Creep Failure and Damage Mechanism of Inconel 718 Alloy at 800–900 °C

Abstract

The creep behavior and damage mechanisms of 718 alloys were investigated at 800–900 °C in air. The fracture morphology and microstructure evolution were observed by optical, scanning and transmission electron microscope. Besides, the creep damage tolerance (λ) and creep strain evolution curve were also calculated. The results showed that the creep curves of 718 alloys at 800 or 850 °C consisted of primary and tertiary stages, while the steady-state region became apparent at 900 °C. The apparent creep activation energy of 718 alloy was in the range from 446.3 to 491.8 kJ/mol. The alloy presented ductile fracture at 800 °C due to the nucleation, growth and linkage of creep voids. However, the failure of alloys at 850 or 900 °C presented necking to a point due to the microstructure degradation. Further investigations showed the softening of materials and the loss of mechanical performance could be mainly attributed to the coarsening or decrease of strengthening precipitates. Above 850 °C, it was found that γ′ phases would dissolve into matrix and stress promoted the re-dissolution of γ′ phases or led to the break of δ phases. Moreover, the creep strain evolution curves indicated that 718 alloys kept a relative stable state at 800–900 °C when the strain fraction was below 1.