Evaluation of Microstructural Deterioration for a Directionally Solidified Ni-Based Superalloy by X-ray Computed Tomography

The evaluation of the microstructural deterioration was carried out by x-ray computed tomography for the first-stage blades, manufactured using a directionally solidified Ni-based superalloy (Ni-SA) retired from a land gas turbine, after service. The x-ray computed tomography was used to analyze the whole blade as well as, a segment of the top of blade with the purpose for obtaining results in a micrometer scale, which can be contrasted with the results of microstructural characterization. In order to evaluate the internal deterioration of the component, it was necessary to determine the in-service operation conditions of first-stage blades. Thus, the temperature distribution in the component during the start-up cycle was obtained, considering a transitory regime and using the conjugate heat transfer technique by means of Fluent ANSYS software. Besides, the mechanical stresses present in the component were calculated using finite element method implemented in ANSYS software. The main results of this work identified that the presence of cracking in the blade is associated with the occurrence of creep microcavities and coarsened particles of MC-type carbides, which showed debonding from the γ-matrix of the Ni-SA. The origin of the cracking was determined to occur in the cooling channels of the component.