Characterization of microstructural evolution in a nickel base superalloy

Sammanfattning

The microstructural evolution of a nickel-based superalloy is investigated in this study duringshort and long-term thermal exposure. The focus of the study is on the coarsening of the γ´phase over various operating temperatures and exposure times, as well as changes in volumefraction. This γ´ phase is the basis for the high-temperature strength of the alloy and is thestrengthening phase. The basis of the test material consisted of a neutral reference sample and30 heat-treated samples. These were processed and analyzed using various techniques,including specially designed sample preparation programs, scanning electron microscopy(SEM), and image analysis in MIPAR, where the quantification of γ´ area fraction and equivalentdiameter was performed. Evaluation and further development of models based on classical TIDC (Trans-interfacediffusion-controlled) theory and LSW (Lifshitz-Slyozov-Wagner) theory were performed at hightemperatures and long exposure times to enhance reliability. The modified version of the LSWmodel showed significantly less error, as indicated by higher R^2 and lower RMSE values,compared to the standardized LSW model. This indicates increased accuracy of predicted γ´-size. The model results show promising implications for predicting the operating temperatures ofcomponents taken out of service. An investigation of Vickers hardness measurements was also performed, which showed acorrelation between mechanical properties, γ´ morphology, and carbide formation. These resultsdemonstrated that optimal γ´ size, rather than maximum volume, was indicated by bestdislocation inhibition, as distinguished by temporary hardness peaks. The result of all the work is that microstructural changes for the alloy studied can be predictedthrough a validated predictive model, which can lead to forecasts for process optimization andlife assessment in industrial components. The investigation in this study also reveals potentialfor future development of experimental procedures and model advancements underindependent isothermal conditions, aiming to enhance the accuracy of the temperatureestimation method.