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Age-hardening reactions in a type III dental gold alloy
Biomaterials 22 (2001) 1433}1438
Age-hardening reactions in a type III dental gold alloy Hyung-I Kim *, Young-Keun Kim , Myoung-Ik Jang , Kunihiro Hisatsune, Amal Abd El Samad Sakrana Department of Dental Materials, College of Dentistry, Pusan National University, 1-10 Ami-dong, Seo-gu, Pusan 602-739, South Korea Department of Dental Materials Science, Nagasaki University School of Dentistry, 1-7-1 Sakamoto, Nagasaki 852-8588, Japan Received 14 July 1998; accepted 29 September 2000
Abstract The age-hardening reactions in a commercial type III dental gold alloy were studied by means of hardness test, X-ray di!raction study and scanning and transmission electron microscopic observations. The hardening was attributed to the formation of the metastable AuCu 1 type ordered phase in the grain interior by the isothermal ageing at 225 and 4503C at which two hardness peaks were observed by the isochronal ageing. By ageing at 4503C, the hardening did not begin immediately because the incubation period was required. The age hardening at 2253C was characterized by a slow growth rate of the metastable AuCu 1 type ordered phase. The overageing with softening which occurred following prolonged ageing at 4503C was due to the formation of the lamellar structure composed of the Ag-rich and AuCu 1 type ordered phases at grain boundaries. 2001 Published by Elsevier Science Ltd. Keywords: Dental gold alloy; Age hardening; Metastable phase; Overageing; Lamellar structure
1. Introduction Dental casting gold alloys should contain gold and platinum-group metals of more than 75 wt% according to the American Dental Association speci"cation No. 5. It is well known that the dental casting gold alloys develop signi"cant changes in mechanical properties * such as hardness, strength and ductility * during appropriate heat treatment. These alloys were developed largely by empirical methods, which resulted in the production of the extremely complex combinations of alloys, frequently containing "ve or more constituents. They are essentially ternary system of gold, silver and copper. Platinum and palladium are added to the ternary Au}Ag}Cu system for the greater strength and higher corrosion resistance. The addition of platinum or palladium to ternary Au}Ag}Cu system gives rise to conspicuous age hardening [1,2]. Most dental gold alloys containing platinum and/or palladium exhibit conspicuous age-hardening characteristics [3}6]. Prasad et al. [3] studied the age hardening of a commercial type III dental gold alloy, and suggested
* Corresponding author. Fax: #82-51-255-7804. E-mail address: [email protected] (H.I. Kim).
that the hardening was primarily the result of a precipitation phenonmenon. Yasuda and Ohta [4] studied the age hardening of a commercial dental gold alloy by an isochronal ageing study, and concluded that the age hardening arose primarily from the formation of an AuCu 1 type ordered phase. Hisatsune et al. [5] and Tani et al. [6] investigated the age hardening of commercial type IV dental casting gold alloys by an isothermal ageing study, and reported that the formation of a metastable AuCu 1 type ordered phase within grains contributed to the hardening. Type IV dental gold alloys can be signi"cantly hardened because they are designed to be age-hardenable. Although the American Dental Association speci"cation No. 5 does not always require the age-hardenability in type III dental gold alloys, most of them are age-hardenable. The present study was carried out to elucidate the age-hardening reactions in a commercial type III dental gold alloy with small amounts of platinum and palladium.
