Study on the Mechanism Transformation and Structural Instability of Temperature-Induced Creep in High Nb-TiAl Alloy

A systematic study was conducted on the creep response, evolution of the microstructure, and fracture behavior of the Ti-44Al-8Nb-0.2W-0.2B-0.1Y alloy following isothermal forging, under test conditions of 240 MPa and 780–840 °C. The results show that the alloy exhibits strong temperature sensitivity in the range of 800 - 820℃. At lower temperatures ranging from 780 to 800℃, the alloy exhibited a longer creep life (105.2 h and 179.9 h respectively) and a smaller creep strain (9.8% and 12.3%). When the temperature rises to 820 - 840℃, the creep life sharply decreases to 24.87 - 14.38 hours, while the creep strain increases to 24.9 - 22.4%. According to the microstructural analysis, the as-received structure features a combination of lamellar colonies and equiaxed grains. During the creep process at 780℃/240 MPa, the main deformation mechanism is the bending, coarsening and edge decomposition of lamellar colonies. Dynamic recovery, together with discontinuous dynamic recrystallization within the equiaxed grain region, serves as the primary softening mechanism. The equiaxed grains region is prone to generating micro-pores, which is a weak link in the alloy's creep process. Under the high-temperature creep condition of 840℃/240 MPa, the lamellar structure undergoes severe deformation. The γ lamellae undergo extensive dynamic recrystallization through sliding, rotation, spherification and fracture at the phase interface, forming fine near-γ equiaxed structures, while a high proportion of subgrain boundaries are also formed. The microstructure of the alloy transforms into a mixture of residual lamellar and equiaxed grains. The sharp decline in the creep performance of the alloy within the temperature range of 800-820℃ is closely related to the transformation of the microscopic deformation mechanism, which has shifted from being dominated by dislocation slip and dynamic recovery to being mainly characterized by phase boundary migration, dynamic recrystallization and grain boundary sliding.