Age-hardening characteristics in an AuCu-14at.%Ag alloy

Journal of Alloys and Compou,uks, 176 (1991) 2 6 9 - 2 8 3 JAL 0018

269

Age-hardening characteristics in an AuCu-14at.%Ag alloy Kunihiro Hisatsune and Koh-Ichi Udoh Department of Dental Materials Scienze, Nagasaki U~iversity School of Dentistry, 7-1 Sakarnoto-machi, Neugasaki (Japan)

Bambag Irawan Sosrosoedirdjo Department of Dental Materials, Faculty of Denti.stv~j, University of Indonesia, Salemba 4, Jakarta (Indonesia)

Toshihiko Tani and Katsuhiro Yasuda Department of Dental Materials Science, Nagasaki University School of Dentistry, 7-1 Sakarnoto-machi, Nagasaki (Japan) (Received April 22, 1991)

Abstract A g e - h a r d e n i n g c h a r a c t e r i s t i c s a n d a s s o c i a t e d p h a s e t r a n s f o r m a t i o n s were studied by electrical resistivity, h a r d n e s s tests, X-ray a n d e l e c t r o n diffraction, 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 a n d s c a n n i n g e l e c t r o n m i c r o s c o p y o b s e r v a t i o n s in a n A u C u - 1 4 a t . % A g p s e u d o b i n a r y alloy. T h e critical t e m p e r a t u r e of o r d e r i n g was d e t e r m i n e d to b e 6 5 0 K a n d t h e s p i n o d a l t e m p e r a t u r e w a s e s t i m a t e d to be 6 4 0 K. Age h a r d e n i n g at 6 2 3 K was a t t r i b u t e d to n u c l e a t i o n a n d g r o w t h p r o c e s s e s of t h e AuCu II' m e t a s t a b l e o r d e r e d p h a s e e v e n t h o u g h t h e a g i n g t e m p e r a t u r e w a s j u s t b e l o w the s p i n o d a l t e m p e r a t u r e . W h i l e h a r d e n i n g in t h e l o w e r t e m p e r a t u r e r a n g e t o o k place at two stages, t h e f o r m e r was due to s p i n o d a l d e c o m p o s i t i o n a n d t h e latter w a s b r o u g h t a b o u t b y o r d e r i n g of AuCu I' a n d / or AuCu II' m e t a s t a b l e p h a s e s d e p e n d i n g on a g i n g t e m p e r a t u r e . Overaging w a s i n d u c e d by g r a i n b o u n d a r y p r e c i p i t a t i o n of AuCu I a n d / o r AuCu II o r d e r e d a n d silver-rich a2 stable phases.

1. I n t r o d u c t i o n In recent years phase transformation in commercial dental gold alloys has been studied considerably to elucidate the age-hardening mechanism [ 1 - 6 ]. However, explication of the mechanisms of age hardening in commercial dental gold alloys is very difficult, because the alloys have an extremely complex combination of constituents, frequently containing five or more constituents. Therefore, it is necessary to study age-hardening characteristics and the associated phase transformation in Au-Cu-Ag ternary alloys, since commercial dental gold alloys are composed of gold, copper and silver as essential constituents. It has been known that the Au-Cu-Ag ternary system exhibits three distinguishable phase changes in certain composition regions, i.e. ordering based on AuCu or CuaAu, precipitation induced by a nucleation and growth

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mechanism and spinodal decomposition. It is also well known that these phase changes bring about a significant age hardening in the alloys depending on their composition, especially on the atomic ratio of gold and copper, with appropriate heat treatment. Figure 1 shows an isothermal section at 573 K of the plausible Au-Cu-Ag ternary phase diagram which was constructed by superimposing the theoretical phase diagram calculated using the cluster variation method by Yamauchi e t a l . [7] on experimental data [8-14]. In the phase diagram, the dots show the composition of the experimental alloys which were examined mainly using transmission electron microscopy (TEM) and selected area electron diffraction (SAED) as well as X-ray diffraction (XRD) techniques. The double circle exhibits the composition of the alloy used in the present study. The full lines correspond to the miscibility gap of two-phase decomposition and the three-phase triangles. The spinodal locus of 02G/OC 2 = 0 (G, free energy; C, composition of solute) is supposed to be located at the position shown as a broken line. The shaded area indicates the composition region corresponding to commercial dental gold alloys; therefore, it is thought that the present alloy is one of a representative composition of commercial dental gold alloys. Thus, the present study is carried out to elucidate the age hardening mechanism and characteristics in the AuCu-14at.% Ag alloy as a representative composition for commercial dental gold alloys.

