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ATOM PROBE ANALYSIS OF G-P ZONES AND PRECIPITATE COMPOSITIONS IN AlZnMg ALLOYS
S. Ortner, C. Grovenor, G. Smith
To cite this version:
S. Ortner, C. Grovenor, G. Smith. ATOM PROBE ANALYSIS OF G-P ZONES AND PRECIPITATE
COMPOSITIONS IN AlZnMg ALLOYS. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-355-
C6-360. �10.1051/jphyscol:1987658�. �jpa-00226866�
JOURNAL DE PHYSIQUE
Colloque C6, suppliment au n O 1 l , Tome 48, novembre 1987
ATOM PROBE ANALYSIS OF G-P ZONES AND PRECIPITATE COMPOSITIONS IN AlZnMg ALLOYS
S.R. Ortner, C.R.M. Grovenor and G.D.W. Smith
Department of Metallurgy and Science of Materials, Parks Road, Oxford, OX1 3PH, U.K.
A b s t r a c t . APFIM experiments have been used t o c h a r a c t e r i s e t h e e a r l y s t a g e s of p r e c i p i t a t i o n i n high p u r i t y and commercial AlZnMg a l l o y s . The development of G-P zones has been followed. T h e i r composition, and t h o s e of i n t e r m e t a l l i c phases i n a commercial a l l o y , 7018, have been s t u d i e d a f t e r a v a r i e t y of h e a t t r e a t m e n t s .
1. INTRODUCTION.
The decomposition of s u p e r s a t u r a t e d s o l i d s o l u t i o n s i n AlZnMg a l l o y s i s known t o t a k e p l a c e v i a t h e formation of m e t a s t a b l e phases. Evidence h a s been found f o r t h e e x i s t e n c e of s o l u t e c l u s t e r s , vacancy-solute complexes, G-P zones and i n t e r m e d i a t e monoclinic/hexagonal phases, ( v a r i o u s l y known a s r ) ' , X and R) [ I ] , p r i o r t o t h e formation of t h e s t a b l e MgZn2 ( q ) o r ( A 1 , z ~ ) ~ ~ M g ~ ~ (T) phases. APFIM i s a technique which i s i d e a l l y sui'ted t o s t u d y t h i s kind of complex decomposition process, and v a l u a b l e i n f o r m a t i o n on t h e s t r u c t u r e and composition of m e t a s t a b l e p r e c i p i t a t e s i n A1-Cu and A1-Ag a l l o y s has a l r e a d y been r e p o r t e d from FIM and APFIM experiments
[2,3]. The i n v e s t i g a t i o n of s m a l l p r e c i p i t a t e p a r t i c l e s i n t h e s e a l l o y s i s f a c i l i t a t e d by t h e f a c t t h a t r e g i o n s of high copper o r s i l v e r c o n t e n t image more b r i g h t l y t h a n t h e aluminium-rich matrix. U n f o r t u n a t e l y , n e i t h e r z i n c n o r magnesium image b r i g h t l y i n AlZnMg specimens [ 4 ] , s o i t i s n o t p o s s i b l e t o l o c a t e G-P zones o r o t h e r m e t a s t a b l e Zn
+
Mg-rich p r e c i p i t a t e s i n a FIM image.I n t h i s work, t h e VG FIMlOO APFIM h a s been used t o s t u d y t h e e a r l y s t a g e s of decomposition i n both a high p u r i t y AlZnMg a l l o y and a commercial a l l o y (7018).
F i e l d i o n t i p s were p o l i s h e d i n c o l d 25% HNO3 i n MeOH o r CH3COOH/HN03/H3P04
s o l u t i o n s , and were imaged i n A r , which p e r m i t t e d c o n t r o l l e d removal of t h e a n o d i c oxide film. During a n a l y s i s , t h e r e s i d u a l gas p r e s s u r e was l e s s t h a n T, and t h e specimen temperature below 80K. A p u l s e f r a c t i o n of 25% was used i n a l l t h e experiments d e s c r i b e d i n t h i s paper.
