**1. Introduction**

Metallic biomaterials used for dental applications, which are called dental alloys, and such alloys require a high corrosion resistance because the pH and temperature vary widely in the oral environment where foods and beverages are taken in. These alloys also require biocom‐ patibility in order to prevent an allergic reaction to the metals. Dental alloys are mainly used to make devices for filling cavities and as substitutes for teeth that are lost because of decay and periodontal disease. A variety of dental devices have been developed, which include metallic fillers, inlays, crowns, bridges, clasps, dentures, dental implants composed of a fixture and an abutment, and fixed braces (train tracks). These forms of dental restoration, customshaped for an individual, are made by casting; therefore, the castability of alloys is another requirement for dental applications.

Dental alloys are mainly classified into two groups: precious and nonprecious metals. Suitable alloys are employed according to the intended use. Alloys of precious metals such as gold (Au), palladium (Pd), and silver (Ag) are usually employed because of their high corrosion resistance, biocompatibility, and castability, as compared to those of nonprecious metals. Precious alloys are grouped into high-carat alloy (high-precious or -nobility alloy) and lowcarat alloy (low-precious or –nobility alloy). The high-carat alloy contains more than 75 % precious metals. Non-precious metal alloys such as stainless steels, cobalt-chromium, nickelchromium, and titanium alloys are also commonly used.

Among the dental alloys, precious alloys are widely used. Au alloys have been commonly used in dental applications from past to the present, and many commercial variations of alloy compositions have been developed, despite their high cost. American Dental Association classifies these Au alloys on the basis of their mechanical properties. Many studies have been carried out to improve the mechanical properties of the Au alloys containing copper (Cu) (Au– Cu–Pd [1, 2], Au–Ag–Pd–In [3], Au–Cu–Zn [4], Au–Cu–Zn–Ag [5], and Au–Ag–Cu–Pd [6]

© 2013 Hieda et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

alloys), with the main focus on the microstructural changes produced by heat treatment. However, recent trends have shown that low-carat dental alloys (Ag and Pd alloys) are attracting much attention as alternatives to Au alloys because of their lower price. Thus, Ag alloys such as Ag–Pd–Cu–Au [7-17], and Ag–Cu–Pd–Au [18, 19] alloys have been developed for commercial applications. The hardness of these alloys increases with aging or solution treatment, and many studies have reported on the behaviors of these alloys in response to various heat treatments and the various mechanisms. Dental casting of Ag–20Pd–14.5Cu-12Au alloy (mass%) has been developed and used widely in Japan. In general, this alloy is subjected to aging treatment (AT) at around 673 K after solution treatment (ST) at 1023 K in order to enhance its mechanical strength. Recently, it has been reported that the mechanical strength of Ag–20Pd–14.5Cu–12Au alloy is significantly enhanced when the alloy is subjected to ST at temperatures higher than 1073 K and subsequently water quenched without any AT [9-16]. The Vickers hardness of this alloy increases with an increase in the cooling rate after ST [11]. This unique hardening behavior and the increase in mechanical strength induced by the hightemperature ST have been explained in terms of the precipitation hardening caused by the precipitation of an L10-type ordered β′ phase.

This chapter describes with focusing on the relationship between the unique hardening behavior exhibited by the as-solutionized dental Ag–Pd–Cu–Au alloys and the corresponding microstructural changes. Other mechanical properties (fatigue, fretting-fatigue, and friction wear properties) and corrosion properties are also described.

### **2. Age hardening behavior of Ag–Pd–Cu–Au alloys**

Ag-Pd alloys have complete miscibility in all composition ratios. Addition of Cu leads to age hardening in these alloys. In the early researches during 1960–70s, mechanisms for age hardening of Ag-Pd-Cu alloys were proposed [20-22]. According to them, the age hardening was caused by the formation of a CuPd ordered phase (L20-type) [20], precipitation of a Curich α1 phase [21], and phase separation of α solid solution to a CuPd ordered phase (L20-type) and Ag-rich α2 phase [22]. More recently, Au has been added to Ag-Pd-Cu alloys to increase their corrosion resistance. Therefore, since 1980 to the present, many studies have focused on the age-hardening mechanism of the Ag-Pd-Cu-Au alloys (Table 1) [8, 19, 23, 24]. Ohta et al. [23] reported that the precipitation of a L10-type face-centered tetragonal CuPd ordered platelet (β') inside the grains and discontinuous precipitation of the Ag-rich α2 phase and CuPd ordered phase (β) in the grain boundary regions enhance the hardening of Ag-Pd-Cu-Au alloys with a low Au content (Au = 10 mass%). According to their report, TEM images of the precipitates (β and α2) along the grain boundaries in the alloys show strain contrast and moiré fringes, which indicates that β and α2 phases are coherent with each other. Solution hardening behavior was also found in alloys subjected to ST at 1223 K followed by slow quenching (SQ) before aging. Researches since 2000 have focused on the age hardening behavior of the Ag-Pd-Cu-Au alloys as a function of the treatment duration [8, 19]. It was found that during the early stage of the aging, diffusion and aggregation of Cu atoms from the Ag-rich α phase occur, and in the later stage, the hardness of the alloy decreases because of the coarsening of the Cu-rich lamellar precipitates [8, 19]. At a Cu concentration of 20 mass%, the CuPd ordered phase (β) does not exhibit any change after aging and thus does not contribute to the age hardening [19].