2. E x p e r i m e n t a l details The alloying constituents used in the present study had a purity of better than 99.99%. An alloy of composition Au-43at.%Cu-14at.% Ag was prepared by melting in a high frequency induction furnace. Test pieces for hardness measurements and scanning electron microscopy (SEM) examination were

271

cold rolled to sheets. Foils of 0.1 m m t h i c k n e s s w e r e u s e d for electrical resistivity m e a s u r e m e n t s and TEM o b s e r v a t i o n s . Discs o f 3 m m d i a m e t e r w e r e p u n c h e d out f r o m the s h e e t for TEM study. All t e s t p i e c e s w e r e well a n n e a l e d at 973 K for 1.8 ks to obtain a s i n g l e - p h a s e solid solution and t h e n q u e n c h e d into i c e - b r i n e . Electrical resistivity w a s m e a s u r e d by a p o t e n t i o m e t r i c m e t h o d on a q u e n c h e d thin foil s p e c i m e n during c o n t i n u o u s h e a t i n g and cooling at a c o n s t a n t rate of 1 , 7 × 10 -3 K s ~ f r o m r o o m t e m p e r a t u r e to the solution t r e a t m e n t t e m p e r a t u r e . T e s t p i e c e s for h a r d n e s s m e a s u r e m e n t w e r e isot h e r m a l l y a g e d at 393, 473, 573, 623 and 6 7 3 K for v a r i o u s p e r i o d s a f t e r solution t r e a t m e n t . SEM o b s e r v a t i o n s w e r e m a d e on s p e c i m e n s w h i c h w e r e e t c h e d in an a q u e o u s solution of 10% p o t a s s i u m c y a n i d e a n d 10% a m m o n i u m p e r s u l p h a t e after polishing by s t a n d a r d m e t a l l o g r a p h i c t e c h n i q u e s . T h e discs w h i c h w e r e a g e d at different t e m p e r a t u r e s for v a r i o u s p e r i o d s of t i m e to p r o d u c e p h a s e t r a n s f o r m a t i o n s w e r e e l e c t r o t h i n n e d to t r a n s p a r e n c y by a double-jet t e c h n i q u e in a solution of 35 g c h r o m i u m trioxide in 2 0 0 ml acetic acid a n d 10 ml distilled water. A 2 0 0 kV e l e c t r o n m i c r o s c o p e e q u i p p e d with a s p e c i m e n - t i l t i n g device w a s e m p l o y e d . The XRD s t u d y w a s c a r r i e d on the p o w d e r a g e d s p e c i m e n s with nickel-filtered Cu K a radiation.

3. R e s u l t s

3.1. Electrical resistivity An a n i s o t h e r m a l h e a t i n g a n d cooling e x c u r s i o n on a s o l u t i o n - t r e a t e d a n d q u e n c h e d s p e c i m e n w a s carried out to identify t e m p e r a t u r e r e g i o n s h a v i n g significant c h a n g e s . In Fig. 2, c u r v e a s h o w s resistivity c h a n g e s r e p r e s e n t e d b y the ratio of the resistivity at a given t e m p e r a t u r e a n d t h a t at the solution t r e a t m e n t t e m p e r a t u r e ( 9 7 3 K). Curve b is the a s s o c i a t e d t e m p e r a t u r e derivative

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272 curve, dp/dT, of curve a. Here, p and T a r e electrical resistivity and temperature respectively. The heating curve of resistivity as shown in Fig. 2 yields three significant inflection points (curve a). These inflection points suggest phase transformation at 790 K, 650 K and 593 K in the alloy present. In curve b, two sharp double peaks at 580 K and 640 K and an appreciable peak at around 780 K show up. The double and neighbouring peaks are thought to be characteristics of o r d e r - d i s o r d e r phase transformation and a two-phase decomposition respectively. As seen in Fig. 2, these resistivity changes can be attributed to AuCu I (598 K), AuCu II (650 K) ordering and a two-phase decomposition (790 K) of the alloy investigated.

