Journal of Alloys and Compounds 339 (2002) 144–148
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Partial phase diagram for the AuCu–Zn pseudobinary system Hyo-Joung Seol*, Takanobu Shiraishi, Yasuhiro Tanaka, Eri Miura, Yasuko Takuma, Kunihiro Hisatsune Department of Dental Materials Science, Nagasaki University School of Dentistry, Nagasaki 852 -8588, Japan Received 5 November 2001; accepted 3 December 2001
Abstract Zinc addition to AuCu was found to have significant effects on the structural change of AuCu. A partial phase diagram for the AuCu–Zn pseudobinary system was constructed based on electrical resistivity measurements and X-ray diffraction study. The addition of Zn to AuCu stabilized the AuCu II superstructure and extremely enlarged the AuCu II single-phase region in the Au–Cu–Zn system. The order–disorder transition temperature was slightly lowered with the addition of Zn to AuCu but the AuCu II↔AuCu I transition temperature was greatly lowered. The antiphase domain size of the AuCu II superstructure markedly decreased with increasing Zn content. The lattice parameter a decreased and c increased with Zn addition, thus the axial ratio, c /a, considerably increased. 2002 Elsevier Science B.V. All rights reserved. Keywords: Dental alloys; X-ray diffraction; Phase diagram
1. Introduction Zinc is usually added to dental casting gold alloys as a deoxidizer and has been used to improve the castability of alloys. But even with small Zn addition, the mechanical properties of an original alloy will be changed. There are some studies reporting improvement of mechanical properties at 37 8C by Zn addition to equiatomic AuCu alloy [1,2]. According to Ohta et al. [1], the age-hardening rate is closely related to the melting temperature of the alloy, and the addition of Ga, Al or Zn with low melting point greatly promoted the age-hardening rate of equiatomic AuCu alloy at 37 8C. In their study, the corrosion resistance for clinical use was enough when 6 at.% Zn was added to AuCu equiatomic alloy. A study on applying the shape memory property of Au–Cu–Zn alloys to dental use was also reported [3]. But further study on the dental use of the AuCu–Zn system is still insufficient. Since the mechanical properties of gold alloys are greatly related to their phase transformation [4] and to understand phase transformation occurring in the AuCu–Zn added alloy, the phase diagram of the AuCu–Zn system needs to be constructed. In the present study, a partial phase diagram for the *Corresponding author. E-mail address:
[email protected] (H.-J. Seol).
AuCu–Zn pseudobinary system with Zn content of up to about 20 at.% was determined to examine the changes in the stable phase with Zn content. Crystal structural changes occurring by Zn addition to AuCu were also examined.
2. Materials and methods
2.1. The chemical compositions of alloys The chemical compositions of alloys used in the present study were equiatomic AuCu with additions of 0, 0.67, 2.03, 5.43, 9.80 and 19.61 at.% Zn. These alloys are named after the rounded value of their Zn content as listed in Table 1. The shapes of all the alloys were plate-like with the size of 53730.5 mm 3 .
2.2. Electrical resistivity ( ER) measurement By cold rolling the above-mentioned plate samples, sheets of size 232530.1 mm 3 were obtained for electrical resistivity measurement. They were solution-treated at 500 8C for 30 min under an argon atmosphere and rapidly quenched into ice brine. The electrical resistivity changes during continuous heating in vacuo up to 500 8C at a rate
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01989-2
H.-J. Seol et al. / Journal of Alloys and Compounds 339 (2002) 144 – 148 Table 1 Chemical compositions of alloys used (analyzed values) Alloys
Zn0 Zn1 Zn2 Zn5 Zn10 Zn20
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radiation was used as an incident beam and to cut the CuK b radiation, a diffracted-beam monochromator was used.
Composition (at.%) Au
Cu
Zn
49.48 49.60 49.01 47.33 45.08 40.24
50.52 49.73 48.96 47.24 45.12 40.15
0 0.67 2.03 5.43 9.80 19.61
of 0.1 8C / min was measured using direct current potentiometric method with four terminals.