The age-hardening mechanism of Ag-Pd-Cu-Au alloys with an Au content of 20 mass% was also investigated, and it was concluded that β' and the discontinuous Ag-rich α2 phase contribute to the age hardening of these alloys [24]. Fig.1 shows the hardness (Hv) of the precipitates measured independently inside the grains (inter grain) and along the grain boundaries (nodule) [24]. These curves indicate that there are two hardening stages of the alloys in terms of aging temperature: the formation of β' phase in the grain interior at low aging temperatures and the precipitation of the Ag-rich α<sup>2</sup> phase along the grain boundaries at high aging temperatures. It was also found that the Cu concentration influences the formation of β' phase in these alloys (Fig. 2) [24]. The hardness (Hv) of each alloy increases with an increase of the Cu concentration, i.e. the volume fraction of β' phase, at any temperatures.


**Table 1.** Compositions of Ag-Pd-Cu-Au alloys (mass%).

alloys), with the main focus on the microstructural changes produced by heat treatment. However, recent trends have shown that low-carat dental alloys (Ag and Pd alloys) are attracting much attention as alternatives to Au alloys because of their lower price. Thus, Ag alloys such as Ag–Pd–Cu–Au [7-17], and Ag–Cu–Pd–Au [18, 19] alloys have been developed for commercial applications. The hardness of these alloys increases with aging or solution treatment, and many studies have reported on the behaviors of these alloys in response to various heat treatments and the various mechanisms. Dental casting of Ag–20Pd–14.5Cu-12Au alloy (mass%) has been developed and used widely in Japan. In general, this alloy is subjected to aging treatment (AT) at around 673 K after solution treatment (ST) at 1023 K in order to enhance its mechanical strength. Recently, it has been reported that the mechanical strength of Ag–20Pd–14.5Cu–12Au alloy is significantly enhanced when the alloy is subjected to ST at temperatures higher than 1073 K and subsequently water quenched without any AT [9-16]. The Vickers hardness of this alloy increases with an increase in the cooling rate after ST [11]. This unique hardening behavior and the increase in mechanical strength induced by the hightemperature ST have been explained in terms of the precipitation hardening caused by the

This chapter describes with focusing on the relationship between the unique hardening behavior exhibited by the as-solutionized dental Ag–Pd–Cu–Au alloys and the corresponding microstructural changes. Other mechanical properties (fatigue, fretting-fatigue, and friction

Ag-Pd alloys have complete miscibility in all composition ratios. Addition of Cu leads to age hardening in these alloys. In the early researches during 1960–70s, mechanisms for age hardening of Ag-Pd-Cu alloys were proposed [20-22]. According to them, the age hardening was caused by the formation of a CuPd ordered phase (L20-type) [20], precipitation of a Curich α1 phase [21], and phase separation of α solid solution to a CuPd ordered phase (L20-type) and Ag-rich α2 phase [22]. More recently, Au has been added to Ag-Pd-Cu alloys to increase their corrosion resistance. Therefore, since 1980 to the present, many studies have focused on the age-hardening mechanism of the Ag-Pd-Cu-Au alloys (Table 1) [8, 19, 23, 24]. Ohta et al. [23] reported that the precipitation of a L10-type face-centered tetragonal CuPd ordered platelet (β') inside the grains and discontinuous precipitation of the Ag-rich α2 phase and CuPd ordered phase (β) in the grain boundary regions enhance the hardening of Ag-Pd-Cu-Au alloys with a low Au content (Au = 10 mass%). According to their report, TEM images of the precipitates (β and α2) along the grain boundaries in the alloys show strain contrast and moiré fringes, which indicates that β and α2 phases are coherent with each other. Solution hardening behavior was also found in alloys subjected to ST at 1223 K followed by slow quenching (SQ) before aging. Researches since 2000 have focused on the age hardening behavior of the Ag-Pd-Cu-Au alloys as a function of the treatment duration [8, 19]. It was found that during the early stage of the aging, diffusion and aggregation of Cu atoms from the Ag-rich α phase occur, and

precipitation of an L10-type ordered β′ phase.

516 Advances in Biomaterials Science and Biomedical Applications

wear properties) and corrosion properties are also described.

**2. Age hardening behavior of Ag–Pd–Cu–Au alloys**

Figure 1

**Figure 1.** Anisothermal age hardening curve of Ag-25.2Pd-9.88Cu-20Au alloy.

**Figure 2.** Effect of Cu concentration (at%) on hardness (Hv) of Ag-30.32Pd-9.66Cu-5.04Au (A), Ag-25.4Pd-12.82Cu-9.96Au (B), Ag-28Pd-9.12Cu-12.04Au (C), and Ag-25.2Pd-9.88Cu-20Au (D) alloys. SQ and RQ indi‐ cate specimens subjected to slow quenching and rapid quenching after ST, respectively.

**Figure 3.** (a) Diffraction pattern, (b) TEM image, and (c) HRTEM image of Ag-Pd-Cu-Au alloy subjected to ST at 1023 K for 3.6 ks followed by aging treatment at 623 K for 1.8 ks.

Figure 3 shows a diffraction pattern, a transmission electron microscopy (TEM) image, and a high resolution TEM (HRTEM) image of β' phase formed in the Ag–20Pd–14.5Cu–12Au alloy subjected to ST at 1023 K for 3.6 ks followed by AT at 623 K for 1.8 ks [25]. β' phase is platelet shape with the size of about 10 nm long, which precipitates parallel to {200} crystal plane of matrix. A lattice constant of a-axis matches with that of the matrix, which exhibits coherent with the matrix [25].