3.2. Age-hardening characteristics Figure 3 shows changes in hardness during isothermal aging at various temperatures. It is obvious that predominant age hardening occurs below 623 K which is lower than the critical temperature T¢ = 650 K for ordering. In the range 6 2 3 - 6 7 3 K the hardening curve exhibits an S shape which implies the occurrence of nucleation and growth processes by aging. On the contrary, the hardening curves caused by aging below 573 K represent a two-stage hardening after exhibiting a rapid increase in hardness at an early stage of aging. Thus, it is assumed that two different phase transformation processes o ccu r in aging, associated with hardening at a temperature between 623 K and 573 K. A marked decrease in hardness, however, is found after a lengthy aging period by he t e r ogeneous processes at the grain boundaries as will be shown later.

3.3. X-ray diffraction study An XRD study was carried out to elucidate the sequence of phase changes associated with age hardening. Figure 4 represents changes in diffraction profiles of the 331 and 420 fundamental diffraction reflections during aging at 673 K for various periods. The solution-treated (ST) specimen, designated 350 393K o 473K • 573K 300 ~ ~ 623K •

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av, gives an f.c.c, diffraction p a t t e r n with a lattice p a r a m e t e r a = 0 . 3 9 0 7 nm. T w o s e t s of diffraction p e a k s of the 331 and 4 2 0 reflections, d e s i g n a t e d as a~ a n d a2 p h a s e s , s h o w u p in diffraction profiles d u r i n g aging. S u p e r l a t t i c e reflections a n d the s i d e - b a n d s w e r e n o t o b s e r v e d at p o s i t i o n s e x p e c t e d in t h e XRD p a t t e r n . T h e lattice p a r a m e t e r s of the a~ a n d a2 p h a s e s w e r e a(al) = 0 . 3 8 9 7 n m a n d a(a2)= 0 . 4 0 5 6 n m respectively. Thus, it is s u g g e s t e d t h a t a t w o - p h a s e d e c o m p o s i t i o n t a k e s place during a g i n g at 673 K. F i g u r e 5 s h o w s c h a n g e s in the 111 and 2 0 0 f u n d a m e n t a l and t h e 001 a n d 1 l 0 s u p e r l a t t i c e reflections during a g i n g at 623 K for v a r i o u s p e r i o d s .

274 Superlattices formed in this temperature range are identified as metastable AuCu II' and stable AuCu II with orthorhombic long-period superstructures. The Miller indices used here for AuCu II' and AuCu II ordered phases are expressed as domain size M = 5 according to Johansson and Linde [15]. Although the two-phase decomposition takes place at a relatively early stage of aging, the metastable AuCu II' ordered phase disappears with lengthy aging. Finally, a stable AuCu II ordered phase is observed together with stable el and ex2 phases. No shifts in peaks are found in the diffraction profiles for all phases generated by aging at 623 K. Different changes in X-ray diffraction profiles show up after aging at 573 K, as seen in Fig. 6. Shifts in superlattice reflections are visible in diffraction profiles of metastable AuCu I' with f.c.t, structure and AuCu II' with long-period-ordered phases. This will be caused by variations in the composition of AuCu I' a n d / o r in the antiphase domain size of AuCu II' phases during aging at 573 K. Furthermore, this provides evidence for the h o m o g e n e o u s process of phase changes in this temperature range. In Fig. 6, a h eter o g en e ous process is also revealed in the later stage of aging. This is a sign that metastable AuCu I' and AuCu II' ordered phases coexist with stable AuCu I and AuCu II as well as with the a2 phase after aging at 573 K for 6 Ms. The metastable AuCu I' phase was only found as an ordered phase together with ao phase after aging at 523 K for 300 ks. However, one must assume that the a0 phase will change into a~ and ~2 phases by further aging even if in this t e m p e r a t u r e range.

