Evolution of the Microstructure and Mechanical Properties of a Ni-base Superalloy during Service

The degradation of a first-stage Industrial Gas Turbine blade was addressed to understand the intrinsic failure mechanisms that undergo during service. The system under study is composed by an Ni-based SA + B2-NiAl CVD coating. The turbine blade was in service by 120,000 hours at 900°C in air. The analysis was performed on different cross section along the blade height. The mechanical behavior and crystal structure of the SA were studied by Rockwell Hardness testing (RHT) with a diamond indenter and by X-Ray Diffraction (XRD), respectively. The SA microstructure was analyzed by optical microscopy (OM) and Scanning electron microscopy (SEM) after metallographic preparation. The g and g´ phase fraction were identified and quantified by image analysis. Rafted g´ precipitates distributed in the matrix are present in the SA due to thermo-mechanical fatigue at operating conditions. The presence of secondary carbides (e.g., M₂₃C) with irregular form, large size and without preferential crystallographic alignment was observed by mean of OM and SEM. Micro-cavities close to the carbides, with a size of about 40 mm were also observed. In general, the hardness of the SA was around 39 HRC. Relevant variations from 26 to 41 HRC along the SA are strongly correlated to the microstructure and chemical composition. Finally, this study was carried out with the porpouse to identify the critical parameters that promote microstructural changes and consequently affect the mechanical behavior in this turbine blade. Thermodynamic and kinetic calculations using ThermoCalc and DICTRA were performed to be compared and discuss with the experimental results.