Influence of Re on the phase constituent of a NiCoCrAlY coating alloy

Abstract

MCrAlY alloy has served as overlay coatings or bond coats in thermal barrier coating systems in gas turbine engines, and its phase constituents play a vital role in determining the performance of these coating systems. In order to further understand the influence of Re on the phase constituents of MCrAlY coatings, the phase evolution of a Ni-20Cr-10Al-20Co coating alloy with 0-9% (mass fraction) Re addition was predicted the Thermo-Calc thermodynamic software and TTNi7 Ni-based superalloy database. Based on the calculation results, the four NiCoCrAlY alloys with different levels of Re (0, 3%, 6% and 9%) were prepared, and their phase constituents were experimentally investigated in the temperature range of 800-1250°C. Both the calculation result and the experimental microstructural observation indicated that the addition of Re slightly increased the fraction of β-NiAl phase, but dramatically enhanced the amounts of σ and α-Cr phases. Based on the combined analysis of microstructural observation and EPMA composition identification, it was proposed that the increased amounts of σ and α-Cr phases should result from the changed partition of Cr in all phases caused by Re addition. The σ phase is a low temperature stable phase, and would change to α-Cr phase as temperature rises. The transformation temperature of σ → α-Cr phase increases as the addition of Re increases. The experimental results also showed that the γ'-Ni3Al→ γ-Ni transformation temperature decreases with the addition of Re. In addition, the calculation results were compared with the experimental results and were found to be in reasonable agreement with the experimental observations from the aspect of theMCrAlY phase evolution. Some deviations of the calculation and experiment results about the precise phase-change temperatures, such as γ'-Ni3Al→ γ-Ni and σ → α-Cr phase transformation temperatures, were observed, and the reason were discussed in the light of both limited availability of thermodynamic database and experimental problems. © Copyright;.