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275

3.4. T r a n s m i s s i o n electron m i c r o s c o p y a n d selected a r e a electron d i ffr a ctio n e x a m i n a t i o n s F i g u r e s 7(a) and 7(b) r e p r e s e n t t h e d a r k field i m a g e s p r o d u c e d b y using t h e 2 0 0 , , a n d 2 2 0 ~ r e f l e c t i o n s r e s p e c t i v e l y , t a k e n f r o m a s p e c i m e n a g e d at 6 7 3 K for 180 ks. An SAED p a t t e r n c o r r e s p o n d i n g to the central a r e a of Figs. 7(a) a n d 7(b) is s h o w n in Fig. 7(c). T h e SAED p a t t e r n s h o w s the c o e x i s t e n c e of the ae' p h a s e a n d d i s o r d e r e d solid solution of the ao phase, b o t h f.c.c, in s t r u c t u r e . No s u p e r l a t t i c e reflections are o b s e r v e d in the SAED p a t t e r n . F r o m d a r k field i m a g e s , it is o b v i o u s t h a t the a2' p h a s e is f o r m e d on the {100} p l a n e s o f t h e m a t r i x o~ p h a s e as a plate-like p r e c i p i t a t e by a n u c l e a t i o n and g r o w t h m e c h a n i s m . This is b e c a u s e not only the c h a r a c t e r i s t i c satellite or s i d e - b a n d s in SAED p a t t e r n s but also the m o d u l a t e d s t r u c t u r e in TEM i m a g e s w e r e not o b s e r v e d at an early s t a g e of aging. F i g u r e 8 s h o w s TEM m i c r o g r a p h s a n d SAED p a t t e r n t a k e n f r o m a s p e c i m e n a g e d at 6 2 3 K f o r 100 ks. T h e SAED p a t t e r n s h o w n in Fig. 8(d) i n d i c a t e s t h e f o r m a t i o n o f a m e t a s t a b l e AuCu I' and AuCu II' o r d e r e d p h a s e s w h i c h are a r r a n g e d with their c a x e s as t h r e e orientational v a r i a n t s X, Y a n d Z; f o r e x a m p l e , the Z varianL m e a n s its c axis is parallel to the Z direction, i.e. the incident e l e c t r o n b e a m . This configuration is o b s e r v e d

Fig. 7. TEM images and an SAED pattern taken for the specimen aged at 673 K for 180 ks; (a) the dark field image formed by using the 220a~ spot; (b) the dark field image formed by using the 220~z spot; (c) SAED pattern corresponding to (a) and (b).

276

Fig. 8. TEM images and an SAED pattern taken for the specimen aged at 623 K for 100 ks: (a) the dark field image formed by using the 110 superlattice spot; (b) the dark field image formed by using the 001x superlattice spot; (c) the dark field image formed by using the 001y superlattice spot; (d) SAED pattern taken from the central area of the images.

