Oxide morphology and spalling model for NiAl
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C Y C L I C oxidation s t u d i e s in the MCrA1Y s y s t e m s have shown that the b e n e f i c i a l effects of active m e t a l additions a r e due to m i n u t e effects at the A1203 oxidem e t a l i n t e r f a c e - i n c l u d i n g oxide pegging via Y203 and the p r e v e n t i o n of void f o r m a t i o n , x,2 While i n f e r i o r to some NiCrA1Y alloys, NiA1 has also proved itself to be a v e r y oxidation r e s i s t a n t m a t e r i a l . Studies at the NASA Lewis R e s e a r c h C e n t e r have dealt with m a p p i n g of cyclic oxidation r e s i s t a n c e in the NiCrA1 s y s t e m and have shown that Ni-40 at. pct A1 is n e a r the bounda r y for a c c e l e r a t e d oxidation. 3,4 W o r k h e r e has also b e e n c o n c e r n e d with the case of v a r i o u s oxide s p a l l i n g models b a s e d on a) a continuous loss of oxide, 5 b) a s t e p - p r o c e s s a s s u m i n g a constant f r a c t i o n of oxide t h i c k n e s s s p a l l e d each cycle, 6 and c) a n o t h e r s t e p p r o c e s s a s s u m i n g a c o n s t a n t f r a c t i o n of oxide a r e a s p a l l e d each cycle.7 The p u r p o s e of the p r e s e n t work was to study in d e t a i l the A1203 oxide morphology and o x i d e - m e t a l i n t e r f a c e of a n i c k e l - r i c h NiA1 alloy with the hope of p r o v i d i n g i n f o r m a t i o n r e l e v a n t to s p a l l i n g t h e o r i e s in the MCrA1Y s y s t e m . A second p u r p o s e was to study the k i n e t i c s of s p a l l i n g as a c r i t i c a l e v a l u a t i o n of one of the s t e p - p r o c e s s s p a l l models as well as to obtain the e x p e r i m e n t a l data. MATERIALS AND PROCEDURE The m a t e r i a l used for this p r o g r a m was made f r o m a l u m i n u m shot and n i c k e l 270 sheet, both 99.99 pct p u r e . A 2 x 7 x 7 cm c a s t i n g was produced by t u n g s t e n a r c m e l t i n g in a r g o n . The charge c o m p o s i t i o n was 40 at. pct a l u m i n u m ; wet c h e m i c a l a n a l y s i s of the c a s t i n g was 41.8 • 0.2 at. pct (24.8 wt pct). The c a s t i n g was s e c t i o n e d and m a c h i n e d into 0.3 • 1.5 • 2 cm test couJAMES L. SMIALEKis Research Engineer, NASA Lewis Research Center, Cleveland,Ohio 44135. Manuscript submitted May 27, 1977. METALLURGICALTRANSACTIONSA
pons. S u r f a c e s were polished through 3 ~t d i a m o n d except for long t e r m (500 and 1500 h) cyclic oxidation s p e c i m e n s which were f i n i s h e d with 600 g r i t e m e r y . F u r n a c e oxidation t e s t s were r u n in 1 arm a i r at 1100 ~ • 5~ Cyclic t e s t s involved 1, 20, and 50 h heating cycles, a l l with 20 m i n cooling to ~65~ Morphologies of r e s u l t a n t oxide and m e t a l s u r f a c e s w e r e e x a m i n e d in a s c a n n i n g e l e c t r o n m i c r o s c o p e equipped with an e n e r g y d i s p e r s i v e s p e c t r o g r a p h i c (EDS) unit. A c c e l e r a t i n g voltage was 25 kV. Cyclic t e s t e d s p e c i m e n s were e x a m i n e d after i n t e r v a l s r a
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