiB and Ti₂AlC follow the well-established orientation relationship as [11¯1¯]_TiB//[12¯10]_Ti2AlC, (110)_TiB//(101¯0)_Ti2AlC. Furthermore, the creep rupture is dominated by the initiation and coalescence of cavities as well as the propagation of cracks.