clearly in t h e d a r k field i m a g e s s h o w n in Figs. 8(a), 8(b) a n d 8(c). It is also c l e a r t h a t the o r d e r e d p h a s e s c o n s i s t o f thin p l a t e l e t s a n d block-like regions. T h e AuCu I' o r d e r e d p h a s e d i s a p p e a r e d in f u r t h e r aging. It w a s t h e r e f o r e n o t a m e t a s t a b l e o r d e r e d p h a s e in this t e m p e r a t u r e range. As a result of lengthy a g i n g at 623 K, the m e t a s t a b l e AuCu II' o r d e r e d p h a s e i n c r e a s e d in size. Figure 9 exhibits TEM m i c r o g r a p h s a n d an SAED p a t t e r n t a k e n f r o m a s p e c i m e n a g e d at 623 K f o r 1 Ms. In t h e SAED p a t t e r n it is o b v i o u s t h a t a m e t a s t a b l e AuCu II' o r d e r e d p h a s e with t h r e e o r i e n t a t i o n a l v a r i a n t s o f its c-axis is f o r m e d . Reflections f r o m the m e t a s t a b l e a2' p h a s e are also distinguishable. B e c a u s e d a r k field i m a g e s a r e p r o d u c e d f r o m the s a m e area, it is p o s s i b l e to distinguish t h e s e p h a s e s in the m i c r o s t r u c t u r e f r o m e a c h other. As s e e n in Figs. 9(a) a n d 9(b), block-like d i s o r d e r e d a2' fills the s p a c e b e t w e e n AuCu II' o r d e r e d p h a s e regions. In addition, thin p l a t e l e t s o f A u C u II' a r e f o u n d inside the block-like a2'. T h u s it is p o s t u l a t e d t h a t t h e block-like a2' p h a s e m a y be c o v e r e d b y the thin p l a t e l e t s o f the A u C u II' o r d e r e d p h a s e (Fig. 9(c)). This m i c r o s t r n c t u r e a n d a t o m i c configu r a t i o n w e r e a n a l y s e d b y high r e s o l u t i o n e l e c t r o n m i c r o s c o p y , a n d t h e results will b e p u b l i s h e d e l s e w h e r e [16].

277

Fig. 9. TEM images and an SAED pattern taken for the specimen aged at 623 K for 1 Ms: (a) the dark field image formed by using the 220~, spot; (b) the dark field image formed by using the 220(AuCu II'x,v) fundamental spot; (c) the dark field image formed by using the 110(AuCu II'z) superlattice spot; (d) SAED pattern taken from the central area of the images.

Figure 10 shows TEM micrographs and an SAED pattern produced from a specimen aged at 573 K for 18 ks. The SAED pattern reveals the formation of the AuCu I' and AuCu II' ordered phases. Dark field TEM images (Figs. 10(a), 10(b) and 10(c)) suggest that these ordered phases are formed as platelets on ~100} planes. Although the diffraction spots arising from the a2' phase are indistinguishable in the SAED pattern, it can be deduced that the 200, 020, 220 and equivalent fundamental spots for the ordered phases coincide almost with the equivalent spots from the a~' phase. As a result of further aging at 573 K, the diffraction spots from the a2' phase became visible in SAED patterns.

3.5. Scanning electron microscopy observations To clarify changes in microstructure corresponding to phase transformations, especially in a grain interior and at the grain boundary, SEM observations were made on specimens aged at 673, 623 and 573 K for various periods. For specimens aged at 673 K, a characteristic plate-like precipitate was observed in the interior of the grain after prolonged aging. A lamellar structure was also found primarily along grain boundaries, which later extruded into the interior of the grain.

278

Fig. 10. TEM images and an SAED pattern taken for the specimen aged at 573 K for 1.8 ks: (a) the dark field image formed by using the 110 superlattice spot; (b), (c) dark field images formed by using the 001 x and 001y supperlattice spots respectively; (d) SAED pattern taken from the central area of the images.

