The effect of an air environment on the creep and rupture behavior of a nickel-base high temperature alloy
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effect of an oxidizing environment on the c r e e p and r u p t u r e behavior of high t e m p e r a t u r e alloys has not been extensively, or even consistently, documented in the l i t e r a t u r e . F o r example, t h e r e a r e conflicting r e p o r t s that an a i r environment leads to a f a s t e r c r e e p r a t e and s h o r t e r rupture life in nickel and a n i c k e l - b a s e alloy x and that an a i r environment leads to a slower c r e e p r a t e and longer rupture life in some i r o n - b a s e a l l o y s and in Inconel X. s Recently, it has been demons t r a t e d that an a i r environment has a profound effect on the low cycle fatigue behavior at elevated t e m p e r a t u r e s for Udtmet 500. 3,4 In the r e c e n t work it was shown that the f a i l u r e mode in a i r at 1088 K (1500~ is g r a i n boundary c r a c k i n g (emanating f r o m the s u r face) due to s t r e s s - a s s i s t e d oxidation of g r a i n boundar i e s . s On the other hand, the behavior at this t e m p e r a t u r e In a vacuum of ~ 10-a t o r r was that c h a r a c t e r i s t i c of r o o m t e m p e r a t u r e : c r a c k nucleation in slip bands and t r a n s g r a n u l a r c r a c k propagation. 4 An analogous study of a s i m i l a r m a t e r i a l t e s t e d in static loading has a p p a r e n t l y not yet been c a r r i e d out. Since n i c k e l - b a s e alloys a r e g e n e r a l l y evaluated by c r e e p and rupture testing tn a i r , and a r e used in an oxidizing environment in gas t u r b i n e s , it s e e m s p r u dent to obtain a c l e a r e r idea of the effects of an a i r environment In this r e g a r d . The p r e s e n t study was c a r r i e d out for this purpose. A common n i c k e l - b a s e turbine blade alloy, Udimet-700, was s e l e c t e d as a typical p o l y c r y s t a l l i n e m a t e r i a l used in both wrought and c a s t form. It will be shown that under c e r t a i n conditions an a i r environment acts in a way s i m i l a r to that o b s e r v e d in fatigue, s'4 In this case a s t r e s s r u p ture t e s t b e c o m e s analogous to a high t e m p e r a t u r e s t r e s s c o r r o s i o n test.
P. N. CHAKU, formerly Research Fellow, Department of Metallurgy
and MaterialsScienceand LRSM, Universityof Pennsylvania,Philadelphia, is now with Midvale-HappenstallCo., Philadelphia,Pa. 19140. C. J. McMAHON,JR., is AssociateProfessor,Department of Metallurgy and MaterialsScience,Universityof Pennsylvania,Philadelphia,Pc. 19174. Manuscript submitted February 27, 1973. METALLURGICAL TRANSACTIONS
EXPERIMENTAL PROCEDURE The alloy was p u r c h a s e d f r o m Special Metals, Inc. as 1.78 cm (0.7 In.) diam. bar stock; the composition Is shown in Table I. This m a t e r i a l was machined d i r e c t l y to make wrought s p e c i m e n s and was also r e m e l t e d In vacuum and i n v e s t m e n t - e a s t (at the General E l e c t r i c R e s e a r c h and Development c e n t e r ) to make c a s t s p e c i m e n s . The s p e c i m e n shape Is shown In F
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