Thermodynamic investigation into the equilibrium phases in the NiCoCrAl system at elevated temperatures

NiCoCrAlY alloys, commonly used as the bond coat layer in thermal barrier coatings, have been widely studied through experimentation. It is generally agreed that the major phases in the NiCoCrAlY are γ-Ni(Co,Cr) matrix and β-NiAl, with occasional presence of γ′-Ni3Al, (Co, Cr)-rich σ, and/or α-Cr precipitates, depending on the chemical composition and the temperature. It is noted, however, that the chemical compositions of the NiCoCrAlY alloys vary from one study to another. The present work reviews the literature on the experimental descriptions of the phases present in NiCoCrAlY and the corresponding phase transformations. To complement this literature review and, more importantly, to allow for consideration of alloy compositions not yet studied experimentally, equilibrium phases within the NiCoCrAl quaternary system are investigated in terms of the change in phase fractions as a function of temperature, isotherms at elevated temperatures (700-1100 °C) with a wide and continuous variation in composition (Co, Cr or Al content), and the phase compositions, through computation by Thermo-calc®. The results are in good general agreement with the experimental findings, thus validating the approach. The γ′ and the σ phases are emphasized in the analysis, because of their significant contributions to the thermo-mechanical properties of the NiCoCrAlY alloys. Results indicate that an increase in Co content leads to decreased stability of the γ′ phase in the temperature range of 700-1100 °C; while appropriate Co additions increase the stability of the σ phase, with the appearance of a four-phase (γ + γ′ + σ + β) co-existing region, at temperatures below 1000 °C.