T h e SEM m i c r o g r a p h s o f Fig. 1 1 d e m o n s t r a t e a g r o w t h o f the lamellae by c o n s u m p t i o n of the interior of the grain c a u s e d b y a g i n g at 6 2 3 K. A W i d m a n s t a t t e n - l i k e s t r u c t u r e w h i c h s u g g e s t s f o r m a t i o n o f n e t w o r k s of parallel p l a t e l e t s on certain lattice p l a n e s is o b s e r v e d in grains (Figs. l l ( a ) a n d 1 l ( b ) ) . A f t e r p r o l o n g e d aging, the W i d m a n s t a t t e n s t r u c t u r e c h a n g e s into a different m i c r o s t r u c t u r e a s s e e n in Fig. l l ( c ) o r Fig. l l ( d ) , w h i c h is an e n l a r g e m e n t p a r t o f Fig. 1 l ( c ) . It c a n be s e e n t h a t the m i c r o s t r u c t u r e is c o m p o s e d o f p a r t i c l e s of cuboidal s h a p e w h i c h a r e similar to etch pits. T h e r e f o r e , it is s u g g e s t e d t h a t t h e s e c u b o i d a l p a r t i c l e s c o r r e s p o n d to the block-like a2' r e g i o n as indicated in the TEM m i c r o g r a p h s a n d SAED p a t t e r n o f Fig. 9. C h a n g e s in the SEM m i c r o s t r u c t u r e a f t e r a g i n g at 5 7 3 K are r e p r e s e n t e d in Figs. 12(a), 12(b) a n d 12(c). The o c c u r r e n c e o f twinning is o b s e r v e d at a n early s t a g e o f aging, while t h e twin s t r u c t u r e d i s a p p e a r s a n d a l t e r n a t e s to a s t a i r - s t e p a r r a n g e m e n t of c u b o i d a l b l o c k s as s e e n in Fig. 12(d) which is an e n l a r g e m e n t of p a r t of Fig. 12(c) at a l a t e r s t a g e o f aging. F r o m the XRD a n d TEM studies, it is e x p e c t e d t h a t this s t a i r - s t e p s t r u c t u r e c o n s i s t s

279

Fig. 11. Changes in SEM images caused by aging at 623 K for various periods: (a), (b), (c) aging periods of 60 ks, 1.8 Ms and 6 Ms respectively; (d) enlargement of part of (c).

of c u b o i d a l block-like a2' p h a s e s s u r r o u n d i n g the m a t r i x AuCu I' and AuCu II' o r d e r e d p h a s e s . Lamellar p r e c i p i t a t e s are also f o u n d at the grain b o u n d a r y .

4. D i s c u s s i o n

T a b l e 1 s u m m a r i z e s the r e s u l t s o b t a i n e d in the p r e s e n t study. Notwiths t a n d i n g t h a t d i s c r e p a n c i e s in sensitivity to h e a t t r e a t m e n t actually exist b e c a u s e of t i m e lags due to d i f f e r e n c e s in the sizes of the s p e c i m e n s a m o n g t h e e x p e r i m e n t a l m e t h o d s , the results exhibit g o o d a g r e e m e n t for critical t e m p e r a t u r e s , c o e x i s t i n g p h a s e s in e a c h t e m p e r a t u r e range, a n d m i c r o s t r u c tures. TEM a n d SAED studies m a k e it p o s s i b l e to d e t e c t p h a s e t r a n s f o r m a t i o n s o c c u r r i n g in t h e interior of grains d e p e n d i n g on m i c r o s t r u c t u r e a n d crystal s t r u c t u r e . SEM o b s e r v a t i o n s reveal grain b o u n d a r y p r e c i p i t a t i o n b y a hete r o g e n e o u s m e c h a n i s m . On the c o n t r a r y , XRD studies reveal a w h o l e p r o c e s s o f p h a s e t r a n s f o r m a t i o n s o c c u r r i n g in the alloy, if the e x p e r i m e n t s are c a r r i e d out b y c a r e f u l i s o t h e r m a l aging. Thus, s e q u e n c e s o f p h a s e t r a n s f o r m a t i o n s in the p r e s e n t alloys at v a r i o u s t e m p e r a t u r e s are as follows.

280

Fig. 12. Changes in SEM images caused by aging at 573 K for various periods: (a), (b), (c) aging periods of 60 ks, 600 ks and 18 Ms respectively; (d) enlargement of part of (c).

(1) At 673 K, homogeneous

ao

(f.c.c.)

) al'

(f.c.c.)+a~'

(f.c.c.)