2. RESULTS ON HIGH PURITY AlZnMg.
The bulk composition of t h i s a l l o y was A 1 7.3XZn 2.6%Mg by weight, and l i e s i n t h e a
+
q+
T t e r n a r y phase f i e l d of t h e e q u i l i b r i u m diagram. T h i s a l l o y was chosen because previous work had i n d i c a t e d t h a t G-P zones a r e r e a d i l y formed d u r i n g ageing a t 150°C a f t e r s o l u t i o n t r e a t m e n t a t 46S°C and preageing a t room temperature, [5].Samples aged a t 150°C f o r 0.5, 1 and 2.5 hours have been s t u d i e d . F i g u r e 1 shows a t y p i c a l FIM image of a high p u r i t y AlZnMg t i p i n argon a t about 70K. No s i g n of p r e c i p i t a t e s was found i n any of t h e s e images, s o t h e APFIM experiments were c a r r i e d o u t by probing ' b l i n d ' , u s u a l l y c l o s e t o (100) o r (111) poles. Some evidence was found f o r t h e s e g r e g a t i o n of s o l u t e atoms t o low i n d e x planes a t temperatures above 100K. F i g u r e 2 shows a l a d d e r diagram through a G-P zone which c o n t a i n e d 15 Zn and 16 Mg atoms. We e s t i m a t e t h a t t h i s number of s o l u t e atoms corresponds t o a s i n g l e p l a n e of atoms a t t h e e v a p o r a t i o n v o l t a g e of t h i s specimen, but cannot determine whether t h e G-P zone c o n s i s t s of a s i n g l e pure Zn
+
Mg plane o r two o r more p l a n e s of mixed A 1+
Zn+
Mg. F i g u r e 3, by c o n t r a s t , shows a G-P zone which a p p a r e n t l yArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987658
C6-356 JOURNAL
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PHYSIQUEc o n t a i n s a p u r e aluminium p l a n e between two s o l u t e - r i c h r e g i o n s . T h i s may w e l l b e a 3 p l a n e s t r u c t u r e of t h e k i n d s e e n by Hono e t a1 121 i n A1-Cu a l l o y s . Some l a r g e r s o l u t e - r i c h r e g i o n s were a l s o d e t e c t e d .
T a b l e 1 g i v e s t h e r e s u l t s of e x t e n s i v e a n a l y s e s of samples h e a t - t r e a t e d a t 150°C.
We show t h e approximate t h i c k n e s s of t h e G-P zones (assuming t h a t zones of t h e form shown i n F i g u r e 2 a r e s i n g l e p l a n e s ) , and t h e i r Mg t o Zn r a t i o s . It i s c l e a r from t h e s e d a t a t h a t t h e s i z e of t h e m e t a s t a b l e p r e c i p i t a t e s g r a d u a l l y i n c r e a s e s w i t h a g e i n g t i m e , b u t t h a t t h e Zn t o Mg r a t i o remains c o n s t a n t a t 1:1, a l t h o u g h t h e r e i s c o n s i d e r a b l e s c a t t e r i n t h i s r a t i o between i n d i v i d u a l p a r t i c l e s . Dunkeloh e t a 1 [ 8 ] i n t e r p r e t t h e i r SAXS d a t a a s showing t h a t t h e Zn c o n t e n t of G-P zones i n a s i m i l a r a l l o y i s 40
-
45 a t % . T h i s , combined w i t h o u r d a t a on t h e r a t i o of Zn t o Mg i n t h e zones, s u g g e s t s t h a t t h e zones c o n t a i n v e r y l i t t l e aluminium. It i s a l s oobserved t h a t t h e m a t r i x s t i l l c o n t a i n s s i g n i f i c a n t c o n c e n t r a t i o n s of Zn and Mg even a f t e r 2.5 hours a t 150°C
-
l e v e l s of 2 a t % of b o t h t h e s e s o l u t e e l e m e n t s a r e q u i t e commonly d e t e c t e d .TABLE 1. G-P ZONES I N HIGH PURITY AlZnMg SPECIMENS.