2.3. X-ray diffraction study For the X-ray diffraction (XRD) study, powder specimens which passed through a 200-mesh screen were obtained by filing the homogenized plate samples. After being sealed in an evacuated silica capsule and solutionized at 500 8C for 30 min, they were annealed for long periods at various temperatures and then quenched into ice brine. To examine the structural changes in AuCu with Zn contents and the stable phases, XRD profiles were recorded by an X-ray diffractometer (RAD-rA, Rigaku, Tokyo, Japan) which was operated at 45 kV and 130 mA. CuK a
3. Results and discussion
3.1. Variations of electrical resistivity Fig. 1 shows variations of electrical resistivity as a function of temperature for all the alloys tested during continuous heating up to 500 8C at a rate of 0.1 8C / min. The electrical resistivity value was represented by the ratio of resistivity at any given temperature to the final resistivity value at 500 8C, and then each curve was rearranged to avoid overlapping. The order–disorder transition temperature, indicated by a single arrow, is systematically lowered with Zn addition to AuCu. A step around 390 8C for the Zn0 alloy, indicated by a double arrow, corresponds to the phase transformation point from AuCu I to AuCu II. A similar step was observed in the Zn1 and Zn2 alloys. However, this step disappeared in the alloys containing Zn of about 5 at.% or more. This suggests that the phase transformation point from AuCu I to AuCu II shifted. Considering that the electrical resistivity of the AuCu II phase is higher than that of the AuCu I phase [5] and that the magnitude of changes in electrical resistivity in Fig. 1 diminished with Zn addition, the present electrical resistivity results suggest a broadening of the AuCu II phase region towards the lower temperature side with Zn addition.
3.2. Crystal structure
Fig. 1. Variation of electrical resistivity as a function of temperature for all the alloys.
To further study the effect of Zn addition to AuCu, an XRD study was done for all the alloys equilibrated at various temperatures for periods ranging from 5000 to 70 000 min. Fig. 2 shows typical XRD patterns of the alloy Zn5 equilibrated at various temperatures. At 200 8C, the coexisting phases (AuCu I1AuCu II) were stable, and at 250 8C, single-phase AuCu II was stable. At 380 8C and 400 8C, the coexisting phases (AuCu II1a) and the a phase became stable, respectively. Considering the XRD pattern at 200 8C, the temperature range in which the phases (AuCu I1AuCu II) coexist is greatly lowered compared to the Zn0 alloy. The XRD patterns at 250 8C and 370 8C showed the same stable phase, i.e. single-phase AuCu II. This means that Zn addition expands the AuCu II phase region as suggested from the electrical resistivity results. In the ternary phase diagram of the Au–Cu–Zn system at 200 8C assessed by Prince et al. [6], by adding about 3 at.% Zn to the equiatomic AuCu, the coexisting phases (AuCu I1AuCu II) became stable. In the case that more than 4 at.% Zn was added, single-phase AuCu II became stable. But in our results, the coexisting phases (AuCu I1AuCu II) appeared
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Fig. 2. XRD patterns of the alloy Zn5 equilibrated at various temperatures.
when 5.4 at.% Zn was added, and the study of Choi et al. [7] supports our results. According to Choi et al. [7], the coexisting phases (AuCu I1AuCu II) were stable with 6.1 at.% Zn addition to equiatomic AuCu at 200 8C. In all the XRD patterns in Fig. 2, there is overlapping of the 002 fundamental reflection and the 0, 221 / 2M, 1 superlattice reflection. The notation M reflects the periodic antiphase domain size in the AuCu II superlattice, as will be explained in the following subsection. Fig. 3 is the XRD patterns showing characteristics of the AuCu II phase for various Zn content alloys. All the XRD patterns in Fig. 3 show 1, 161 / 2M, 0 superlattice reflections which are characteristic of the AuCu II phase. With increasing Zn content, the space between the peak positions of the 1, 121 / 2M, 0 and 1, 111 / 2M, 0 superlattice reflections, which are marked by circles, systematically increases. The peak positions of 0, 261 / 2M, 1 (triangles) and 1, 161 / 2M, 2 (squares) also changed in the same way. Of these, the 0, 221 / 2M, 1 superlattice reflection moved to lower angles passing through the 002 fundamental reflection with increasing Zn content. In the same manner, the 1, 121 / 2M, 2 superlattice reflection moved to lower angles. For the Zn20 alloy, this superlattice reflection overlapped with the 0, 211 / 2M, 1 superlattice reflection which moved to higher angles with Zn content. The 200 fundamental reflection splits into two reflections, i.e. the 200 and 020 reflections in high Zn content alloys. The splitting of the 200 reflection starts in the Zn5 alloy, and the degree of splitting apparently increases with increasing Zn content. It should be noted that the peak position of the 002 fundamental reflection moved to lower angles with increasing Zn content.