(within interior

of the

heterogeneous

grain) , al (f.c.c.)+a2 (f.c.c.) (at grain boundaries) (2) At 623 K, homogeneous

ao

> AuCu II'

(LPAPB o r d e r e d ) + a 2 '

(within interior of the

heterogeneous

grain) ~ A u C u II' +AuCu II + a~ (tic.c0 + a2' + a2 , AuCu II + al + a2 (at grain boundaries) where LPAPB denotes long period antiphase boundary. (3) At 573 K, homogeneous

a0

> AuCu

I'

(f.c.t.)+AuCu

II'+a,/

(within interior of the heterogeneous

grain)-~AuCu I' +AuCu I +AuCu II' +AuCu II + a2' + a2 I + AuCu II + a2 (at grain boundaries) (4) At 523 K,

~ AuCu

homogeneous

ao

~ AuCu I ' + a2' (within interior of the g r a i n ) ~ A u C u I' +AuCu heterogeneous

I + a 2 ' +o/2 ) AuCu I - ~ a 2 (at grain boundaries) Phases designated with a prime, e . g . AuCu II', 0/e', indicate metastable phases formed in the interior of grains; phases without a prime are equilibrium phases formed at grain boundaries. Age hardening in the alloy occurs depending

28l TABLE 1 Summary of the results obtained by various experimental methods

T(K)

P/PT

--800K-

(z o

Hv

TEM and SAED

XRD

SEM

790K (x,1 AuCu I1'+ ~2 (N & G process)

+ --700K-O~2

No hardening, .... ( {:z~ + a 2 ) .... 673K

-

0/,,I+ 0(,2

---

Grain 1 boundary reaction )

648K 650K Sigmoidal IWidmanstatten' AuCu II .ha..rden!.n.g. rAuCu II'+AuCu II~ AuCu I1'+ (Z2 Grain (Splnodal) boundary + 623K ~ + ~I+ (~2 , AuCu r+AuCu I1' reaction --600K+ 0(;2 598K Two stage Twinning AuCu I'+AuCu I1' Hardening Grain AuCu I+AuCu I1+ boundary AuCu I 573K reaction 568K + CY,2. j Two stage + Hardening AuCu I'+ ~2 523K

/

- - 5 0 0 K -

(Z2

--40OK-

282

on the sequence of these phase transformations which itself depends on temperature. In Fig. 2, there were two distinct hardening modes, i.e. a sigmoidal hardening curve obtained by aging at 623 K and a two-stage hardening curve obtained by aging below 573 K. It is thought that sigmoidal hardening is caused by a nucleation and growth process of phase transformations, i.e. formation of the AuCu II' metastable LPAPB ordered phase. Although characteristic side-bands or satellite reflections arising from spinodal decomposition are not observed in XRD or SAED patterns during the early stage of aging at 623 K, it is expected that the spinodal locus is located at around 640 K, because TEM and SAED studies revealed that the growth rate of the AuCu II' phase was considerably lower for aging at 643 K than at 633 K. Thus it is thought that ordering at 643 K is brought about by a nucleation and growth mechanism while ordering at 633 K is accelerated by the spinodal decomposition. Nevertheless, aging at 623 K exists just below the spinodal locus; a rapid increase in hardness is not observed (Fig. 2). Miyazaki et al. [17] dealt with a theoretical analysis of two-phase decomposition derived from the Fourier expression of the non-linear diffusion equation on the basis of the Cahn-Hilliard flux equation. They presented composition profiles of a solute with continuing aging for various alloys which had a composition between the centre of the miscibility gap and just on the spinodal locus. Aging proceeded by a tendency to a nucleation and growth process during aging as was indicated by rectangularly shaped composition profiles. According to Miyazaki et al., it was thought that hardening at 623 K was brought about by a nucleation and growth process of a twophase decomposition accompanied by AuCu II' ordering of the copper-rich metastable phase. When aging was carried out below 573 K, the first hardening stage was induced by spinodal decomposition as was indicated by a rapid increase in hardness during the initial stage. The second hardening stage was attributed to strain induced by ordering of the metastable AuCu I' a n d / o r AuCu II' phases depending on aging temperature. The drastic decrease in hardness at a later stage of aging coincides with the occurrence of grain boundary precipitates composed of AuCu I and/or AuCu II and the a~ stable phases as was observed in SEM micrographs.

Acknowledgment The authors gratefully acknowledge the financial support of the present study through the Japanese Ministry of Education, Science and Culture, Grant-in-Aid for Scientific Research (B) (01460241).

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