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Age-hardening behavior in a palladium-base dental porcelain-fused alloy
177
MATERIALS CHARACTERIZATION 25:177-184 (1990)
Age-Hardening Behavior in a Palladium-Base Dental PorcelainFused Alloy
KUNIHIRO HISATSUNE, MASAYUKI HASAKA,* BAMBANG IRAWAN SOSROSOEDIRDJO, AND KOICHI UDOH
Nagasaki University School of Dentistry, Nagasaki 852, Japan, and *Faculty of Engineering, Nagasaki University, Nagasaki 852, Japan
Age hardening in a Palladium-base porcelain-fused alloy was investigated by hardness tests, x-ray diffraction, scanning, and transmission electron microscopy. In the early stages of aging, a metastable ordered phase with a face-centered tetragonal structure based on Pd3In was formed within the grains. Hardening was due to coherency strain between the metastable structure and the matrix with a face-,centered cubic structure. In the later stages, a lamellar structure, consisting of a stable ordered phase with a face-centered tetragonal structure based on Pd3In and a stable phase with a face-centered cubic structure, grew from the grain boundaries. Finally, the lamellae covered the whole grain and resulted in softening.
Introduction Various porcelain-fused alloys have been used in the dental field, and several high-nobility alloys are available for this specific application. These high-nobility alloys contain different combinations of Au, Pt, and Pd as major constituents, and Ag, Sn, In, and Fe as minor constituents. Au-based porcelain-fused alloys are substantially hardened by aging [I], with the elements Fe, In, and Sn playing an important role in the agehardening process [2]. In the last few years, Pd-based alloys have been used instead of the Au-based alloys because of increases in the cost of Au. In general, their compositions consist of 90 wt.% (Pd + Ag) and 10 wt.% (Sn + In). Huget et al. [3], O'Brien and German [4], and Biederman et al. [5] have reported the existence of age-hardening behavior in the Pdbased alloys but have not discussed the mechanisms that might be involved. The aim of the present study was to clarify the characteristics of the aging reaction and to elucidate the age-hardening mechanisms in a Pdbased porcelain-fused alloy. © Elsevier Science Publishing Co., Inc., 1990 655 Avenue of the Americas, New York, NY 10010
1044-5803/90/$3.50
178
K. Hisatsune et al.
Experimental The alloy used in this study was a Pd-based dental porcelain-fused alloy with the composition, in wt.%, 55Pd-36Ag-5Sn-4In (BYRON, Nippon Shiken Dental Co., Ltd., Tokyo, Japan). The alloy was solution-treated at 1000°C for 30 min, then quenched into ice brine. Isothermal aging was subsequently carried out at 500, 600, or 700°C for times up to 105 rain. The age-hardening characteristics were studied with the use of microVickers hardness tests, x-ray diffraction (XRD), and scanning and transmission electron microscopy (SEM and TEM, respectively). The hardness tests were made with the use of a diamond pyramid indentor with a 200g load, the hardness number being calculated as the average of five indentations. The XRD study was carried out on filed and appropriately heat-treated powder specimens, with the use of an x-ray diffractometer and C u - K a radiation. SEM specimens were prepared with the use of a standard metallographic technique, with final etching being done with a freshly prepared aqueous solution of 10% potassium cyanide and 10% ammonium persulfate. TEM specimens were electrolytically thinned by a double jet method, the electrolyte being a mixture of 35 g CrO3, 200 mL acetic acid, and I0 mL distilled water.
Results and Discussion AGE-HARDENING BEHAVIOR Figure 1 shows the variation of hardness with time for specimens aged at 500, 600, and 700°C. The open triangle denotes the hardness of the original solution-treated and quenched material. Initially, the hardness increased as the aging progressed, actually doubling during aging at 500 and 600°C. Later, the hardness reached a plateau, and softening began. As will be shown later, this type of age-hardening behavior is associated with aging reactions within the grains and at the grain boundaries. MICROSTRUCTURES Figure 2 shows SEM images of the microstructures of specimens aged for various times at 600 or 700°C. Figure 2a is typical of the microstructure seen after 1,000 min at 600°C, when maximum hardness had been achieved. Some striations are visible within the grains, together with a small amount of precipitate at the grain boundary. This grain boundary component was not visible after aging for 100 min at 600 °C, at which
Age Hardening in Pd-Base Alloy
/ 300
!