2. Materials and methods The alloy used in this study was a commercial type III dental casting gold alloy with nominal composition of 74.0 wt% Au !13.5 wt% Ag }7.0 wt% Cu !2.4 wt%
0142-9612/01/$ - see front matter 2001 Published by Elsevier Science Ltd. PII: S 0 1 4 2 - 9 6 1 2 ( 0 0 ) 0 0 3 0 1 - X
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H.I. Kim et al. / Biomaterials 22 (2001) 1433}1438
Pt !2.0 wt% Pd !1.0 wt% Zn !0.1 wt% Ir (Degulor C, Degussa, Germany). The atomic composition of the alloy was calculated from the nominal composition to be 57.1 at% Au !19.0 at% Ag !16.8 at% Cu !1.9 at% Pt !2.9 at% Pd !2.3 at% Zn !0.1 at% Ir. Plate specimens for hardness test were "rst solutiontreated at 7503C for 30 min under an argon atmosphere to obtain a single phase and then quenched in ice brine. The specimens were isochronally aged in the 150&5503C range for 10, 20, 50 min, and were isothermally aged for the various ranges of time up to 50,000 min in a molten salt bath at 225 and 4503C corresponding to the temperatures of the two hardness peaks in the isochronal agehardening curves. Hardness measurements were made on the heat-treated plate specimens using a micro-Vickers hardness tester with a 300 gf load and a 10 s holding time. Vickers hardness results were obtained as the average values of "ve indentations. X-ray di!raction (XRD) study was carried out on the heat-treated powder specimens using an X-ray di!ractometer (D/Max-2400, Rigaku Corp., Japan). The X-ray di!ractometer was operated at 40 kV and 50 mA with nickel-"ltered Cu K & radiation. ? The heat-treated plate specimens for scanning electron microscopy (SEM) were prepared by utilizing a standard metallographic technique. The "nal etchant used was a freshly prepared aqueous solution of 10% KCN and 10% (NH ) S O . The SEM observations were made with a scanning microscope (JSM-5400, JEOL, Japan) operating at 20 kV. Discs of 3 mm diameter punched out of the cold-rolled and heat-treated sheet of 0.1 mm thickness for transmission electron microscopy (TEM) were electrothinned by a double-jet technique in a solution of 35 g of CrO , 200 ml of CH COOH and 10 ml of distilled water. A 200 kV electron microscope (H-800, Hitachi Co., Japan) equipped with a specimen tilting device was employed.
3. Results and discussion 3.1. Age-hardening characteristics Fig. 1 shows the isochronal age-hardening curves of the present alloy. The isochronal age-hardening curves display two clear hardness peaks, the "rst at about 2253C and the second at about 4503C. This behavior is similar to that found in a type IV dental gold alloy [4]. The specimens were isothermally aged at 225 and 4503C corresponding to the temperatures of the two hardness peaks in the isochronal age-hardening curves. The results of the isothermal age hardening are shown in Fig. 2. The empty squares represent the results of hardness in the grain interior for the specimen aged at 4503C, which have been measured with a 25 gf load. The hard-
Fig. 1. Isochronal age-hardening curves for 10, 20 and 50 min.
Fig. 2. Isothermal age-hardening curves at 225 and 4503C.
ness increased gradually with ageing time from the initial stage of ageing and continued progressively to increase up to 50,000 min by the isothermal ageing at 2253C. By the isothermal ageing at 4503C, the hardness did not increase in the early stages of ageing but then increased drastically and reached the maximum value. After showing the maximum hardness at 200 min, the hardness decreased gradually for the time. However, the hardness in the grain interior continued to increase. 3.2. Hardening behavior at 2253C Fig. 3 shows the SEM photograph of the specimen aged at 2253C for 50,000 min. Although the specimen aged at 2253C for 50,000 min attained the considerable hardness (Fig. 2), characteristic changes of the microstructure could not be con"rmed in the SEM photograph. Accordingly, "ne structure changes in the grain interior are assumed. To investigate a microstructural change from the single phase during isothermal ageing at 2253C, XRD
H.I. Kim et al. / Biomaterials 22 (2001) 1433}1438
Fig. 3. SEM photograph of the specimen aged at 2253C for 50,000 min.
