Age hardening in a dental white gold alloy

“43

AGE HARDEKISG

K. HISATSUNE*, Department Futrrtoha

(Received

?vl. OHTA,

of Dental S1’7 iJapan) August

IN A DENTAL

FYHITE GOLD ALLOY

T. SHIRAISHI

Materials

and

Engineering,

hl. YA>lXNE

School

of Dentistry,

Kyushu

University,

16, 19Sl)

Summary _4ge hardening in a dental white gold alloy was investigated by means of hardness tests, X-ray diffraction, and scanning and transmission electron microscopy observations. Hardening at 300 “C is due to both the precipitation of a metastable face-centred tetragonal (f.c.t.) ordered phase AuCu-I’ in the grain interior and the formation of fine nodules consisting of an f.c.t. ordered phase AuCu-I, an f.c.c. phase 01~and a b.c.c. ordered phase CuPd at the grain boundaries. The alloy aged at 500 “C was hardened by the formation of XuCu-I’, which was coherent with the matris, and was softened by the nodular reaction at the grain boundary.

1. Introduction In dentistry, two kinds of white gold alloys are used, although the name “white gold” is somewhat of a misnomer. One contains 5 - 20 wt.% =\u and is not a gold-base but a silver-base alloy. The other is the alloy containing 28 - 50 wt.% Au, which may be called a low carat gold alloy or a dental white gold alloy. Both alloys exhibit age-hardening characteristics. The aging behaviour and the hardening mechanisms of the silver-base alloys have already been studied [ 1 - 71. Ohta et al. [ 51 concluded that the hardening is due to the combination of the formation of a face-centred tetragonal (f.c.t.) ordered phase CuPd, which is coherent with the matrix, and a nodular structure, which consists of an f.c.c. silver-rich phase and a b.c.c. ordered phase CuPd. In contrast, high carat gold alloys are hardened by the formation of AuCu-I or AuCu-II type structures [8 - 111. However, there have been few studies of the dental white gold alloys which have a composition between those of the age-hardenable silver-base and gold-base alloys. The aim of this study is to clarify the age-hardening characteristics in a dental white gold alloy, i.e. a kind of low carat gold alloy. *Present address: Department School of Dentistry, Nagasaki 852,

0022-5088/82/0000-0000/$02.75

of Dental Japan.

Materials

@ Elsevier

Science,

Sequoia/Printed

Nagasaki

University

in The

Netherlands

TABLE The

1

nominal

composition

of the dental

white

gold

alloy

Amount

(wt.?)

30

29

22

16

1

Amount

(at.%)

16.4

29.0

22.3

30.6

1.7

2. Experimental

details

The alloy used in this study is a dental white gold alloy (Ishifuku Dental White 30, Ishifuku Metal Industry Co. Ltd., Japan). The nominal composition is shown in Table 1 where it can be seen that copper is the most. abundant element in the alloy. The alloy was first solution treated at 850 “C for 30 min and then quenched in iced brine. Isothermal annealing was carried out at 300, 400, 500 and 650 “C for up to lo6 min. The age-hardening characteristics were examined by means of micro-Vickers hardness tests and scanning and transmission electron microscopy. Hardness tests were made using a diamond pyramid hardness indenter with a 500 gf load. The dimensions of the specimens were about 12 mm X 4 mm X 2 mm. Measurements were performed on all the specimens in the solution-treated condition and again after various aging treatments. The Vickers hardness results are the averages of at least five indentations. The specimens for scanning electron microscopy were prepared using standard metallographic techniques. A freshly prepared aqueous solution of 20% potassium cyanide and 20% ammonium persulphate was utilized for the final etching of the samples. The specimens for transmission electron microscopy were thinned by a jet method. The electrolyte used was a mixture of chromic acid and glacial acetic acid, as recommended by Fisher and Marcinkowski [ 121 for Au-Cu alloys. Observations were made with a JEIM-1000 microscope operating at 1000 kV (High Voltage Electron Microscopy Laboratory, Kyushu University). Both bright and dark field observations were made and combined with selected area diffraction patterns.