"Z
./
/.7
~so
FIG. 1.
I
I
I
~oo°¢l~l - - t ~ i E ~ - ° - - w -
f ./i / \ = [ I / ; d/I i • \ i _ ~ _ _ ~ o-o-/o..m / ~ ~m ! o/0
1OO
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L / - - -5 J
,
t ~,2°°
179
p\
'
--
\t
! ~
IZoo\ ' 7
,7 0
1
2
Aging
Time
3 , log t
4
5
6
(min)
Variations of micro-Vickers hardness with aging time.
Fro. 2. SEM photographs of the specimens aged at 600°C for 103 min (a), 3 × 104 min (b), 105 min (c), and at 700°C for 104 min (d).
180
K. Hisatsune et al.
point the hardness had increased by 50%. Thus, it can be inferred that the hardening resulted from reaction within the grain. As the aging time increased beyond 1,000 min, the grain boundary component developed (Fig. 2b,c) and finally covered whole grains. A fine cross-stripe structure could be seen within the grains after 30,000 min at 600°C (Fig. 2c), but this was eventually consumed by the grain boundary component. Similar observations, but at shorter times, were observed with aging at 700°C, and the lamellar structure was found to cover whole grains after 10,000 min (Fig. 2d). From the above observations, it is concluded that the hardening was caused by intragranular reactions and that the later softening arose from a grain boundary reaction. A similar type of mechanism of age-hardening has been recognized in a low Au alloy I6].
GRAIN INTERIOR REACTION Figure 3 shows the variation of the XRD profile with aging time. Little change in the profile was observed in the initial 100 min at 600°C, but a weak and broad peak appeared in the vicinity of 20 -- 47.5 ° at longer times. This diffraction peak was visible as early as 300 min into the aging, as can be seen in the slow scanning profile shown in Fig. 4. This is identified as the 002 reflection of a face-centered tetragonal (fct) structure and probably corresponds to a grain interior component. The 002 reflection shifted to the higher angle side with increasing aging time (Fig. 4) but finally vanished as the microstructural change seen in Fig. 2 was completed. This metastable phase is referred to as "beta prime" (f3'), to distinguish it from the stable fct 13 phase described subsequently. Figure 5 shows a bright-field TEM image and its selected area diffraction pattern of the specimen aged at 600°C for 100 min. The pattern indicates a [001] zone. The subscripts X and Y refer to the reflections from the regions with the c-axis in the X and Y directions, respectively. Figure 5 reveals that the metastable 13' phase of ordered fct structure formed with two variants within the grain and coexisted coherently with the matrix oto of face-centered cubic (fcc) structure. Thus, the hardening is attributable to the coherency strain between the fct and fcc structures. The striations seen within the grains in Fig. 2a must be due to strain contrast arising from the elastic strain induced by a tetragonality of the 13' phase, as seen in a C u - B e alloy [7]. Figure 6 shows a bright-field TEM image of the specimen with maximum hardness, achieved by aging for 1,000 min at 600°C. The grain interior component is rather large and stretches in a
Age Hardening in Pd-Base Alloy
181
0{11111j~ 111
o/,1200 ~' 002
M *C k ]
J ~lOOO0~ln~
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40 FIG. 3.
200 !
45 2e (deg.)
I
50
Variations of x-ray line profiles with aging time.
(110) direction, as similarly observed in an Au-based alloy [8]. In the later aging stages, this component could be observed by SEM (see the fine cross-stripe structure in Fig. 2b). Consequently, the age-hardening continued with the growth of the grain interior component. The reaction resulting in the hardening can be characterized by the following sequence: ao(fcc) --~ a'(fcc) + 13'(ordered fct)
GRAIN BOUNDARY REACTION As described in the previous sections, the formation of lamellae was initiated at the grain boundaries considerably before maximum hardness was obtained and developed as the aging progressed. As indicated by the arrows in Fig. 3, this component was detected by XRD at the lower-angle side of the 200 reflection from the matrix so. This peak increased in in-
182
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/~ 002
./1•
'
FIG. 4.
