Microstructural and Micromechanical Origins of In-Service Cracking in Nickel-Based Superalloy Hot Section Components from Nigerian Gas Turbines

This study investigates the microstructural and micromechanical origins of in-service cracking in hot section components, specifically first-stage turbine blades and nozzle guide vanes, removed from gas turbines at power generation facilities in Lagos and Abuja. Gas turbine availability is critical to Nigerian electricity generation, and the premature cracking of hot section superalloy components results in costly unplanned outages and accelerated replacement expenditure. Cracked components from planned maintenance shutdowns are systematically examined by fractographic analysis, metallographic sectioning, EBSD grain orientation mapping, SEM-EDX compositional analysis, and nanoindentation hardness profiling across the crack vicinity. Synchrotron X-ray diffraction measurements of residual stress are performed on selected specimens at the European Synchrotron Radiation Facility through an international collaboration. Microstructural degradation mechanisms documented include gamma prime rafting, topologically close-packed phase precipitation, oxidation penetration, and thermal barrier coating spallation. The fracture mode at crack tips is characterised as a combination of oxidation-assisted intergranular cracking and fatigue-driven transgranular propagation. A microstructurally informed finite element model with crystal plasticity constitutive equations is developed to simulate the stress concentration at grain boundary triple junctions in the degraded microstructure, identifying the mechanistic role of gamma prime depletion in crack initiation. Keywords: nickel superalloy, turbine blade, in-service cracking, crystal plasticity, gas turbine Nigeria.