Fig. 4. XRD patterns of the specimens solution-treated at 7503C for 30 min, and aged for 10 and 10,000 min at 2253C.
patterns were also taken from the powder specimens solution-treated at 7503C and aged at 2253C for various times. Fig. 4 shows the XRD patterns taken from the powder specimens solution-treated and aged for 10 and 10,000 min at 2253C. The solution-treated specimen,
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which is designated as phase, gives an XRD pattern of the face-centered cubic (f.c.c.) lattice. However, any characteristic changes during isothermal ageing at 2253C were not detected from the results of the XRD pattern. TEM observations of the specimens aged at 2253C were made in order to clarify whether there were any changes of the microstructure in the grain interior during the ageing, which could not be recognized by the XRD study. Fig. 5 shows the selected-area electron di!raction (SAED) pattern and the 001 dark "eld (DF) image of the V specimen aged at 2253C for 100 min. The 001 , 001 and V W 110 superlattice spots were observed in the SAED patX tern. They were identi"ed as the AuCu 1 ordered phase with three variants by the di!raction analysis. The DF image reveals the formation of very "ne ordered structure in the grain interior. This structure was identi"ed as the metastable AuCu 1 type ordered phase with a facecentered tetragonal (f.c.t.) lattice. However, it is very close to a f.c.c. structure. From the SAED pattern, the axial ratio (c/a) of the AuCu 1 phase was determined to be approximately 0.99. According to Porter and Easterling [7], it is often possible to form a coherent nucleus of a metastable phase if the "nal product phase have di!erent crystal structures. It is considered that the ordered phase with small amount of tetragonality can be produced easily in the matrix f.c.c. phase. Therefore, the formation of the AuCu 1 structure with an axial ratio close to unity produces small coherency strain, and the hardening is relatively small as can be seen in Fig. 2. Fig. 6 shows the SAED pattern and the 001 DF image V of the specimen aged at 2253C for 50,000 min. With increasing ageing time this ordered structure developed and formed platelets on the matrix 1 0 0 planes, from which the SAED pattern made the streaks along the 0 0 1 directions. The axial ratio of this ordered phase was determined from the SAED pattern to be approximately 0.93. This means that the axial ratio of the metastable AuCu 1 type ordered phase with a f.c.t. lattice gradually decreases with ageing time in the grain interior. It is considered that the development of the AuCu 1
Fig. 5. SAED pattern (A) and 001 DF image (B) taken from the specimen aged at 2253C for 100 min. V
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H.I. Kim et al. / Biomaterials 22 (2001) 1433}1438
Fig. 6. SAED pattern (A) and 001 DF image (B) taken from the specimen aged at 2253C for 50,000 min. V
Fig. 7. Changes in the XRD pattern during isothermal ageing at 4503C.
phase can increase the coherency strain and obtain an additional hardening as can be seen in Fig. 2. This grain interior reaction must contribute to the hardness increase of the specimen aged at 2253C. Hisatsune et al. [5] and Tani et al. [6] reported that the formation of a metastable AuCu 1 phase within grains contributed to the hardening in commercial type IV dental gold alloys with platinum and palladium. Yasuda and Ohta [4] reported that the hardening of a commercial dental gold alloy was considered to arise primarily from the formation of an AuCu 1 type ordered phase, and thought that the hardening was brought about by the strains generated in the nucleation period by the tetragonality of the AuCu 1 type superlattice. The hardening by the formation of the metastable AuCu 1 phase
in the grain interior has been reported in commercial dental gold alloys containing palladium [8]. It was reported that the hardening was due to the introduction of coherency strains at the interface between the metastable AuCu 1 type ordered platelets and the matrix. 3.3. Hardening behavior at 4503C Fig. 7 shows the changes in the XRD pattern during isothermal ageing at 4503C. With increasing ageing time, the 111 and 200 di!raction peaks of the phase shifted gradually toward the low di!raction angle. And then the 111 di!raction peak of the AuCu 1 ordered phase began to be detected from 200 min of ageing, and increased in intensity by prolonged ageing. The product phases
H.I. Kim et al. / Biomaterials 22 (2001) 1433}1438
Fig. 8. SEM photographs of the specimens aged at 4503C for 200 min (A), 5,000 min (B) and 50,000 min (C).