AGEING TIME 150°c
NUMBER OF SOLUTE NUMBER OF ZONES RATIO OF Zn TO Mg
PLANES I N ZONE ANALYSED (Mean 7 S.D.)
3 . RESULTS FROM COMMERCIAL ALLOY 7018.
7018 c o n t a i n s 4.5-5.5% Zn, 0.7-1.5% Mg, 0.15-0.5% Mn, 0.1-0.25% Z r , O.Z%max C r , 0.3Xmax S i , 0.45%max F e , 0.15%max T i and 0.25%max Cu by weight. I n such a complex a l l o y we expect a g r e a t e r d i v e r s i t y of phases t o be found, i n c l u d i n g t h e Al-Zr p a r t i c l e s which a s s i s t i n g r a i n r e f i n e m e n t and have been a n a l y s e d i n t h e APFIM work of S a k u r a i e t a 1 [ 6 ] on A1-Li a l l o y s . Although t h i s a l l o y hardens a t room
t e m p e r a t u r e , no Zn
+
Mg G-P zones were found a f t e r n a t u r a l ageing. However, a s i n g l e copper r i c h zone was found i n m a t e r i a l aged a t room t e m p e r a t u r e , which i s a s u r p r i s i n g d i s c o v e r y g i v e n t h e low c o n c e n t r a t i o n of copper i n t h e a l l o y . I n m a t e r i a l h e a t e d f o r 16 hours a t 150°C a f t e r s o l u t i o n t r e a t m e n t a t 460°C (T6 c o n d i t i o n ) , Zn+
Mg G-P zones of one and two s o l u t e p l a n e s were d e t e c t e d . These zones showed a l a r g e s c a t t e r i n Zn t o Mg r a t i o , from 0.7-
2.3, w i t h a n a v e r a g e around 1, a s found i n t h e h i g h p u r i t y m a t e r i a l .Large, b r i g h t l y imaging p r e c i p i t a t e s c o n t a i n i n g t r a n s i t i o n m e t a l s were f r e q u e n t l y found i n t h e s e samples. F i g u r e 4 shows a Ne FIM image from a specimen which c o n t a i n e d two t y p e s of p r e c i p i t a t e s , one w i t h a v e r y w e l l d e f i n e d c r y s t a l l o g r a p h y , and t h e o t h e r i n which t h e p o l e s a r e very p o o r l y r e s o l v e d . (The aluminium m a t r i x does n o t image a t a l l a t t h e low t i p v o l t a g e s needed t o o b t a i n good images of t h e p r e c i p i t a t e d phases.) APFIM a n a l y s i s of t h e b r i g h t e r p a r t i c l e s showed t h a t they c o n t a i n e d h i g h l e v e l s of Zr and s m a l l e r q u a n t i t i e s of Zn and Ti. F i g u r e 5 shows a c o m p o s i t i o n p r o f i l e a c r o s s one such p a r t i c l e . Although t h e a l l o y i s i n t h e a
+
A13Zr phase f i e l d i n t h e e q u i l i b r i u m diagram, t h e s e p a r t i c l e s do n o t seem t o be A13Zr. We do n o t b e l i e v e , under t h e c o n d i t i o n s of a n a l y s i s used i n t h i s s t u d y , t h a t s u f f i c i e n t dc e v a p o r a t i o n of t h e aluminium c o u l d have o c c u r r e d t o modify t h e
composition of t h e s e p a r t i c l e s from AlgZr t o t h e composition of approximately A1l(Zr,Ti,Zn)l which i s observed i n t h e p a r t i c l e shown i n F i g u r e 5. T h i s i s n o t i n agreement w i t h t h e work of S a k u r a i e t a 1 [ 6 ] who assumed t h a t very s i m i l a r , b r i g h t l y
imaging p a r t i c l e s were t h e e q u i l i b r i u m Al-Zr phase. These p r e c i p i t a t e s a r e s u g g e s t e d t o a c t a s h e t e r o g e n e o u s n u c l e a t i o n s i t e s f o r MgZn2 [ 7 ] , b u t , even though we a n a l y s e d many Al-Zr p a r t i c l e s , s e g r e g a t i o n of Mg and Zn t o t h e i n t e r f a c e w i t h t h e m a t r i x was n o t d e t e c t e d . S e g r e g a t i o n of o t h e r t r a n s i t i o n e l e m e n t s t o t h i s i n t e r f a c e was sometimes observed. F i g u r e 6 is a mass spectrum from an Al-Zr p r e c i p i t a t e showing s m a l l peaks from C r , Mn, S i and F e , a l l of which were s e g r e g a t e d t o t h e i n t e r f a c e between t h e p a r t i c l e and t h e m a t r i x .
