GH4151 is a heavy alloy, hard-to-deform Ni-based superalloy with service temperatures reaching up to 750-800 ^oC. It is an important candidate material for high-temperature alloys used in the turbine disks of the new-generation advanced aeroengines. During service, the rapid superposition of temperature and stress makes turbine disks susceptible to damage. This study explores the use of hard-to-deform GH4151 alloy used for turbine disks. The tensile behavior of the GH4151 alloy within a temperature range of 23-950 ^oC was investigated using advanced techniques such as SEM, TEM, EDS, and EPMA. The microstructural changes, deformation microstructure, and their impact on the fracture mechanism were analyzed, and the fracture failure mechanisms of the alloy at various temperatures were elucidated. The results indicate that yield strength and tensile strength initially decrease gradually, followed by a rapid decline with increase in experimental temperature. Meanwhile, elongation after fracture of the alloy decreased initially and increased with increasing experimental temperature. The fracture mode transitioned from a mixed fracture to an intergranular fracture. Further research showed that during tensile testing at temperatures of 23-550 ^oC, deformation primarily occurred in the γ channels, with a significant accumulation of dislocations at the γ/γ′ interfaces. This led to the tearing of the γ/γ′ interfaces and the formation of microvoids, which in turn generated a transgranular fracture. The intergranular fracture within the mixed-fracture mode is attributed to the stress concentration at MC carbide interfaces, resulting in the formation of voids. During tensile testing at temperatures of 650-800 ^oC, cracks were initiated via an intergranular fracture, and propagated through mixed-fracture modes. Deformation occurred simultaneously in the γ channels and the γ′ phase. Dislocation pile-up at the grain boundaries accelerated the enrichment of O atoms toward the elastic stress fields or the enrichment of defects at the grain boundaries under high-temperature and high-stress conditions. Such enrichment led to dynamic embrittlement of the grain boundaries, causing intergranular fracture, which reduced the elongation after fracture. As the strain increased, crack propagation was accelerated, reducing the time available for the O atoms to dynamically embrittle the grain boundaries. When the accumulation of dislocations at the γ/γ′ interfaces reached a critical value, crack propagation shifted to a mixed-fracture mode dominated by transgranular fracture. During tensile testing at 950 ^oC, cracks were initiated and propagated via intergranular fracture. The morphology of the γ′ phase changed to an approximately spherical shape, reducing the hindrance to dislocation motion. This reduction did not lead to the coalescence of microvoids at the γ/γ′ interfaces, and thus, no transgranular fracture occurred in the samples tested at 950 ^oC. Because of the decrease in tensile strength at 950 ^oC, the external stress applied was reduced, and crack propagation slowed down, elongation was increased.