Thermodynamic calculation and experimental analysis of critical phase transformations in HVOF-sprayed NiCrAlY-coating alloys

Most of the current high temperature protective NiCrAlY-type coatings for stationary gas turbines are configured for base-load operation. During service at elevated temperatures, phase transformations occur which ultimately lead to a microstructural evolution and thus might have a substantial influence on service relevant materials properties. In consequence, these coating alloys need to be adapted for operation in high-cyclic and/or peak-loading conditions. The present investigation is focusing on the microstructural evolution of nine different NiCrAlY-coatings in the temperature range of up to 1150 °C. The study especially aims at determining the v-(NiAl) and g'-(Ni3Al) phase transformation, which can have a substantial impact on the coefficient of thermal expansion, or the materials deformation behaviour respectively. For this, a combination of computational thermodynamics (Thermo-Calc 2015b, TCNi6 database) validated by dilatometry and X-ray diffraction was used. In addition, the obtained results were cross-checked using scanning electron microscopy and energy dispersive spectroscopy on water quenched, free-standing coating alloy specimens. In general, a good agreement between the thermodynamic equilibrium calculations, dilatometry and X-ray diffraction, as well as the microstructure analysis after water quenching was found with respect to the critical phase transformations of a-(Cr), v-(NiAl) and g'-(Ni3Al) precipitates in the g-(Ni) matrix. The great potential of computational thermodynamics combined with dilatometry and X-ray diffraction for the evaluation of critical phase transformations was proven for the different coating alloys. Using this methodology, novel and very promising NiCrAlY-coating compositions, without detrimental phase transformations around 1000 °C, were identified for the application on heavy-duty gas turbines hot gas path components. This manuscript presents a summary of the main results.