I
I
46
47
I
48 28 (deg.)
I
49
I
50
Variations of x-ray line profile of the specimen aged at 600°C with aging time.
tensity with increasing aging time, and others from the corresponding phases were clearly visible after prolonged aging (105 rain) at 700°C. This component was identified as an a~ phase with fcc structure and an ordered [3 phase with fct structure. It is evident that these are the stable phases corresponding to the lamellae developed from the grain boundary, be-
FK;. 5. TEM micrograph (a) and its diffraction pattern (b) of the specimen aged at 600°C for 10z min.
Age Hardening in Pd-Base Alloy
FIG. 6.
TEM micrograph of the specimen aged at 600°C for
183
10 3
rain.
cause the SEM image (Fig. 2d) of the specimen aged at 700°C for 105 min shows that lamellae covered the whole grain. In the intermediate stages of aging, two reflections were detected in the vicinity of 20 = 48.5 ° (Fig. 4). This profile indicated the coexistence of a metastable ~' phase and a stable ~ phase, corresponding to the grain interior and grain boundary components, respectively. From the lattice constant and alloying elements, both the 13' and 13 phases with fct structure must be based on the ordered fct Pd3In reported by Harris et al. [9]. From this, it is deduced that the grain boundary reaction resulting in softening can be characterized by the sequence: c~'(fcc) + 13'(ordered f c t ) ~ ~(fcc) + 13(ordered fct)
Conclusions
The age-hardening behavior of a Pd-based dental porcelain-fused alloy has been investigated by hardness measurements, XRD, SEM, and TEM observations. The reactions during isothermal aging at 500,600, and 700°C can be classified into two types, those occurring within the grains and those at the grain boundaries. In the initial stages of aging, a fct ordered phase based on Pd3In was formed metastably within the grains. Thereafter, lamellae with fcc and fct structures were produced at the grain boundaries and grew into the grains as the aging progressed. Finally, entire grains were covered with these lamellae. The hardening in the early stages can be attributed to the coherency strain between the metastable fct and fcc structures formed inside the grains. Later, the formation of
184
K. Hisatsune et al.
the stable fct and fcc structures via the grain boundary reaction resulted in softening. References 1. R. M. German, Hardening reactions in a high-gold content ceramo-metal alloy, J. Dent. Res. 59:1960-1965 (1980); W. J. O'Brien, J. E. Kring, and G. Ryge, Heat treatment of alloys to be used for the fused porcelain technique, J. Pros. Dent. 14:955-960 (1964). 2. K. F. Leinfelder, W. J. O'Brien, G. Ryge, and C. W. Fairhurst, Hardening of high-fusing gold alloys, J. Dent. Res. 45:392-396 (1966). 3. E. F. Huget, N. Dvivedi, and H. E. Cosner, Jr., Characterization of gold-palladiumsilver and palladium-silver for ceramic-metal restorations, J. Pros. Dent. 36:58-65 (1976). 4. L. A. O'Brien and R. M. German, Compositional effects on Pd-Ag dental alloys, J. Dent. Res. 63:175 (1984). 5. R. R. Biederman, R. M. German, and J. R. Toran, The physical metallurgy of a Pd-Au dental alloy, in Precious Metals 1981 (E. D. Zysk, ed.), Pergamon, New York (1982), pp. 423-431. 6. K. Hisatsune, M. Ohta, T. Shiraishi, and M. Yamane, Age-hardening in a dental white gold alloy, J. Less-Comm. Met. 83:243-253 (1982). 7. L. E. Tanner, Diffraction contrast from elastic shear strains due to coherent phases, Phil. Mag. 14:111-130 (1966). 8. M. Ohta, T. Shiraishi, M. Yamane, and K. Yasuda, Age-hardening mechanism of equiatomic AuCu and AuCu-Ag pseudo-binary alloys, Dent. Mat. J. 2:10-17 (1983). 9. I. R. Harris, M. Norman, and A. W. Bryant, A study of some palladium-indium, platinum-indium and platinum-tin alloys, J. Less-Comm. Met. 16:427-440 (1968). Received May 25, 1989; accepted Januao" 15, 1990.
