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A MICROSTRUCTURAL STUDY OF Co-20 wt.%
Cr-10 wt.% Al USING APFIM AND TEM
M. Burke, J. Caola
To cite this version:
M. Burke, J. Caola. A MICROSTRUCTURAL STUDY OF Co-20 wt.% Cr-10 wt.% Al US- ING APFIM AND TEM. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-245-C7-249.
�10.1051/jphyscol:1986742�. �jpa-00225936�
JOURNAL D E PHYSIQUE
Colloque C7, supplement au n o 11, Tome 47, Novembre 1986
A MICROSTRUCTURAL STUDY OF Co-20 wt.% Cr-10 wt.% A1 USING APFIM AND TEM
M.G. BURKE and J.R. CAOLA
Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261, U.S.A.
ABSTRACT- CoCrAl alloys, which are widely used as oxidation-resistant c o a t i n g s o n high t e m p e r a t u r e n i c k e l - b a s e s u p e r a l l o y t u r b i n e components, h a v e been examined by APFIM and AEM. The complex microstructures that are developed in these a l l o y s are readily amenable to APFIM-TEM characterization. In this paper, we report microstructura 1 and microchemica 1 observations of the phases present i n a C o - 2 0 wt.%Cr-10 wt.% A 1 a 1 l o y ( n o m i n a l c o m p o s i t i o n ) wi-th particular emphasis on the combined TEM-APFIM identification of fine second phase precipitates.
INTRODUCTION
T h e C o C r A l a l l o y c o a t i n g s a r e c o m m e r c i a l l y i m p o r t a n t a s protective coatings to reduce the in-service oxidation of nickel-base superall oy components. CoCrAl a ll0yS are Fenera1 1 y thought to be composed of two major phases: a(
,
an fcc solid solution which is rich in Co and Cr; and9 ,
a B2 (CsC1) ordered structure based upon CoA1[1].Since the microstructure is known to affect the oxidation resistance of the material, we have conducted a detailed investi~ation into the composition, morphology and distribution of the various phases in a cast Co
-
20wt.% Cr - 10wt.% A1 alloy [2,3]. In this paper, we report o n a segment of this investigation that has used APFIM and T E M to examine in detail the composition of the major constituents d and4 ,
and that has used these techniques to identify and characterize a previously unreported secondary distribution of fine precipitates with the (3 constituent.
EXPERIMENTAL
T h e C o - 20wt.% C r - 10wt.% A 1 a l l o y e x a m i n e d i n t h i s investiqation was prepared by vacuum arc meltinq and drop castinq into a water-cooled copper chi1 1. After annealinq at 1423 K for 100 hrs, the material was air-cooled. APFIM specimens were prepared by a two stage e lectropo 1 ishing technicue [ 4
?.
A1 1 atom ?robe ana 1 yses were conducted a t 50 K and usinq a 15% pulse fraction. Transmission electron microscopy was performed on a JEOL 200CX operated at 200 k V .Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986742
C7-246 JOURNAL DE PHYSIQUE
RESULTS AND DISCUSSION
T h e m i c r o s t r u c t u r e p r o d u c e d u p o n a i r - c o o l i n q f r o m 1 4 2 3 K c o n s i s t e d o f a two p r i m a r y c o n s t i t u e n t s :
4
( f c c ) a n d$
( C s C 1 ) . D e t a i l e d TEM e x a m i n a t i o n of t h e A l - r i c h p h a s e f u r t h e r r e v e a l e d t h e p r e s e n c e of c o a r s e p l a t e - l i k e p r e c i p i t a t e s and a u n i f o r m d i s t r i b u t i o n of f i n e p r e c i p i t a t e s . T h i s i s i l l u s t r a t e d i n t h e c e n t e r e d d a r k f i e l d TEM m i c r o g r a p h i n F i q u r e 1. T h e d i a m e t e r o f t h e s e f i n e d i s c - l i k e p r e c i p i t a t e s was d e t e r m i n e d from TEM m i c r o g r a p h s t o b e-
20 nm, a n d a n a v e r a g e d i s c t h i c k n e s s of a p p r o x i m a t e 1 y 1.4 nm was c a l c u l a t e d f r o m t h e s t r e a k i n g o b s e r v e d i n t h e e l e c t r o n d i f f r a c t i o n p a t t e r n s [ 2 , 3 ] .u o e i l d and @ p h a s e s w e r e examined i n t h e APFIM. A neon i o n image a n d a m a s s s p e c t r u m o b t a i n e d f r o m t h e & p h a s e i s shown i n F i c u r e 2.