C r , Mn, S i and Fe a r e a l s o found d u r i n g APFIM a n a l y s i s of t h e second kind of
p a r t i c l e shown i n F i g u r e 4. The p r o p o r t i o n s of C r , Mn and Fe vary from p a r t i c l e t o p a r t i c l e , b u t t h e o v e r a l l c o m p o s i t i o n i s always c l o s e t o t h a t of t h e bulk phase AlgSil.g(Fe,Mn,Cr)2. These p a r t i c l e s can be very l a r g e , o f t e n f i l l i n g most of t h e imaging a r e a of a f i e l d i o n t i p , F i g u r e 7 .
4 . CONCLUSIONS.
APFIM a n a l y s i s h a s been used t o d e t e c t Zn
+
Mg-rich G-P zones i n b o t h h i g h p u r i t y and commercial ALZnMg a l l o y s , and t o show t h a t t h e r a t i o of Zn t o Mg i n t h e s e zones i s roughly c o n s t a n t r e g a r d l e s s of zone s i z e and a g e i n g time. Whether t h e l a r g e r p a r t i c l e s s h o u l d be c o n s i d e r e d t o be q' r a t h e r t h a n G-P zones i s u n c l e a r , b u t i t seems u n l i k e l y t h a t a G-P zone can c o n t a i n a s many a s 5 p l a n e s e n r i c h e d w i t h Zn and Mg. There i s evidence t h a t g ' forms a s a r e s u l t of a n u c l e a t i o n and growth p r o c e s s which does n o t depend on t h e p r e s e n c e of G-P zones [ 8 ] , but t h e l a r g e r p a r t i c l e s found i n t h i s work seem t o form by c o n t i n u e d growth of t h e one o r two p l a n e G-P zones. The p r e c i s e sequence of m e t a s t a b l e phase growth i n t h e s e a l l o y s i s o b v i o u s l y worthy of more d e t a i l e d i n v e s t i g a t i o n .O t h e r i n t e r m e t a l l i c phases have been found i n t h e commercial a l l o y , a s might have been expected; a non-equilibrium phase of approximate composition AlZr, c o n t a i n i n g s i g n i f i c a n t amounts of Zn and T i , and a n A l - r i c h phase c o n t a i n i n g Cr, Mn, S i and Fe.
5. ACKNOWLEDGEMENTS.
It i s a p l e a s u r e t o acknowledge t h e c o n t r i b u t i o n of t h e APFIM Group i n Oxford t o t h e work r e p o r t e d i n t h i s paper, and P r o f e s s o r S i r P e t e r H i r s c h FRS i s thanked f o r t h e p r o v i s i o n of l a b o r a t o r y f a c i l i t i e s . One of t h e a u t h o r s , SO, i s s u p p o r t e d by RARDE ( C h e r t s e y )
.