3. Results and discussion 3.1. Hardening characteristics The results of the microhardness tests are shown the specimen aged at 650 ‘C, all other measurements increase in hardness. The time to attain the maximum with increasing aging temperature. A rapid increase in

in Fig. 1. Apart from represented a large hardness decreased hardness can be seen

150'

tl

J 0

1 Agcvng

Fig. 1. Isothermal and 630 ‘C.

2 Tome

harclness

3 4 log t ; man)

curves

5

of the

6

dental

white

gold

alloy

aged

at 300,

400,

500

for the specimen aged at 500 ‘C, compared with the gradual increase at 300, 400 and 650 “C. For the specimens aged at 300 and 400 ‘C, this increase was closely followed by softening. However, for the specimen aged at 500 “C the peak hardness was maintained for a relatively long annealing time and this was followed by a fall in the hardness after aging for about 100 min. In contrast, the maximum hardness of the specimen aged at 650 “C! was very small. 3.2. =Ige hardening at 300 ‘C Figures 2(a), 2(b) and 2(d) show the microstructure of the specimen aged at 300 “C for lo3 min, 3 X lo4 min and 6 X lo5 min respectively. Aging at 300 “C did not produce any microstructural change until 100 min had elapsed, although a considerable increase in hardness was observed. Accordingly, some changes in the grain interior are assumed. Figures 3(a), 3(b) and 3(c) show the transmission electron micrograph, the diffraction pattern and the key diagram for a specimen aged at 300 “C for 100 min. The diffraction pattern (Fig. 3(b)) suggests that an AuCu-I structure with three right-angled variants (X, Y, Z), which have an axial ratio of 0.88, was formed. Figure 3(a) is the dark field image obtained using the 110, reflection. It is apparent that the fine ordered phase AuCu-I is dispersed in the grain interior. CC’hen the ordered AuCu-I structure is formed in the disordered matrix, a considerable amount of strain may be introduced since the structure transforms from an f.c.c. to an f.c.t. lattice. Furthermore, Fig. 3(c) reveals that the .4uCu-I phase was coherent with the matrix. This coherency produces some coherency strains. Consequently, it is deduced that the hardening of the specimen aged at 300 ‘C for 100 min is due to an increase in the coherency strain. Such features have also been reported in Au-Cu binary alloys [ 131 and Au-Ag-Cu ternary alloys [S - lo].

246

Fig. 2. Scanning white gold alloy min.

electron micrographs showing microstructural aged at 300 “C for (a) 1000 min, (h) and (c) 3

x

changes in the dental lo4 min and (d) 6 X

105

Further aging resulted in the formation of nodules along the grain boundaries as shown in Fig. 2(a), and these developed with increasing aging time. However, the hardness increased continuously, as shown in Fig. 1. Figure 2(b) shows the microstruct~~re corresponding to the peak hardness (aged for 3 x 10’ min). From the above observations it is concluded that the hardness peak was maintained when the nodules and the early products coexisted. The hardening is thought to arise because the nodules are very fine (Fig. 2(c)). Thereafter, the nodules developed by consuming the grain interior. Prolonged aging was accompanied by softening. It is obvious that the softening is due to coarsening of the nodules, as shown in Fig. 2(d). 3.3. Age hardening at 500 “C Figure 4 shows the variations in microstructure of a specimen aged at 500 “C for varying periods. Figures 4(a), 4(b) and 4(c) correspond to the microstructure of specimens aged for 10 mitt, 100 min and 10000 min respectively. Xging for 10 min produced a striation structure in the grain interior, as shown in Fig. 4(a). With increasing aging period, Iarge twins were observed (Fig. 4(b)). Aging for 100 min produced some nodules along the grain boundaries, as shown in Fig. -l(b). Further aging resulted in the devel-

Fig. 3. (a) Transmission electron micrograph of the dental white gold alloy aged at 300 ‘C for 100 min (dark field image taken at the 110~ spot); (b) 001 diffraction pattern; (c) key ciiagram for (hf.