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were identi"ed as the Ag-rich phase with a f.c.c. structure and the AuCu 1 type ordered phase with a f.c.t. structure in the XRD pattern. When the alloy was isothermally aged at 4503C, the hardness did not increase in the early stage of ageing. It is thought that the main factor limiting hardening is associated with incubation period. The incubation period is required before nucleation begins. The incubation period is increased and the nucleation rate is decreased just below temperature at which the second phase is completely dissolved. In this case, the solution is only slightly oversaturated. The rate of nucleation is, accordingly, very slow [9]. The compositional boundary of the present alloy exists around the solid-state miscibility gap in the Ag}Au}Cu ternary system [10] and the "nal volume fraction of the AuCu 1 type ordered phase is much less than that of the Ag-rich phase at 4503C as can be seen in Fig. 7. Fig. 8 shows the SEM photographs of the specimen aged at 4503C for 200, 5000 and 50,000 min. Nodular products of a lamellar structure were predominantly formed at grain boundaries. These grain boundary products increased and grew into the grain interior with ageing time, although the growth rate was slow. The decrease in hardness occurred after showing the maximum hardness by prolonged ageing, although the hardness in the grain interior kept on increasing as seen in the age-hardening curves (Fig. 2). This overageing with softening coincided with the formation of the grain boundary products of a lamellar structure, as observed in the SEM photograph of the specimen aged for 200 min corresponding to the maximum hardness at 4503C. This implies that the softening is attributed to the formation of the grain boundary products, and depresses the hardening in the grain interior. Yasuda and Ohta [4] reported that the grain boundary products did not contribute to the age hardening in a commercial dental gold alloy. Hisatsune et al. [5] reported that the grain boundary reaction contributed to the softening in a commercial type IV dental casting gold alloy. Fig. 9 shows the SAED pattern and the 001 DF image V of the grain interior of the specimen aged at 4503C for
Fig. 9. SAED pattern (A) and 001 DF image (B) taken from the specimen aged at 4503C for 200 min. V
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H.I. Kim et al. / Biomaterials 22 (2001) 1433}1438
200 min. As well as by ageing at 2253C, the metastable AuCu 1 type ordered phase with three variants was formed in the grain interior. The streaks along 0 0 1 directions are clearly seen in the SAED pattern, and the formation of the plate-like ordered phase on the matrix 1 0 0 planes is clearly seen in the DF image. The axial ratio of this ordered phase was determined from the SAED pattern to be approximately 0.92. Grain interior reaction at 4503C is analogous to that at 2253C. The age hardening at 2253C was characterized by a slow growth rate of the metastable AuCu 1 type ordered phase. From the above results, it was con"rmed that the metastable AuCu 1 type ordered phase which had been formed in the phase of the grain interior was trans formed into the Ag-rich and AuCu 1 ordered phases at grain boundaries during isothermal ageing in this alloy. Thus, it can be concluded that the hardening is attributed to the formation of the metastable AuCu 1 type ordered phase in the grain interior by the isothermal ageing at 4503C and the overageing with softening is due to the formation of the grain boundary products composed of the Ag-rich and AuCu 1 type ordered phases.