The c o m p o s i t i o n o f t h i s p h a s e was measured t o b e Co-9.4+/- 0 . 6 a t . % A 1 - 2 3 . 9 + / - 1 . O a t . % C r . T h i s i s c o n s i s t e n t w i t h t h e c o m p o s i t i o n o f &, o b t a i n e d by e n e r g y d i s p e r s i v e x - r a y a n a l y s i s , Co - - 1 2 a t . % A 1 - -26 a t . % C r [ l ] . Both t h e a t and t h e o r d e r e d . @ a r e imaged i n t h e f i e l d - i o n m i c r o q r a p h o f F i g u r e 3 . Atom p r o b e a n a l y s i s was c o n d u c t e d i n e a c h p h a s e a n d a l o n g t h e i n t e r f a c e . T h e m a s s s p e c t r u m a c q u i r e d f r o m t h e o r d e r e d @ p h a s e r e v e a l e d a c o m p o s i t i o n o f Co - 38.7
+ / -
1.7 a t . % A l - 5.1+/-
0.6 a t . % C r . I n t e r e s t i n g l y , s u l p h u r was d e t e c t e d i n t h e v i c i n i t y o f t h e oC/@ i n t e r f a c e , b u t n o t i n t h e @ p h a s e . The m a s : s p e c t r u m o b t a i n e d f r o m a t o m p r o b e a n a l y s i s i n t h e p h a s e n e a r t h e i n t e r f a c e i s p r e s e n t e d i n F i c u r e 4.S m a l l d a r k 1 y i m a q i n q p r e c i p i t a t e s w e r e o b s e r v e d w i t h i n t h e o r d e r e d (3 p h a s e ( s e e t h o s e i d e n t i f i e d i n F i g u r e 5 ). Atom p r o b e a n a l y s i s o f t h i s a r e a r e v e a l e d t h a t t h e s e r e q i o n s w e r e d e p l e t e d i n a l u m i n u m , w i t h a n a p p r o x i m a t e c o m p o s i t i o n o f Co
-
5 a t . % A 1 - 27.2 a t . % C r . T h i s a g r e e d v e r y w e l l w i t h STEM e n e r g y d i s p e r s i v e m i c r o a n a l y s i s o f t h e c o a r s e ? l a t e - l i k e p r e c i p i t a t e s . T h e m i c r o c o m p o s i t i o n a l i n f o r m a t i o n c o u p l e d y i t h t h e d e t a i l e d e l e c t r o n d i f f r a c t i o n r e s u l t s and a n a l y s i s [ 2 ] i n d i c a t e t h a t t h e c o a c s e p l a t e - l i k e and f i n e d i s c - s h a p e d p r e c i p i t a t e s i n t h e @ p h a s e w e r e € Co.SUMMARY
Atom p r o b e f i e l d - i o n microscopy a n d t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y h a v e b e e n s u c c e s s f u l l y employed i n t h e c h a r a c t e r i z a t i o n o f a Co- 20 a t . % A L - 20 a t . % C r a 1 l o y a n d h a v e i d e n t i f i e d p r e v i o u s l y u n i d e n t i f i e d c o a r s e p l a t e s a n d f i n e d i s c - 1 i k e p r e c i p i t a t e s of Co i n a (3 B2-ordered A l - r i c h m a t r i x . F u r t h e r r e s e a r c h i s i n p r o g r e s s .
ACKNOWLEDGEMENTS
The a u t h o r s a r e q r a t e f u l t o D r . G.H. M e i e r f o r h i s encouragement an?
s u p p o r t ; D r . A. A. Ashary f o r s u p p l y i n g t h e c a s t a 1 l o y ; D r . J. S p r a c u e f o r a r r a n g i n q t h e u s e o f t h e N a v a l R e s e a r c h L a b TEM; a n d D r . S.S.
B r e n n e r , D i r e c t o r of t h e Atom P r o b e L a b o r a t o r y , f o r u s e of l a b o r a t o r y f - c i l i t i e s .
1. J.A. S p r a q u e , V. P r o v e n z a n o , a n d F.A. Smidt,Jr.,Thin S o l i d F i l m s , 95 ( 1 9 8 2 ) p.57.
-
2. J.R. Caola, Scripta Net., 20 (1986) 2 . 5 1 9 .
3. J.R. C a o l a , P r o c . 4 4 t h A n n u a l E M S A ed. b y G.W. B a i l e y ( S a n Francisco Press, 1986) in press.
ci2itates in the matrix.
8
a (fcc)
t h e coa
MASS SPECTRUM
r s e a n d
m/e AMU
Firlure 2. A ) N e o n i o n i z a c e of t h e
d
phase, a n d B ) c o r r e s p o n d i n ? m z s s spectrum.C7-248 JOURNAL RE PHYSIQUE
F i c u r e 3 . FIM n i c r o q r a p h showin? t h e p r e s e n c e of
d
and @ p h a s e s .MASS SPECTRUM
m/e AMU
F i r l u r e 4 . A t o m ?robe P a s s s p e c t r u m o b t a i n e d f r o m a n a l y s i s o f O(
n e a r t h e
d /g
i n t e r f a c e .- 3 .
e l c u r e 5 . A ) F I M m i c r o c r a p h d e 2 i c t i n c t h e s m a l l d a r k l y i m a q i n c p r e c i p i t a t e s i n t h e Q ( C s C 1 ) phase.
