Factors affecting adhesion of oxide scales on alloys
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Fig. 1--Microstructure of Ni-15.1 Ta-6.7 A1 after oxidation at 1200~ for 5 rain. in 1 atm of air. (a) Alloy structure showing 7-7' eutectic, 7' colony boundaries (arrows) and 3" denuded zone. (b) Structure similar to (a) emphasizing the highly i r regular nature of the alloy surface and revealing the cratered protrusions (arrows). (c) Scanning electron micrograph a c centuating the continuous A1203 layer (arrows) beneath the intermediate, spinel zone. d u r i n g o x i d a t i o n . F u r t h e r m o r e , the s i t e s of t h e s e c a v i t i e s c o r r e s p o n d with r e g i o n s w h e r e the A12Os l a y e r f i r s t b e c o m e s continuous a s can be s e e n in F i g s . 2 and 3, w h e r e the t o t a l oxide t h i c k n e s s is s i g n i f i c a n t l y VOLUME 5, JULY 1974-1685
Fig. 2 - - P a r t i a l l y detached s c a l e f r o m Ni-15.1 Ta-6.7 A1 oxidized at 1200~ for 100 h, in 1 arm of a i r showing d e p r e s s i o n s in the mixed oxide scale c o r r e s p o n d i n g with the p r o t r u s i o n s
from the alloy substrate.
Fig. 3 - - D e p r e s s i o n in the mixed oxide scale of the s p e c i m e n shown in Fig. 2 r e v e a l i n g extensive porosity in the A1203
layer (arrow). s m a l l e r at s i t e s where c a v i t i e s a r e p r e s e n t . It is p r o posed that during oxidation s m a l l voids a r e nucleated by v a c a n c y condensation at those s i t e s where the A120s i n i t i a l l y b e c o m e s continuous, and that these voids grow during subsequent oxidation into s i z e a b l e c a v i t i e s . F u r t h e r , it is believed that these c a v i t i e s a r e initiation s i t e s for oxide s p a l l a t i o n under the influence of t h e r m a l l y induced s t r e s s e s . The oxide s c a l e f r o m the alloy doped with 0.04 Y had the s a m e l a y e r e d s t r u c t u r e as that found on the undoped alloy, but it also contained Y20 s p a r t i c l e s . The Y203 was usually confined to those a r e a s where y t t r i d e p r e c i p i t a t e s had been p r e s e n t in the alloy. In general, the oxide s c a l e on the a l l o y doped with 0.04 Y was quite uniform and the e x t r e m e l y thin AI~O3 s c a l e a p p e a r e d to have become continuous at all s i t e s on the alloy s u r face at roughly the s a m e t i m e , Fig. 6. Some i r r e g u l a r ities were o b s e r v e d on the alloy s u r f a c e but these i r r e g u l a r i t i e s g e n e r a l l y did not have smooth tops and v e r y few e x a m p l e s of i n t e r f a c i a l c a v i t i e s were d e tected. However, in spite of this i m p r o v e m e n t in the p e r f e c t i o n of the o x i d e / s u b s t r a t e interface, the oxide s c a l e s p a l l e d f r o m this alloy during cooling, which 1686-VOLUME 5, JULY 1974
Fig. 4--Smooth, e r a t e r e d p r o t r u s i o n s and r i d g e s in the alloy s u b s t r a t e of the s p e c i m e n shown in Fig. 2. (a) G e n e r a l area. (b) Higher magnification p h o t o m i c r o g r a p h of boxed a r e a in (a). I m p r i n t s of A1203 g
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