References [1] Wise EM, Crowell WS, Eash JT. The role of the platinum metals in dental alloys, I & II. Trans AIME 1932;99:363}412. [2] Wise EM, Eash JT. The role of the platinum metals in dental alloys, III. Trans AIME 1933;104:276}307. [3] Prasad A, Eng T, Mukherjee K. Electron microscopic studies of hardening in type III dental alloy. Mater Sci Eng 1976;24: 179}85. [4] Yasuda K, Ohta M. Age-hardening characteristics of a commercial dental gold alloy. J Less-Common Met 1980;70:75}87. [5] Hisatsune K, Nakagawa M, Udoh K, Sosrosoedirdjo BI, Hasaka M. Age-hardening reactions and microstructures of a dental gold alloy with palladium and platinum. J Mater Sci Mater Med 1990;1:49}54. [6] Tani T, Udoh K, Yasuda K, Van Tendeloo G, Van Landuyt J. Age-hardening mechanisms in a commercial dental gold alloy containing platinum and palladium. J Dent Res 1991;70:1350}7. [7] Porter DA, Easterling KE. Phase transformations in metals and alloys. 2nd ed. London: Chapman & Hall, 1992, p. 271. [8] Yasuda K, Udoh K, Hisatsune K, Ohta M. Structural changes induced by ageing in commercial dental gold alloys containing palladium. Dent Mater J 1983;2:48}58. [9] Reed-Hill RE. Physical metallurgy principles. 2nd ed. New York: Van Nostrand, 1973, pp. 361}2. [10] Prince A, Raynor GV, Evans DS. Phase diagrams of ternary gold alloys. London: The Institute of Metals, 1990, p. 31}3.
Age-hardening reactions in a type III dental gold alloy Hyung-I Kim *, Young-Keun Kim , Myoung-Ik Jang , Kunihiro Hisatsune, Amal Abd El Samad Sakrana Department of Dental Materials, College of Dentistry, Pusan National University, 1-10 Ami-dong, Seo-gu, Pusan 602-739, South Korea Department of Dental Materials Science, Nagasaki University School of Dentistry, 1-7-1 Sakamoto, Nagasaki 852-8588, Japan Received 14 July 1998; accepted 29 September 2000
Abstract The age-hardening reactions in a commercial type III dental gold alloy were studied by means of hardness test, X-ray di!raction study and scanning and transmission electron microscopic observations. The hardening was attributed to the formation of the metastable AuCu 1 type ordered phase in the grain interior by the isothermal ageing at 225 and 4503C at which two hardness peaks were observed by the isochronal ageing. By ageing at 4503C, the hardening did not begin immediately because the incubation period was required. The age hardening at 2253C was characterized by a slow growth rate of the metastable AuCu 1 type ordered phase. The overageing with softening which occurred following prolonged ageing at 4503C was due to the formation of the lamellar structure composed of the Ag-rich and AuCu 1 type ordered phases at grain boundaries. 2001 Published by Elsevier Science Ltd. Keywords: Dental gold alloy; Age hardening; Metastable phase; Overageing; Lamellar structure
1. Introduction Dental casting gold alloys should contain gold and platinum-group metals of more than 75 wt% according to the American Dental Association speci"cation No. 5. It is well known that the dental casting gold alloys develop signi"cant changes in mechanical properties * such as hardness, strength and ductility * during appropriate heat treatment. These alloys were developed largely by empirical methods, which resulted in the production of the extremely complex combinations of alloys, frequently containing "ve or more constituents. They are essentially ternary system of gold, silver and copper. Platinum and palladium are added to the ternary Au}Ag}Cu system for the greater strength and higher corrosion resistance. The addition of platinum or palladium to ternary Au}Ag}Cu system gives rise to conspicuous age hardening [1,2]. Most dental gold alloys containing platinum and/or palladium exhibit conspicuous age-hardening characteristics [3}6]. Prasad et al. [3] studied the age hardening of a commercial type III dental gold alloy, and suggested
* Corresponding author. Fax: #82-51-255-7804. E-mail address: [email protected] (H.I. Kim).
that the hardening was primarily the result of a precipitation phenonmenon. Yasuda and Ohta [4] studied the age hardening of a commercial dental gold alloy by an isochronal ageing study, and concluded that the age hardening arose primarily from the formation of an AuCu 1 type ordered phase. Hisatsune et al. [5] and Tani et al. [6] investigated the age hardening of commercial type IV dental casting gold alloys by an isothermal ageing study, and reported that the formation of a metastable AuCu 1 type ordered phase within grains contributed to the hardening. Type IV dental gold alloys can be signi"cantly hardened because they are designed to be age-hardenable. Although the American Dental Association speci"cation No. 5 does not always require the age-hardenability in type III dental gold alloys, most of them are age-hardenable. The present study was carried out to elucidate the age-hardening reactions in a commercial type III dental gold alloy with small amounts of platinum and palladium.
