Material characterization of long-term service-exposed GTD-111 nickel based superalloy

In this study, the influence of long-term service exposure on the microstructure of a GTD-111 precipitation hardened Ni-based superalloy extracted from turbine blades after 55,000 and 75,000 h without any interval rejuvenations is investigated. This alloy consists of multi phases including γ, γ′ (Ni3Al) strengthening precipitates, carbides, and eutectic phases. The long-term operation caused dramatic microstructural degradations, such as decomposition of MC carbides, formation of M23C6 carbides along grain boundaries, grain boundary oxidation and cracking, coarsening of primary γ′, dissolving of secondary γ′, and formation of TCP-type phases which profoundly impacted its failure mechanisms. The results showed that after 75,000 h of service, the grain boundaries which reached the surface were oxidized and even some led to crack initiation due to high thermal cyclic loads in the cooling holes. The obtained results also illustrate that with increasing service exposure times, the size of primary γ′ increased to about 1.5–2 µm and secondary γ′ dissolved. Dissolution and coarsening of γ′, widening of grain boundaries, decomposition of carbides, and phase transformations were analyzed by optical and scanning electron microscope and energy dispersive spectroscopy. Furthermore, the coating on the blade's surface is investigated microstructurally to understand its severe degradation due to the high temperature and pressure such as microstructural degradation and cracking, and widening of interdiffusion layer.