The effect of dispersed reactive metal oxides on the oxidation resistance of nickel-20 Wt pct chromium alloys

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EXPERIMENTAL PROCEDURE Alloys P r e p a r e d by Mechanical Alloying (M.A.) The s t a r t i n g m a t e r i a l s included - 3 0 0 mesh size Inco Type 123 nickel powder, - 200 mesh chromium powder and powders of the r e a c t i v e m e t a l oxides ranging from 0.01 to 0.1 #m in diam. The m a t e r i a l s w e r e p r o c e s s e d in a high e n e r g y m i l l (attritor) as d e s c r i b e d by Benjamin 5 and consolidated in a conventional 750 ton e x t r u sion p r e s s at t e m p e r a t u r e s up to 1100~ using e x t r u sions r a t i o s of 12:1 or g r e a t e r . The consolidated a l loys were heat t r e a t e d at t e m p e r a t u r e s ranging from 982 to 1316~ for a maximum t i m e of 2 h. Many of the a l l o y s were also subjected to i n t e r m e d i a t e hot working and additional heat t r e a t m e n t s in o r d e r to obtain c e r tain grain s t r u c t u r e s . The c h e m i c a l compositions of the a l l o y s p r e p a r e d by m e c h a n i c a l alloying together with those of the other a l l o y s investigated a r e given in Table I. Examples of the d i s p e r s o i d d i s t r i b u t i o n s that may be achieved via the mechanical alloying p r o c e s s a r e given in Fig. 1. These r e s u l t s a r e quite s i m i l a r to those found in c o m m e r c i a l TDNiCr sheet shown in Fig. 2. The grain s t r u c t u r e s of the alloys a r e d e s c r i b e d in Table II. A t y p i c a l example of the elongated grain s t r u c t u r e which can be developed in a l l o y s p r e p a r e d by the mechanical alloying technique is shown in Fig. 3. Other Alloys Both the Com Ni-20Cr and TDNiCr w e r e obtained f r o m c o m m e r c i a l s o u r c e s . The f o r m e r was given a 1 h anneal at 982~ p r i o r to t e s t i n g while the l a t t e r was used in the a s - r e c e i v e d condition. The Lab Ni20Cr alloy was produced in the l a b o r a t o r y by vacuum induction m e l t i n g and hot r o l l i n g at 1177~ The c h e m ical composition and d e s c r i p t i o n of m e t a l l o g r a p h i c f e a t u r e s of these m a t e r i a l s a r e p r e s e n t e d in Tables I a n d IL VOLUME 7A, MARCH t976-379

Oxidation T e s t i n g

(a)

I s o t h e r m a l T e s t s in T h e r m a l Balance. Rectangular s h e e t s p e c i m e n s m e a s u r i n g 20.3 m m by 10.2 m m by 1.2 m m w e r e p o l i s h e d to a 600 grit finish, u l t r a s o n i c a l l y c l e a n e d in t o l u e n e and r i n s e d in ethyl a l c o h o l . They w e r e then suspended by a platinum w i r e in a quartz tube. A r e s i s t a n c e wound furnace was r a i s e d to surround the tube so that the s p e c i m e n was e x p o s e d in a uniform hot zone at the d e s i r e d t e m p e r a t u r e (1000, 1100 and 1200~ Dry air w a s p a s s e d o v e r the s p e c i m e n at a v e l o c i t y of 0.42 c m / s . Water vapor was not included, s i n c e p r e v i o u s s t u d i e s 6 had indicated that it has little influence on the oxidation b e h a v i o r of this type of n i c k e l - b a s e a l l o y . The s p