2. Materials and methods The alloy used in this study was a commercial type III dental casting gold alloy with nominal composition of 74.0 wt% Au !13.5 wt% Ag }7.0 wt% Cu !2.4 wt%
0142-9612/01/$ - see front matter 2001 Published by Elsevier Science Ltd. PII: S 0 1 4 2 - 9 6 1 2 ( 0 0 ) 0 0 3 0 1 - X
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H.I. Kim et al. / Biomaterials 22 (2001) 1433}1438
Pt !2.0 wt% Pd !1.0 wt% Zn !0.1 wt% Ir (Degulor C, Degussa, Germany). The atomic composition of the alloy was calculated from the nominal composition to be 57.1 at% Au !19.0 at% Ag !16.8 at% Cu !1.9 at% Pt !2.9 at% Pd !2.3 at% Zn !0.1 at% Ir. Plate specimens for hardness test were "rst solutiontreated at 7503C for 30 min under an argon atmosphere to obtain a single phase and then quenched in ice brine. The specimens were isochronally aged in the 150&5503C range for 10, 20, 50 min, and were isothermally aged for the various ranges of time up to 50,000 min in a molten salt bath at 225 and 4503C corresponding to the temperatures of the two hardness peaks in the isochronal agehardening curves. Hardness measurements were made on the heat-treated plate specimens using a micro-Vickers hardness tester with a 300 gf load and a 10 s holding time. Vickers hardness results were obtained as the average values of "ve indentations. X-ray di!raction (XRD) study was carried out on the heat-treated powder specimens using an X-ray di!ractometer (D/Max-2400, Rigaku Corp., Japan). The X-ray di!ractometer was operated at 40 kV and 50 mA with nickel-"ltered Cu K & radiation. ? The heat-treated plate specimens for scanning electron microscopy (SEM) were prepared by utilizing a standard metallographic technique. The "nal etchant used was a freshly prepared aqueous solution of 10% KCN and 10% (NH ) S O . The SEM observations were made with a scanning microscope (JSM-5400, JEOL, Japan) operating at 20 kV. Discs of 3 mm diameter punched out of the cold-rolled and heat-treated sheet of 0.1 mm thickness for transmission electron microscopy (TEM) were electrothinned by a double-jet technique in a solution of 35 g of CrO , 200 ml of CH COOH and 10 ml of distilled water. A 200 kV electron microscope (H-800, Hitachi Co., Japan) equipped with a specimen tilting device was employed.
3. Results and discussion 3.1. Age-hardening characteristics Fig. 1 shows the isochronal age-hardening curves of the present alloy. The isochronal age-hardening curves display two clear hardness peaks, the "rst at about 2253C and the second at about 4503C. This behavior is similar to that found in a type IV dental gold alloy [4]. The specimens were isothermally aged at 225 and 4503C corresponding to the temperatures of the two hardness peaks in the isochronal age-hardening curves. The results of the isothermal age hardening are shown in Fig. 2. The empty squares represent the results of hardness in the grain interior for the specimen aged at 4503C, which have been measured with a 25 gf load. The hard-
Fig. 1. Isochronal age-hardening curves for 10, 20 and 50 min.