The APFIM f a c i l i t y i n Oxford h a s been s u p p o r t e d by g r a n t s from t h e P a u l I n s t r u m e n t Fund of t h e Royal S o c i e t y .FIGURE 1.
FIM image of h i g h p u r i t y AlZnMg t i p imaged i n Ar a t a b o u t 65K.
JOURNAL DE PHYSIQUE
6. REFERENCES.
11). H.Loffler, 1-Kovacs and J.Lendvai J.Mater.Sci. 182115 1983.
[2]. K.Hono et a1 Scripta Metall. 487 and Phi1os.Mag.A 53 495 1986.
[3]. K.Osamura, T.Nakamura, A.Kobayashi, T.Hashizume and T.Sakurai Acta Metall.
34 1563 1986.
[4]. E.~.~o~es, A.R.Waugh, P.J.Turner, P.F Mills and M.J Southon 1nst.Phys.Conf.
Ser. No. 36 p.343 1977.
[5]. P.Roper and J.W.Martin Z.Metal1. 7 0 4 0 0 1979.
[6]. T.Sakurai, A.Kobayashi, Y.Hasegawa, A.Sakai and H.W.Pickering Scripta Metall.
20 1131 1986.
[7 1. X ~ d e n i s , J-P.Moulin and A.Guilhardis Mem.Sci.Rev.Met. 66 135 1969.
[a]. K.H.Dunkeloh, G.Kra1i.k and V.Gerold Z.Metal1. 65 291 1974.
150
'
V)l o o - g
: Ol
lx i +
i t f N
A 1 Ions
50
G4...,... &
..,...,...r...I...t...,... ...,...r...,...T..,300 400 500
FIGURE 2. A ladder diagram though a G-P zone in high purity AlZnMg. This zone contains 15 Zn and 16 Mg atoms, which can be considered to make up a single solute plane at the evaporation voltage of this specimen.
AI Ions j ,
T' ...I.."".'
...I...,...I... l.f..ff. .I....." ... ...
300 400 500
FIGURE 3. A ladder diagram though a G-P zone in high purity AlZnMg. This zone
contains sufficient Zn and Mg atoms to form two complete solute planes, and these
planes appear to be separated by a single plane of pure Al. G-P zones with this
kind of ladder diagram have been considered to consist of two solute rich planes in
Table 1.
1
FIGURE 4. A Ne FIM imane of b r i e h t l v imaging p r e c i p i t a t e s i n 7018. The upper p a r t i c l e i s of an Al-Zr phase, while t h e lower p a r t i c l e i s r i c h i n C r , Mn, S i and Fe.Number of ions
FIGURE 5. A composition p r o f i l e through a Zr-containing p a r t i c l e i n 7018. The low concentrations of T i and Zn i n t h e p a r t i c l e a r e c l e a r l y shown, and p a r t i c l e s of t h i s kind have a measured composition c l o s e t o A1l(Zr,Zn,Ti)l. We do not believe t h a t they can be the equilbrium A13Zr phase. The other t r a n s i t i o n elements shown i n t h e mass spectrum i n Figure 6 a r e found concentrated a t t h e i n t e r f a c e between t h e Z r - r i c h p a r t i c l e and the matrix.
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PHYSIQUEMass-to-charge Ratio
FIGURE 6. A spectrum from a Z r - c o n t a i n i n g p a r t i c l e i n 7018 i n c l u d i n g t h e i n t e r f a c e w i t h t h e m a t r i x t o which o t h e r t r a n s i t i o n m e t a l s have s e g r e g a t e d .
FIGURE 7. A Ne FIM image of a l a r g e p a r t i c l e c o n t a i n i n g F e , S i , C r , Mn, and aluminium. T h i s image i s formed a t a t i p v o l t a g e a t which t h e A1 m a t r i x i s p a r t i a l l y v i s i b l e .
