The effect of protective coatings on the high temperature properties of a gamma prime-strengthened Ni-base superalloy
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I.
INTRODUCTION
IT is common practice in the gas turbine industry to coat hot components with materials which are resistant to hot corrosion. However, this treatment involves chemical and mechanical changes in the surface and bulk properties of the substrate. Thus, the system formed by both the coating and the coating/substrate must be considered. Basically, the lifetime limits of the components are determined by the properties of the coating/substrate system and by the imposed service loads. Even though the coating is not designed to enhance the strength of the component, its strain behavior and toughness are important with respect to its ability to protect the surface and to prevent crack initiation under mechanical loading. The formation of cracks on the coating surface could proceed inward into the parent material and shorten the life of the component. The importance of the effects of the coatings and post-coating treatments on the mechanical properties of Ni-base superalloys has long been recognized but has only recently been the subject of systematic quantitative evaluation. 1,2 An early indication of the extreme sensitivity of the mechanical properties of these alloys to the substrate/coating system was given by the study of Mazzei 3 on IN-738 and ECY-768 coated using a pack aluminide system. In this work, a dramatic decrease on tensile and stress rupture properties was noted at elevated temperatures. The work of Felix4 on IN-738 LC coated with chemical vapor deposition (CVD) silicon showed that post-coating heat treatments can drastically lower the time to rupture. Betz, 5 working with IN-100 aluminized by a variety of processes, also found a drop in the lifetime of the substrate/coating system. More recently, the influence of coatings on fatigue properties has been demonstrated. 1,6 However, results indicating the beneficial effect of a particular type of substrate/coating system on the mechanical properties of the substrate material are also not unrare. 2'3'5'7 To date, little information has been reported on the effect of coatings on creep and stress rupture properties of Ni-base superalloys. The interaction between the coating and the R. CASTILLO, Senior Metallurgist, and K.P. WILLETT, Manager, Metallurgical Section, are with Westinghouse Canada Inc., Turbine and Generator Division, Hamilton, Ontario, Canada. Manuscript submitted November 16, 1982.
METALLURGICAL TRANSACTIONS A
substrate in a coated Udimet-520 (U-520)* nickel-base *Udimet is a registered trademark of Special Metals Inc.
superalloy system has been studied, and the results are presented in this paper. The effects on high temperature properties, the morphology of the fractures, and the secondary evidence of creep damage were investigated to rationalize the effect of the coating process on the rupture life of the base alloy. II.
EXPERIMENTAL PROCEDURE
The material selected for the study was a wrought Udimet-520 (U-520) nickel-base superalloy obtained from a commercial heat. The nominal composition of this alloy in weight percent is sh
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