Fig. 2. Isothermal age-hardening curves at 225 and 4503C.
ness increased gradually with ageing time from the initial stage of ageing and continued progressively to increase up to 50,000 min by the isothermal ageing at 2253C. By the isothermal ageing at 4503C, the hardness did not increase in the early stages of ageing but then increased drastically and reached the maximum value. After showing the maximum hardness at 200 min, the hardness decreased gradually for the time. However, the hardness in the grain interior continued to increase. 3.2. Hardening behavior at 2253C Fig. 3 shows the SEM photograph of the specimen aged at 2253C for 50,000 min. Although the specimen aged at 2253C for 50,000 min attained the considerable hardness (Fig. 2), characteristic changes of the microstructure could not be con"rmed in the SEM photograph. Accordingly, "ne structure changes in the grain interior are assumed. To investigate a microstructural change from the single phase during isothermal ageing at 2253C, XRD
H.I. Kim et al. / Biomaterials 22 (2001) 1433}1438
Fig. 3. SEM photograph of the specimen aged at 2253C for 50,000 min.
Fig. 4. XRD patterns of the specimens solution-treated at 7503C for 30 min, and aged for 10 and 10,000 min at 2253C.
patterns were also taken from the powder specimens solution-treated at 7503C and aged at 2253C for various times. Fig. 4 shows the XRD patterns taken from the powder specimens solution-treated and aged for 10 and 10,000 min at 2253C. The solution-treated specimen,
1435
which is designated as phase, gives an XRD pattern of the face-centered cubic (f.c.c.) lattice. However, any characteristic changes during isothermal ageing at 2253C were not detected from the results of the XRD pattern. TEM observations of the specimens aged at 2253C were made in order to clarify whether there were any changes of the microstructure in the grain interior during the ageing, which could not be recognized by the XRD study. Fig. 5 shows the selected-area electron di!raction (SAED) pattern and the 001 dark "eld (DF) image of the V specimen aged at 2253C for 100 min. The 001 , 001 and V W 110 superlattice spots were observed in the SAED patX tern. They were identi"ed as the AuCu 1 ordered phase with three variants by the di!raction analysis. The DF image reveals the formation of very "ne ordered structure in the grain interior. This structure was identi"ed as the metastable AuCu 1 type ordered phase with a facecentered tetragonal (f.c.t.) lattice. However, it is very close to a f.c.c. structure. From the SAED pattern, the axial ratio (c/a) of the AuCu 1 phase was determined to be approximately 0.99. According to Porter and Easterling [7], it is often possible to form a coherent nucleus of a metastable phase if the "nal product phase have di!erent crystal structures. It is considered that the ordered phase with small amount of tetragonality can be produced easily in the matrix f.c.c. phase. Therefore, the formation of the AuCu 1 structure with an axial ratio close to unity produces small coherency strain, and the hardening is relatively small as can be seen in Fig. 2. Fig. 6 shows the SAED pattern and the 001 DF image V of the specimen aged at 2253C for 50,000 min. With increasing ageing time this ordered structure developed and formed platelets on the matrix 1 0 0 planes, from which the SAED pattern made the streaks along the 0 0 1 directions. The axial ratio of this ordered phase was determined from the SAED pattern to be approximately 0.93. This means that the axial ratio of the metastable AuCu 1 type ordered phase with a f.c.t. lattice gradually decreases with ageing time in the grain interior. It is considered that the development of the AuCu 1
Fig. 5. SAED pattern (A) and 001 DF image (B) taken from the specimen aged at 2253C for 100 min. V
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H.I. Kim et al. / Biomaterials 22 (2001) 1433}1438
Fig. 6. SAED pattern (A) and 001 DF image (B) taken from the specimen aged at 2253C for 50,000 min. V
Fig. 7. Changes in the XRD pattern during isothermal ageing at 4503C.
