Microstructural degradation and creep life reduction in Allvac 718Plus superalloy after long-term thermal exposures and its creep mechanisms

Allvac 718Plus alloy is a candidate material for high-temperature engines due to its high thermal stability up to 704 degrees C. This study systematically investigated the effects of gamma ' coarsening and sigma formation after long-term thermal exposures on the creep performance of the 718Plus alloy, as well as the associated creep rupture and planar defect-based deformation mechanisms. The results indicated that the formation of sigma particles after thermal exposure had a more significant impact on reducing the creep life than the coarsening of gamma ' particles. This is attributed to the increased susceptibility of stress concentrations and creep microcraks at the eta/sigma interfaces. In nearly all thermally-exposed samples, the dominant creep deformation mechanisms included the formation of superlattice stacking faults (SSFs) in gamma ' and microtwinning, accompanied with stacking faults cutting through both gamma ' particles and the gamma-matrix. The gliding of Shockley partial dislocations on {111} planes, along with Cr segregation at complex stacking faults (CSFs) and twin boundaries, contributed to the formation of CSFs and the thickening of microtwins. Additionally, the creep resistance associated with the planar defects was compromised due to the coarsening of gamma ' particles. Furthermore, the applied creep stresses significantly induced the acceleration of gamma ' coarsening and the nucleation and growth of sigma particles.