High cycle fatigue of tantalum carbide reinforced nickel base eutectics at room temperature
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I. INTRODUCTION SEVERAL directionally solidified eutectic superalloys are currently being assessed for use in critical gas turbine components.1 Prime candidates are nickel and cobalt-base eutectics reinforced with tantalum carbide fibers,2,a called NiTaC and CoTaC respectively. One of the strongest alloys so far developed, for which creep rupture data is available,2 is known as NiTaC13. Unfortunately, like other recently developed high strength derivatives of this alloy, the low Cr content results in poor resistance to oxidation and hot corrosion in a gas turbine environment. 4 Because of this environmental sensitivity, the successful application of these alloys is dependent on the development of compatible protective coatings. This requires a complete assessment of the effect of various types and compositions of coatings and of coating processes on mechanical properties. In particular, high cycle fatigue resistance, which is generally observed to be sensitive to surface condition, 5 is of major concern. In most coating processes, pre- and post-coating treatments introduce changes to the substrate which could influence fatigue life. F o r e x a m p l e , heat t r e a t m e n t s at high t e m p e r a t u r e s (1000 to 1200~ d e s i g n e d to p r o m o t e diffusion between the coating and s u b s t r a t e and thus i m p r o v e the a d h e r e n c e of the coating, could r e s u l t in the p r e c i p i t a t i o n of s i g m a phase. 2 T h i s phase is known to p r o d u c e e m b r i t t l e m e n t in c o n v e n t i o n a l a l loys s and also u n d e r c e r t a i n c i r c u m s t a n c e s in N i T a C a l l o y s .7 To develop a full u n d e r s t a n d i n g of the effect of o v e r lay coatings on high cycle fatigue p e r f o r m a n c e , it is n e c e s s a r y to u n d e r s t a n d the b e h a v i o r of the uncoated D. E. GRAHAM, formerly with Corporate Research and Development, is now Metallurgist,Carboloy Systems Department, General Electric, Detroit, MI 48232. D. A. WOODFORD,is Staff Metallurgist, Corporate Research and Development, General Electric, Schenectady, NY 12301. Manuscript submitted November 27, 1978. METALLURGICALTRANSACTIONSA
alloy as well as to separate the ancillary variables associated with the coating processes. This paper addresses the following questions: I. Since substrate surfaces are usually roughened to improve coating adherence, what is the effect of various surface preparation methods on high cycle fatigue ? 2. What effects do various coating process heat treatments have on high cycle fatigue ? 3. What are the crack nucleation and propagation mechanisms which operate during high cycle fatigue of NiTaC alloys ? The major objective of this study was to provide a framework on which to assess the effect of coatings on these alloys. In addition, a detailed microstructural evaluation of events leading to failure was made during the room temperature testing reported here. The influence of test temperature will be described in a separate paper. In addition to NiTaC-13, a more
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