Thermal stability of phases in a NiCoCrAlY coating alloy
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Wei,a) G.C. Hou, Q. Zheng, X.F. Sun, H.R. Guan, and Z.Q. Hu Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China (Received 10 December 2007; accepted 16 May 2008)
The temperature dependence of the thermal stability in a NiCoCrAlY coating alloy was examined by experimental observation and thermodynamic modeling in the 400–1200 °C temperature range. The results indicated that the thermal stabilities of primary –NiAl, –NiAl/␣–Cr eutectic, and ␥–Ni were slightly temperature dependent, but those of ␥⬘–Ni3Al, –(Cr,Co,Ni), and ␣–Cr were strongly temperature dependent in the annealed NiCoCrAlY specimens. The temperature dependence of the thermal stabilities among ␥⬘–Ni3Al, –(Cr,Co,Ni), and ␣–Cr might be ascribed to the → ␣ transformation at ∼1100 °C and the ␥⬘ → ␥ transformation at ∼800 °C. Further, using Thermocalc associated with TTNi7 database, thermodynamic equilibria were calculated. The modeling results were compared with the experimental results and found to be in reasonable agreement with the experimental observations of –NiAl, –(Cr,Co,Ni), and ␥⬘–Ni3Al. Some deviations observed are discussed in the light of both limited availability of thermodynamic data and experimental problems.
I. INTRODUCTION
Multiphase MCrAlY alloys, mainly consisting of an Al-rich –NiAl phase and an Al-poor ␥–Ni solid solution, are widely used as overlay coatings or as bond coats in thermal-barrier coatings (TBCs) to protect the components in gas-turbine engines against oxidation and corrosion.1 At high operating temperatures, MCrAlY coatings and substrate alloys are highly susceptible to phase transformations, particularly after prolonged exposure. It is known that volume changes, as a result of phase transformations, have an important effect on TBC mechanical stability. Much attention should thus be paid to the transformations induced by coating–substrate interdiffusion and oxidation. Transformation of –NiAl from an ordered B2 structure to L10 martensite was reported during thermal cycling and isothermal exposure when –NiAl of more than 63 at.% Ni was quenched from temperatures below 1000 °C.2–6 Chen et al.7,8 found that L10 martensite was stable at temperatures below ∼600 °C and the –NiAl phase was stable at higher temperature. Pan et al.9 related the increase of intermediate temperature strength of Pt–
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0282 2264
J. Mater. Res., Vol. 23, No. 8, Aug 2008
Al bond coats to the martensite transformation occurring during thermal cyclings. The quantitative measurement revealed that since the molar volume of –NiAl was approximately 2% larger than that of the L10 martensite, the  → L10 transformation caused the volume change, which significantly influenced stress distribution in TBC system. It was also observed in MCrAlY coatings after long-term exposure that ␥⬘–Ni3Al, ␣–Cr,10,11 Cr/Re-rich -phase,12,13 and blocky Ni–Y intermetallic phase14,15 precipitate in a certai
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