MATERIALS CHARACTERIZATION 25:177-184 (1990)
Age-Hardening Behavior in a Palladium-Base Dental PorcelainFused Alloy
KUNIHIRO HISATSUNE, MASAYUKI HASAKA,* BAMBANG IRAWAN SOSROSOEDIRDJO, AND KOICHI UDOH
Nagasaki University School of Dentistry, Nagasaki 852, Japan, and *Faculty of Engineering, Nagasaki University, Nagasaki 852, Japan
Age hardening in a Palladium-base porcelain-fused alloy was investigated by hardness tests, x-ray diffraction, scanning, and transmission electron microscopy. In the early stages of aging, a metastable ordered phase with a face-centered tetragonal structure based on Pd3In was formed within the grains. Hardening was due to coherency strain between the metastable structure and the matrix with a face-,centered cubic structure. In the later stages, a lamellar structure, consisting of a stable ordered phase with a face-centered tetragonal structure based on Pd3In and a stable phase with a face-centered cubic structure, grew from the grain boundaries. Finally, the lamellae covered the whole grain and resulted in softening.
Introduction Various porcelain-fused alloys have been used in the dental field, and several high-nobility alloys are available for this specific application. These high-nobility alloys contain different combinations of Au, Pt, and Pd as major constituents, and Ag, Sn, In, and Fe as minor constituents. Au-based porcelain-fused alloys are substantially hardened by aging [I], with the elements Fe, In, and Sn playing an important role in the agehardening process [2]. In the last few years, Pd-based alloys have been used instead of the Au-based alloys because of increases in the cost of Au. In general, their compositions consist of 90 wt.% (Pd + Ag) and 10 wt.% (Sn + In). Huget et al. [3], O'Brien and German [4], and Biederman et al. [5] have reported the existence of age-hardening behavior in the Pdbased alloys but have not discussed the mechanisms that might be involved. The aim of the present study was to clarify the characteristics of the aging reaction and to elucidate the age-hardening mechanisms in a Pdbased porcelain-fused alloy. © Elsevier Science Publishing Co., Inc., 1990 655 Avenue of the Americas, New York, NY 10010
1044-5803/90/$3.50
178
K. Hisatsune et al.
Experimental The alloy used in this study was a Pd-based dental porcelain-fused alloy with the composition, in wt.%, 55Pd-36Ag-5Sn-4In (BYRON, Nippon Shiken Dental Co., Ltd., Tokyo, Japan). The alloy was solution-treated at 1000°C for 30 min, then quenched into ice brine. Isothermal aging was subsequently carried out at 500, 600, or 700°C for times up to 105 rain. The age-hardening characteristics were studied with the use of microVickers hardness tests, x-ray diffraction (XRD), and scanning and transmission electron microscopy (SEM and TEM, respectively). The hardness tests were made with the use of a diamond pyramid indentor with a 200g load, the hardness number being calculated as the average of five indentations. The XRD study was carried out on filed and appropriately heat-treated powder specimens, with the use of an x-ray diffractometer and C u - K a radiation. SEM specimens were prepared with the use of a standard metallographic technique, with final etching being done with a freshly prepared aqueous solution of 10% potassium cyanide and 10% ammonium persulfate. TEM specimens were electrolytically thinned by a double jet method, the electrolyte being a mixture of 35 g CrO3, 200 mL acetic acid, and I0 mL distilled water.
Results and Discussion AGE-HARDENING BEHAVIOR Figure 1 shows the variation of hardness with time for specimens aged at 500, 600, and 700°C. The open triangle denotes the hardness of the original solution-treated and quenched material. Initially, the hardness increased as the aging progressed, actually doubling during aging at 500 and 600°C. Later, the hardness reached a plateau, and softening began. As will be shown later, this type of age-hardening behavior is associated with aging reactions within the grains and at the grain boundaries. MICROSTRUCTURES Figure 2 shows SEM images of the microstructures of specimens aged for various times at 600 or 700°C. Figure 2a is typical of the microstructure seen after 1,000 min at 600°C, when maximum hardness had been achieved. Some striations are visible within the grains, together with a small amount of precipitate at the grain boundary. This grain boundary component was not visible after aging for 100 min at 600 °C, at which
Age Hardening in Pd-Base Alloy
/ 300
!