3.3. Crystallographic data for the AuCu II phase The crystallographic data for AuCu II phase was obtained from the XRD patterns in Fig. 3. The crystallographic unit cell of AuCu II is shown in Fig. 4 [8], where the lattice parameters a, b9, c and the antiphase domain size M are defined. Fig. 5 shows the changes in the lattice parameters and antiphase domain size for the AuCu II phase with Zn content at the equilibrating temperatures of 340–380 8C. The M value in Fig. 5 was obtained by averaging the M values calculated from the peak positions of the three pairs of superlattice reflections 1, 161 / 2M, 0 and 0, 261 / 2M, 1 and 1, 161 / 2M, 2. Reflections overlapping with others were excluded from the calculation for obtaining the average M value. The lattice parameters a and b9 were calculated from the peak positions of the 200 and 220 reflections in the case that a and b9 were the same. When a and b9 were different, a was calculated from the 200 reflection, and b9 from the 020 and 220 reflections. The c value was calculated from the peak position of the 002 reflection. For the Zn20 alloy, the lattice parameters were calculated from the peak positions of the 020, 200, 002, 220, 022 and 202 reflections. It is shown in Fig. 5 that the M value greatly decreases with Zn addition. For the Zn20 alloy, the M value was approximately 2 and this is less than half the M value of the AuCu equiatomic alloy. The lattice parameter a decreases considerably while b9 shows little change with Zn addition. The lattice parameter c apparently increases with Zn addition, thus the axial ratio, c /a, considerably increases with increasing Zn content. Schubert et al. [9] found in their study of the Au–Cu–
H.-J. Seol et al. / Journal of Alloys and Compounds 339 (2002) 144 – 148
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Fig. 3. XRD patterns showing characteristics of AuCu II phase for all the alloys.
Zn system that the addition of Zn to AuCu makes the domain size smaller, retaining the AuCu II crystal structure. According to Sato and Toth [10], the electron / atom ratio of an alloy controls the stability of the AuCu II superstructure. Such a tendency was also found in the study of Shiraishi et al. [11], showing that a reduction in the electron / atom ratio reduces the phase stability of AuCu II. Regarding the decrease of the antiphase domain size in Au–Cu–Zn alloys, Ogawa et al. [12] reported that the addition of a divalent metal makes the occurrence of out-of-steps more frequent and causes the domain size to decrease continuously. Considering their studies and our results together, it can be said that when Zn, which is a divalent metal, is added to equiatomic AuCu alloy, the electron / atom ratio increases and thus the AuCu II phase becomes more stable and the domain size decreases.
Fig. 4. Crystal structure of the AuCu II superlattice [8].
Fig. 5. Changes in lattice parameters of the AuCu II superlattice with Zn addition.
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4. Conclusion The structural changes occurring with Zn addition to AuCu were studied and a partial phase diagram for the AuCu–Zn pseudobinary system up to 20 at.% Zn was determined. The following results were obtained from this study:
Fig. 6. Partial phase diagram for the system (AuCu) 12x Zn x with x#0.2: (n) a; (♦) AuCu II1a; ([) AuCu II; (d) AuCu I1AuCu II; (s) AuCu I; (3) Tc; *after Choi et al. [7].
