An exploratory study of a zirconium-modified, precipitation-strengthened nickel-30 copper alloy
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THEMONEL* alloy 400, nominally Ni-30 Cu,
is used
*MONEL is a registered trademark of the Intemational Nickel Company Inc.
in m o d e r a t e t e m p e r a t u r e applications where good c o r r o s i o n r e s i s t a n c e is r e q u i r e d . In general, MONEL a l loy 400 is not used in high t e m p e r a t u r e environments b e c a u s e of its low mechanical strength and poor oxidation r e s i s t a n c e at t e m p e r a t u r e s above about 800 K. However, r e c e n t work (Refs. 1, 2 and 3) has identified MONEL alloy 400 as a suitable c a t a l y s t for the r e d u c tion of nitrogen oxide (NOx) from internal combustion engines. In this application the alloy o p e r a t e s at t e m p e r a t u r e s f r o m nominally 975 to 1200 K, and it plays an active r o l e in the NOx reduction p r o c e s s as the MONEL alloy is continuously subjected to oxidation and reduction r e a c t i o n s . One m a j o r p r o b l e m a s s o c i ated with the use of MONEL alloy 400 in c a t a l y t i c r e a c t o r s has been the lack of l o n g - t e r m durability. Data f r o m Ref. 2 indicates that the d u r a b i l i t y is r e l a t e d to grain boundary degradation and subsequent l o s s in strength. Thus a high strength Ni-30 Cu alloy with i m proved grain boundary strength and s t a b i l i t y is d e s i r able for this application. As the oxidation behavior of the alloy is i m p o r t a n t in c a t a l y t i c applications, any attempt to strengthen the b a s e Nt-30 Cu alloy must not affect the o v e r a l l s u r f a c e oxidation c h a r a c t e r i s t i c s . This r e q u i r e m e n t probably p r o h i b i t s use of the high strength a l u m i n u m - m o d i f i e d MONEL alloy K-500 (nominally Ni-30 Cu-3 A1) as a continuous, unreducible alumina s c a l e could be formed. One p o s s i b l e method of strengthening Ni-30 Cu without affecting the oxidation behavior would be to introduce a low volume f r a c t i o n of inert p a r t i c l e s or p r e c i p i t a t e s into the alloy m a t r i x . To this end, an attempt was made to strengthen a nominally Ni-30 Cu alloy by slight a l l o y ing additions of z i r c o n i u m followed by a hydrogen a n neal. This p r o c e d u r e was designed to introduce a d i s JOHN D. WHITTENBERGER is MaterialsEngineer, Lewis Research Center, Cleveland,Ohio 44135. Manuscript submitted February 7, 1974. METALLURGICAL TRANSACTIONS
p e r s i o n of ZrHz p a r t i c l e s in the m a t r i x in the same manner that ZrHz p r e c i p i t a t e s a r e f o r m e d in Mg-0.5 ? a l l o y s (Refs. 4 and 5). While p r e c i p i t a t e s were f o r m e in the z i r c o n i u m - m o d i f i e d Ni-30 Cu alloy, the precipit a t e s were not ZrH~ as intended, but r a t h e r an i n t e r m t a l l i c compound. However, the p r e c i p i t a t e s did increa the elevated t e m p e r a t u r e strength without affecting th oxidation c h a r a c t e r i s t i c s of the base alloy. This r e p o r t d e s c r i b e s the r e s u l t s of an e x p l o r a t o r y study where a t h e
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