"Z
./
/.7
~so
FIG. 1.
I
I
I
~oo°¢l~l - - t ~ i E ~ - ° - - w -
f ./i / \ = [ I / ; d/I i • \ i _ ~ _ _ ~ o-o-/o..m / ~ ~m ! o/0
1OO
~
L / - - -5 J
,
t ~,2°°
179
p\
'
--
\t
! ~
IZoo\ ' 7
,7 0
1
2
Aging
Time
3 , log t
4
5
6
(min)
Variations of micro-Vickers hardness with aging time.
Fro. 2. SEM photographs of the specimens aged at 600°C for 103 min (a), 3 × 104 min (b), 105 min (c), and at 700°C for 104 min (d).
180
K. Hisatsune et al.
point the hardness had increased by 50%. Thus, it can be inferred that the hardening resulted from reaction within the grain. As the aging time increased beyond 1,000 min, the grain boundary component developed (Fig. 2b,c) and finally covered whole grains. A fine cross-stripe structure could be seen within the grains after 30,000 min at 600°C (Fig. 2c), but this was eventually consumed by the grain boundary component. Similar observations, but at shorter times, were observed with aging at 700°C, and the lamellar structure was found to cover whole grains after 10,000 min (Fig. 2d). From the above observations, it is concluded that the hardening was caused by intragranular reactions and that the later softening arose from a grain boundary reaction. A similar type of mechanism of age-hardening has been recognized in a low Au alloy I6].
GRAIN INTERIOR REACTION Figure 3 shows the variation of the XRD profile with aging time. Little change in the profile was observed in the initial 100 min at 600°C, but a weak and broad peak appeared in the vicinity of 20 -- 47.5 ° at longer times. This diffraction peak was visible as early as 300 min into the aging, as can be seen in the slow scanning profile shown in Fig. 4. This is identified as the 002 reflection of a face-centered tetragonal (fct) structure and probably corresponds to a grain interior component. The 002 reflection shifted to the higher angle side with increasing aging time (Fig. 4) but finally vanished as the microstructural change seen in Fig. 2 was completed. This metastable phase is referred to as "beta prime" (f3'), to distinguish it from the stable fct 13 phase described subsequently. Figure 5 shows a bright-field TEM image and its selected area diffraction pattern of the specimen aged at 600°C for 100 min. The pattern indicates a [001] zone. The subscripts X and Y refer to the reflections from the regions with the c-axis in the X and Y directions, respectively. Figure 5 reveals that the metastable 13' phase of ordered fct structure formed with two variants within the grain and coexisted coherently with the matrix oto of face-centered cubic (fcc) structure. Thus, the hardening is attributable to the coherency strain between the fct and fcc structures. The striations seen within the grains in Fig. 2a must be due to strain contrast arising from the elastic strain induced by a tetragonality of the 13' phase, as seen in a C u - B e alloy [7]. Figure 6 shows a bright-field TEM image of the specimen with maximum hardness, achieved by aging for 1,000 min at 600°C. The grain interior component is rather large and stretches in a
Age Hardening in Pd-Base Alloy
181
0{11111j~ 111
o/,1200 ~' 002
M *C k ]
J ~lOOO0~ln~
E
T__J _...j
',..~14oo~,.j- )
\
~
0 ='.
I
40 FIG. 3.
200 !
45 2e (deg.)
I
50
Variations of x-ray line profiles with aging time.
(110) direction, as similarly observed in an Au-based alloy [8]. In the later aging stages, this component could be observed by SEM (see the fine cross-stripe structure in Fig. 2b). Consequently, the age-hardening continued with the growth of the grain interior component. The reaction resulting in the hardening can be characterized by the following sequence: ao(fcc) --~ a'(fcc) + 13'(ordered fct)
GRAIN BOUNDARY REACTION As described in the previous sections, the formation of lamellae was initiated at the grain boundaries considerably before maximum hardness was obtained and developed as the aging progressed. As indicated by the arrows in Fig. 3, this component was detected by XRD at the lower-angle side of the 200 reflection from the matrix so. This peak increased in in-
182
K. H i s a t s u n e et a/.
/~ 002
./1•
'
FIG. 4.