opment of nodules in the grain interior, until eventually all the grains were completely covered with the nodular structures (Fig. 4(c)). This development is analogous to the grain boundary reaction at 300 “C. However, the nodules formed at 500 ‘C were a very different size from those at 300 “C. Aging for 1 min produced a separation into two phases, a met&able f.c.t. phase AuCu-I with two variants and an f.c.c. phase in the grain interior. These phases were coherent with each other. The abrupt increase in hardness is related to the coherency strains between the AuCu-I structure and the f.c.c. structure. Figure 5 shows the dark field image obtained using the OO1,u superlattice spot of the .AuCu-I. It is clear that the formation of very fine AuCu-I platelets, as reported by Yasuda and coworkers 18, 91, is associated with the age hardening of this alloy. The separation into two phases which are coherent with each other gives rise to considerable elastic strains. Thus, the coherency strain developed makes a major contribution to the age hardening in this alloy, in the same way as reported by other workers [S, 13 15 I for gold-base alloys. Further aging produced an increased phase separation. However, aging at 500 “C for 100 min resulted in two different modes (A and B), as shown in Fig. 6. Figure 7 shows transmission electron micrographs of region A. ~\n f.c.c. phase and two kinds of AuCu-I phases with differently oriented c ases

Fig. 4, Scanning electron micrographs showing microstructural changes in dental gotd alloy aged at 500 “C for (a) 10 min. (b) 100 min and (c) 10 000 min.

white

Fig. 5. Transmission electron micrograph of the dental white gotd aby aged at 500 ‘C for I min (dark fieid image taken using the 001 superiattice spot of Au&-I).

(X and Z) can he detected. Figure 7(a), which is a dark field image taken using the 001 spot, demonstrates how the phase has grown compared with that shown in Fig. 5. The black areas in Fig. 7(a) correspond to the matrix phase which maintains coherency with the XuCu-f phase. fn contrast, region B produced twinning to relieve the coherency strains, since the strain increases with the growth of the ordered phase. The diffraction pattern of region B in Figs. S(a) and 8(b) shows the twinning. Figures 9(a) and 9(b),

Fig. 6. Transmission electron micrograph 100 min (bright field image, 001).

Fig. 7. (a) Transmission electron 001,); (b) transmission electron 110~); (c) diffraction pattern;(d)

of the dental

micrograph of region micrograph of region key diagram for (c).

white gold alloy aged at 500

A in Fig. 6 (dark A in Fig. 6 (dark

C for

field image, field image,

cb,

.‘3c*r

024 020, 0202

Fig. 8. (a) Diffraction

_.

pattern

taken from region B in Fig. 6; (b) key diagram

for (a).

---~---_-~-.-._--

Fig. 9. Dark field images from region B in Fig. 6: (a) 001~;

(b) 001~.

which are the dark field images taken at the 001x and OOly superlattice spots respectively of AuCu-I, show a twin relation on the 110 plane. Syutkina and Yakovleva [ 161 have reported that the ordered phase in AuCu grew in the form of needles consisting of alternating plates of twin orientation linked completely coherently on the 110 planes. However, the twin structures are characterized by a very small length. It is believed that the formation of twinning accompanies the decline in mechanical strength (e.g. hardness) as a result of the elimination of coherency strains. As region A in which the coherency strain is stored coexists with region B as discussed above, the hardness of the alloy may be maintained. The hardness plateau for aging at 5OO.“C (Fig. 1) may be due to the competition of these two stages. On prolonged aging the equilibrium phases or + AuCu-I + CuPd are formed in the nodular structure at the grain boundaries. The formation of the nodule coincides with decreasing hardness. In Au-Cu alloys this stage is accompanied by fracture at the grain boundaries [ 171. As the component in

the grain interior is not the stable phase for the dental white gold alloy, it is inferred that the formation of the stable phase nodule produces stress relasation at the grain boundaries. Therefore, the present alloy could stop the grain boundary cracks. 3.4. Age hardening at 650 ‘C No microstructural changes were seen at the hardness peak, but X-ray diffraction studies [IS] have indicated separation into three phases after aging for 90 s. As in the early stage at 300 ‘C, some precipitation may be produced in the grain interior. Figure 10 shows the result of transmission electron microscopy observations of a specimen aged at 650 “C for 90 s. The diffraction pattern (Figs. 10(b) and 10(c)) proved the existence of separation into three phases in the grain interior, i.e. CY+ (Y, (f.c.c.) + (Ye (f.c.c.) + CuPd (b.c.c.). Figure 10(a) is a dark field image taken using the OIOcUrd superlattice spot. Lenticular CuPd phases were observed in the grain interior. As these phases have obvious interfaces with each other, not so much hardening can be anticipated. Further aging produced nodules at grain boundaries (Fig. 11(a)) and coarsened them (Fig. 11(b)). The formation of the nodules was accompanied by softening.