1. The addition of Zn to AuCu stabilizes the AuCu II superstructure. The AuCu II phase region becomes extremely expanded with increasing Zn content. As a result, the greater part of the partial phase diagram is occupied by the single-phase region of AuCu II. 2. The order–disorder transition temperature is slightly lowered by the addition of Zn to AuCu. The AuCu II↔AuCu I transition temperature is strongly lowered. 3. With increasing Zn content, the antiphase domain size M of the AuCu II superstructure markedly decreases. By adding 20 at.% Zn to the AuCu equiatomic alloy, the M value decreases to approximately 2, maintaining the AuCu II superstructure. 4. As to the lattice parameters of the AuCu II superstructure, the a value decreases and the c value increases with Zn addition, thus the axial ratio, c /a, considerably increases with Zn content.
3.4. Phase diagram Fig. 6 shows the partial phase diagram for the system (AuCu) 12x Zn x with x#0.2 which was constructed from the present electrical resistivity measurements and the X-ray diffraction study. The data marked with an asterisk (*) in Fig. 6 were taken from the report of Choi et al. [7]. The vertical extent of the (AuCu II1a) two-phase region reaches up to about 2 at.% Zn and does not change much even with more Zn addition. The boundary line separating the AuCu II single-phase region and the (AuCu II1a) coexisting region is lowered nearly parallel to the order– disorder transition line marked with a cross (3) in Fig. 6. Therefore, the a↔AuCu II transition temperature decreases slightly as a whole. On the contrary, the AuCu II↔AuCu I transition temperature markedly decreases by Zn addition. As a result, the AuCu I single-phase region is considerably limited. As shown in Fig. 6, the AuCu I single-phase region extends only up to about 5 at.% Zn at 200 8C. This means that Zn addition to AuCu greatly reduces the stability of the AuCu I phase. The (AuCu I1AuCu II) two-phase coexistence region is very narrow and broadened a little with Zn content, going down with a steep slope. Thus, the AuCu II single-phase region is greatly expanded towards lower temperatures. Especially with more than 5 at.% Zn, the AuCu II single-phase is stable even at comparatively low temperatures. From the above, it has been clarified that Zn has a good solubility in the AuCu system, and the greater part of the partial phase diagram for the system (AuCu) 12x Zn x with x#0.2 is occupied by a single-phase region of AuCu II.
Acknowledgements One of the authors, Hyo-Joung Seol, would like to acknowledge The Ministry of Education, Culture, Sports, Science and Technology in Japan (Monbukagakusho) for their financial support of this research.
References [1] M. Ohta, T. Shiraishi, M. Nakagawa, S. Matsuya, J. Mater. Sci. 29 (1994) 2083. [2] H.-J. Seol, T. Shiraishi, Y. Tanaka, E. Miura, K. Hisatsune, J. Mater. Sci.: Mater. Med. (in press). [3] T. Suzuki, T. Yoshida, T. Miyasaka, J. Nippon Dent. Univ. 73 (4) (1985) 883. [4] H. Winn, Y. Tanaka, T. Shiraishi, K. Udoh, E. Miura, R.I. Hernandez, Y. Takuma, K. Hisatsune, J. Alloys Comp. 306 (2000) 262. [5] M. Hirabayashi, J. Japan. Inst. Metals B 15 (1951) 565. [6] A. Prince, G.V. Raynor, D.S. Evans (Eds.), Phase Diagrams of Ternary Gold Alloys, Institute of Metals, London, 1990, pp. 244– 250. [7] S.-K. Choi, T. Shiraishi, H.-I. Kim, M. Ohta, M. Nakagawa, Kor. J. Mater. Res. 6 (1996) 235. [8] Japan Institute of Metals (Ed.), Metals Databook, Maruzen Co., Ltd., Tokyo, 1974, p. 54. [9] K. Schubert, B. Kiefer, M. Wilkens, R. Haufler, Z. Metallkde. 46 (1955) 692. [10] H. Sato, R.S. Toth, Phys. Rev. 124 (1961) 1833. [11] T. Shiraishi, M. Ohta, M. Nakagawa, K. Fujii, J. Alloys Comp. 203 (1994) 45. [12] S. Ogawa, D. Watanabe, H. Watanabe, T. Komoda, J. Phys. Soc. Jpn. 14 (7) (1959) 936.
