Effect of Grain Boundary Formation in Microstructural Changes in IN 738 Service Exposed Turbine Blade

Microstructural changes on a Nickel based superalloy service exposed gas turbine blade after 52,000 hours at 720°C temperature was investigated. The study examines the grain boundary growth sizes; the changes with respect to gamma prime (γi) precipitate and phase distributions that occurred during operation. In order to observe the changes some different portions of the turbine blade undergone scanning electron microscope (SEM) to determine the grain size measurement and the EDX was used to evaluate the elemental analysis. The grain growths were discrete in formation along the grain boundary (GB) and little difference in the size of (γi) precipitates existed in different locations. The gradual transition of grain boundary-carbide morphology from the original fine discrete to fine semi-continuous, continuous film-like and finally coarse is observed in the specimens. The changes mostly evolves around two distinct phases which is from the solid to the equilibrium (sigma coarse phase) and then back to the solid (hardening gamma prime precipitate) phase. The grain size of (γi) precipitate varies throughout the surface of the turbine blade and there is a significant difference between the leading and trailing edge. The microstructural changes on the serviced blade can be interpreted to indicate that no obvious microstructural degradation and phase deformation occurred during exposure time.