Fig. 10. (a) Transmission electron micrograph of the dental white gold alloy aged at 65O_“C for 90 s (dark field image taken using the 010 superlattice spot of CuPd); (h) 001 diffraction pattern;(c) key diagram for (b).

Fig. 11. Scanning electron micrographs showing microstructural changes white gold alloy aged at 650 ‘C for (a) 10 min and (b) 10 000 min.

in the dental

4. Conclusions Age hardening in a dental white gold alloy was investigated by means of hardness tests, X-ray diffraction, and scanning and transmission electron microscopy observations. The main results obtained are as follows. (1) Hardening at 300 and 400 “C is due to both the precipitation of a metastable f.c.t. ordered phase AuCu-I’ in the grain interior and the formation of fine nodules including an f.c.t. ordered phase AuCu-I, an f.c.c. phase 01~and a b.c.c. ordered phase CuPd at grain boundaries. (‘2) At 500 22, separation into two phases, an f.c.t. ordered phase A&u-I’ and an f.c.c. phase al’ which are coherent with each other, produced a high hardness. The formation of a nodu!ar structure at the grain boundaries produced softening. (3) At 650 “C the degree of hardening was small in comparison with the specimen aged at temperatures below 650 “C. Thus the AuCu-I phase was not related to the aging characteristics at 650 “C.

Acknowledgments The authors would like to thank Professor K. Yasuda for valuable discussion, Mrs. E. Yarnamura and Miss Y. Johno for assisting with the experiments and the Ishifuku Metal Industry Co. Ltd. for supplying the alloy used in this study. This study was partly supported by a Grant-in-Aid for Scientific Research from the Nakamura Fund.

253

References 1 K. Hisatsune, M. Ohta and M. Yamane. J. Jpn. Sot. Dent. Appar. Mater.. 13 (1972) 97. 2 K. Hisatsune, M. Ohta and M. Yamane, J. Jpn. Sec. Dent. Appar. Mater., 15 (197-L) 116. 3 M. Ohta, K. Hisatsune and M. Yamane, J. Jon. Sot. Dent. Appar. .CJater., 16 (1975) 144. 4 H. Isaka,J. Jpn. Sot. Dent. Appar. Mater.. 18 (1977) 137. 5 &I. Ohta, K. Hisatsune and AI. Yamane, J. Less-Common JJet., 65(1979) Pll. 6 K. Hisatsune, M. Ohta and M. Yamane. J. .Ipn. Sot. Dent. Appar. Mater.. 30 (1979) 228. 7 hl. Ohta, T. Shiraishi, K. Hisatsune and M. Yamane, J. Dent. Res., 59 (1980) 1966. S Y. Kanzawa, K. Yasuda and H. Metahi, J. Less-Common Met., 43 (1975) 121. 9 K. Yasuda and Y. Kanzawa, Trans. Jpn. Inst. Met., 18 (1977) 46. 10 K. Yasuda, H. Metahi and Y. Kanzawa, J. Less-Common Met.. 60 (1978) P65. 11 M. Ohta and K. Yasuda, J. Dent. Res., 59 (1980) 986. 12 R. M. Fisher and M. J. Marcinkowski, Philos. Msg.. 6 (1961) 1355. 13 M. Hirabayashi and S. Weissmann, Acta Metall., 10 (1962) 25. 1-t D. Harker, Trans. 4m. Sot. Met., 32 (1944) 210. 15 V. S. Arunachalam, Ph.D. Thesis, University of Wales, 1965. 16 V. I. Syutkina and E. S. Yakovleva, Phvs. Status Solidi, 21 (1967) 465. 17 V. S. Arunachalam and R. W. Cahn, J. Mater. Sci., 2 (1967) 160. 18 K. Hisatsune, M. Ohta, T. Shiraishi and M. Yamane, J. Dent. Res.. to be published.