Evaluation of microstructural changes of Rene-80 super alloy turbine blades of J85 engine by non-destructive Eddy current method

Nowadays, flying objects engines are harmed and malfunctioned due to aging phenomenon. One of the most important problems lies in the hot part of these engines. Turbines of these engines have creep and metallurgical difficulties which lead to cracks and part failure. In case of one engine blade failure, the whole engine will be out of operational line. Thus, proposing a method for aging determination and longing the lifetime of these engines’ parts can play a significant role in keeping the military helicopters of the country continuously operating. In this study, Eddy current non-destructive method was utilized to evaluate the microstructural changes of Rene80 super alloy turbine blades of J85 engine. The microstructural changes that occurred in this sample by aging phenomenon at a temperature above 800 ℃ for 1600 h under service was studied through destructive and non-destructive routes. To this end, changes of 4 turbine blade parts which were under service for the same periods of times were compared together as well as with an intact blade sample (as the control sample). Metallography studies, scanning electron microscopy (SEM), EDS element analysis, and micro hardness test were done to assess microstructural changes and destructions and phases characterizations. It was found that there was a good correspondence between the results obtained from destructive studies and Eddy current evaluations. The results show that the volume fraction and grain size of the precipitates of the under-service samples were considerably increased compared to the intact specimen and the morphology of 'γ precipitates were transformed from cubic to spherical shape. Likewise, the hardness of the under-service samples was decreased compared to the intact one. The Eddy current results show that the impedance amount of the under-service samples was decreased compared to the control which is in correspondence with the hardness test results. And the induction resistance parameter in Eddy current test closely correlates with hardness test results (R2=0.95). Altogether, it can be concluded that Eddy current non-destructive method is capable of separating different samples of this super alloy and distinguishing of microstructural changes.