phase can increase the coherency strain and obtain an additional hardening as can be seen in Fig. 2. This grain interior reaction must contribute to the hardness increase of the specimen aged at 2253C. Hisatsune et al. [5] and Tani et al. [6] reported that the formation of a metastable AuCu 1 phase within grains contributed to the hardening in commercial type IV dental gold alloys with platinum and palladium. Yasuda and Ohta [4] reported that the hardening of a commercial dental gold alloy was considered to arise primarily from the formation of an AuCu 1 type ordered phase, and thought that the hardening was brought about by the strains generated in the nucleation period by the tetragonality of the AuCu 1 type superlattice. The hardening by the formation of the metastable AuCu 1 phase
in the grain interior has been reported in commercial dental gold alloys containing palladium [8]. It was reported that the hardening was due to the introduction of coherency strains at the interface between the metastable AuCu 1 type ordered platelets and the matrix. 3.3. Hardening behavior at 4503C Fig. 7 shows the changes in the XRD pattern during isothermal ageing at 4503C. With increasing ageing time, the 111 and 200 di!raction peaks of the phase shifted gradually toward the low di!raction angle. And then the 111 di!raction peak of the AuCu 1 ordered phase began to be detected from 200 min of ageing, and increased in intensity by prolonged ageing. The product phases
H.I. Kim et al. / Biomaterials 22 (2001) 1433}1438
Fig. 8. SEM photographs of the specimens aged at 4503C for 200 min (A), 5,000 min (B) and 50,000 min (C).
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were identi"ed as the Ag-rich phase with a f.c.c. structure and the AuCu 1 type ordered phase with a f.c.t. structure in the XRD pattern. When the alloy was isothermally aged at 4503C, the hardness did not increase in the early stage of ageing. It is thought that the main factor limiting hardening is associated with incubation period. The incubation period is required before nucleation begins. The incubation period is increased and the nucleation rate is decreased just below temperature at which the second phase is completely dissolved. In this case, the solution is only slightly oversaturated. The rate of nucleation is, accordingly, very slow [9]. The compositional boundary of the present alloy exists around the solid-state miscibility gap in the Ag}Au}Cu ternary system [10] and the "nal volume fraction of the AuCu 1 type ordered phase is much less than that of the Ag-rich phase at 4503C as can be seen in Fig. 7. Fig. 8 shows the SEM photographs of the specimen aged at 4503C for 200, 5000 and 50,000 min. Nodular products of a lamellar structure were predominantly formed at grain boundaries. These grain boundary products increased and grew into the grain interior with ageing time, although the growth rate was slow. The decrease in hardness occurred after showing the maximum hardness by prolonged ageing, although the hardness in the grain interior kept on increasing as seen in the age-hardening curves (Fig. 2). This overageing with softening coincided with the formation of the grain boundary products of a lamellar structure, as observed in the SEM photograph of the specimen aged for 200 min corresponding to the maximum hardness at 4503C. This implies that the softening is attributed to the formation of the grain boundary products, and depresses the hardening in the grain interior. Yasuda and Ohta [4] reported that the grain boundary products did not contribute to the age hardening in a commercial dental gold alloy. Hisatsune et al. [5] reported that the grain boundary reaction contributed to the softening in a commercial type IV dental casting gold alloy. Fig. 9 shows the SAED pattern and the 001 DF image V of the grain interior of the specimen aged at 4503C for
Fig. 9. SAED pattern (A) and 001 DF image (B) taken from the specimen aged at 4503C for 200 min. V
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200 min. As well as by ageing at 2253C, the metastable AuCu 1 type ordered phase with three variants was formed in the grain interior. The streaks along 0 0 1 directions are clearly seen in the SAED pattern, and the formation of the plate-like ordered phase on the matrix 1 0 0 planes is clearly seen in the DF image. The axial ratio of this ordered phase was determined from the SAED pattern to be approximately 0.92. Grain interior reaction at 4503C is analogous to that at 2253C. The age hardening at 2253C was characterized by a slow growth rate of the metastable AuCu 1 type ordered phase. From the above results, it was con"rmed that the metastable AuCu 1 type ordered phase which had been formed in the phase of the grain interior was trans formed into the Ag-rich and AuCu 1 ordered phases at grain boundaries during isothermal ageing in this alloy. Thus, it can be concluded that the hardening is attributed to the formation of the metastable AuCu 1 type ordered phase in the grain interior by the isothermal ageing at 4503C and the overageing with softening is due to the formation of the grain boundary products composed of the Ag-rich and AuCu 1 type ordered phases.
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