I
I
46
47
I
48 28 (deg.)
I
49
I
50
Variations of x-ray line profile of the specimen aged at 600°C with aging time.
tensity with increasing aging time, and others from the corresponding phases were clearly visible after prolonged aging (105 rain) at 700°C. This component was identified as an a~ phase with fcc structure and an ordered [3 phase with fct structure. It is evident that these are the stable phases corresponding to the lamellae developed from the grain boundary, be-
FK;. 5. TEM micrograph (a) and its diffraction pattern (b) of the specimen aged at 600°C for 10z min.
Age Hardening in Pd-Base Alloy
FIG. 6.
TEM micrograph of the specimen aged at 600°C for
183
10 3
rain.
cause the SEM image (Fig. 2d) of the specimen aged at 700°C for 105 min shows that lamellae covered the whole grain. In the intermediate stages of aging, two reflections were detected in the vicinity of 20 = 48.5 ° (Fig. 4). This profile indicated the coexistence of a metastable ~' phase and a stable ~ phase, corresponding to the grain interior and grain boundary components, respectively. From the lattice constant and alloying elements, both the 13' and 13 phases with fct structure must be based on the ordered fct Pd3In reported by Harris et al. [9]. From this, it is deduced that the grain boundary reaction resulting in softening can be characterized by the sequence: c~'(fcc) + 13'(ordered f c t ) ~ ~(fcc) + 13(ordered fct)
Conclusions
The age-hardening behavior of a Pd-based dental porcelain-fused alloy has been investigated by hardness measurements, XRD, SEM, and TEM observations. The reactions during isothermal aging at 500,600, and 700°C can be classified into two types, those occurring within the grains and those at the grain boundaries. In the initial stages of aging, a fct ordered phase based on Pd3In was formed metastably within the grains. Thereafter, lamellae with fcc and fct structures were produced at the grain boundaries and grew into the grains as the aging progressed. Finally, entire grains were covered with these lamellae. The hardening in the early stages can be attributed to the coherency strain between the metastable fct and fcc structures formed inside the grains. Later, the formation of
184
K. Hisatsune et al.
the stable fct and fcc structures via the grain boundary reaction resulted in softening. References 1. R. M. German, Hardening reactions in a high-gold content ceramo-metal alloy, J. Dent. Res. 59:1960-1965 (1980); W. J. O'Brien, J. E. Kring, and G. Ryge, Heat treatment of alloys to be used for the fused porcelain technique, J. Pros. Dent. 14:955-960 (1964). 2. K. F. Leinfelder, W. J. O'Brien, G. Ryge, and C. W. Fairhurst, Hardening of high-fusing gold alloys, J. Dent. Res. 45:392-396 (1966). 3. E. F. Huget, N. Dvivedi, and H. E. Cosner, Jr., Characterization of gold-palladiumsilver and palladium-silver for ceramic-metal restorations, J. Pros. Dent. 36:58-65 (1976). 4. L. A. O'Brien and R. M. German, Compositional effects on Pd-Ag dental alloys, J. Dent. Res. 63:175 (1984). 5. R. R. Biederman, R. M. German, and J. R. Toran, The physical metallurgy of a Pd-Au dental alloy, in Precious Metals 1981 (E. D. Zysk, ed.), Pergamon, New York (1982), pp. 423-431. 6. K. Hisatsune, M. Ohta, T. Shiraishi, and M. Yamane, Age-hardening in a dental white gold alloy, J. Less-Comm. Met. 83:243-253 (1982). 7. L. E. Tanner, Diffraction contrast from elastic shear strains due to coherent phases, Phil. Mag. 14:111-130 (1966). 8. M. Ohta, T. Shiraishi, M. Yamane, and K. Yasuda, Age-hardening mechanism of equiatomic AuCu and AuCu-Ag pseudo-binary alloys, Dent. Mat. J. 2:10-17 (1983). 9. I. R. Harris, M. Norman, and A. W. Bryant, A study of some palladium-indium, platinum-indium and platinum-tin alloys, J. Less-Comm. Met. 16:427-440 (1968). Received May 25, 1989; accepted Januao" 15, 1990.