1 Introduction

Owing to lower elastic modulus, Au–Pd alloys could not withstand higher corrosion and strength. Adding a small amount of alloy elements can improve the alloy’s elastic modulus. V, Cr, Fe, Zr, and other elements are often used to improve the mechanical and electrical properties of alloys, and thus get high strength and resistivity [1–4]. As an typical element, Zr is often used to improve the alloy’s instability for arc erosion, poor solder wettability, and low catalytic efficiency, but there are few related reports on the phase diagram of Pd–Au–Zr alloy system until now.

The Pd–Au–Zr ternary system consists of three binary systems, Pd–Au, Au–Zr, and Pd–Zr. Okamoto and Massalski [5] studied the Pd–Au binary phase diagram, they found that the Pd–Au binary phase diagram contained liquid, a continuous solid solution phase (Au–Pd), three intermediate phases, Au3Pd, AuPd3, and the existence of ordered phase AuPd at low temperature. Lomello-Tafin et al. [6, 7] reported seven intermediate compounds, Zr3Au, Zr2Au, Zr5Au4, Zr7Au10, ZrAu2, ZrAu3, and ZrAu4 in the Au–Zr system; however, the structures of Zr5Au4 and Zr7Au10 were not determined clearly. They assessed the Au–Zr phase diagram through differential thermal analysis (DTA), X-ray diffraction (XRD), and electron probe micro-analysis (EPMA) technique, including the identification of the liquid equilibrium, and found two new intermediate compounds Zr3Au2 and ZrAu. According to the experimental measurement, Zr5Au4 was not an independent compound and ZrAu phase was an intermediate phase with some homogeneous area. Waterstrat et al. [8] used CALPHAD technique to predict the Pd–Zr phase diagram, which was in good agreement with the experimental data. Li et al. [9] used the CALPHAD technique to reassess the Au–Zr phase diagram; the result was in accord with the experimental data. Guo et al. [10] assessed the Pd–Zr system and set up three sublattice models for order–disorder transformation between Pd–Zr phase and Pd3Zr phase. The results were in good agreement with the experimental results and the self-consistent thermodynamic interaction parameters were obtained. In this paper, the liquid phase projection and isothermal section for the ternary system were predicted through Panda phase calculation software, and the thermodynamic data were based on the research of Pd–Au, Au–Zr, and Pd–Zr binary phases.

2 Experimental

The phase diagram of the Au–Pd–Zr system was calculated by published thermodynamic descriptions for the constitutive binary systems. The Gibbs energy functions for pure elements were taken from the Science Group Thermodata Europe database [11]. The thermodynamic parameters for Pd–Au, Au–Zr, and Pd–Zr systems from Refs. [5, 9, 10], correspondingly, were directly used in this work.

The raw materials were Au (99.99 wt% spongy), Pd (99.99 % wire), and Zr (99.99 %), and the above metals were melted by GP30-CW7 high-frequency induction furnace with vacuum degree of 5 × 10−2 Pa, then argon gas was used for protection during melting. After melted to composition, uniformed entirely, the alloys were cooled to room temperature in the furnace and then annealed in 1,100 °C for 72 and 144 h, respectively.

All the samples were metallographically prepared. After etching, optical microscopes (OM, XWJ02) and scanning electron microscope with energy dispersive spectroscopy (SEM–EDS, S-3400N) were used for microscopic appearance analysis. X-ray diffraction (DMAX-RC) was used for the alloy phase analysis, measured by the target of Cu Kα, 40 kV/40 mA, scanning angle of 0°–90°. The results of XRD were judged by Jade5.0.

3 Results and discussion

3.1 Calculated phase diagram

As shown in Fig. 1, the highest temperature in liquidus surface is 2,550 °C with the extrapolated liquidus projection of the Au–Pd–Zr ternary system, and the calculated liquidus projection shows a large composition region of liquid immiscibility. The dense liquid isotherms indicate that the three-dimensional map is very steep, resulting in large change of temperature within the alloy of liquid compositions. In the middle phase (near the Au–Pd border), the isotherm distribution is sparser, thus the liquid surface of the alloy in three-dimensional figure is relatively flat, and the temperature changes more slowly with the changing composition .

Fig. 1
figure 1

Calculated liquidus projection of Au–Pd–Zr system

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Seven key ternary reactions are shown in this liquidus projection (A–G). The temperature of a ternary eutectic reaction point (A) close to the binary Pd–Zr is 982.59 °C. This temperature represents the lowest temperature of thermodynamic stability of the liquid phase in this ternary system according to the thermodynamic prediction. The seven key ternary reactions and corresponding temperatures are given in Table 1.

Table 1 Key ternary reactions and temperatures corresponding to Points A–G in Fig. 1
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There are 17 phase regions in all isothermal sections at 25 °C as shown in Fig. 2. The alloy is based on Au–Pd matrix and the mole ration of Au to Pd is 1:1, the amount range of Zr is 0–1, the starting point of red line means that the composition is 0.5Au–0.5Pd–0Zr and the end point is 0Au–0Pd–1Zr. The precipitation orders for the five points in red line of Fig. 2 are shown as below according to the vertical section of Fig. 3: Point 1, liquid → Pd3Zr + liquid → Liquid + Pd3Zr + ZrAu2 → ZrAu2 + ZrAu3 + Pd3Zr; Point 2, liquid → Pd3Zr + liquid → liquid + Pd3Zr + ZrAu2 → Pd3Zr + ZrAu2 + PdZrM → ZrAu2 + Pd3Zr + Zr7Au10; Point 3, liquid → PdZrM + liquid + liquid → PdZrM + liquid → liquid + ZrAu + PdZrM → ZrAu + PdZrM + Zr7Au10; Point 4, liquid → PdZrM + liquid + liquid → liquid + PdZrM → ZrAu + PdZrM + Pd4Zr3.

Fig. 2
figure 2

Calculated isothermal section of Au–Pd–Zr system at 25 °C

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Fig. 3
figure 3

Vertical section of Au–Pd–Zr system for five corresponding calculated points in Fig. 2 and enlarged portion of Alloy 3

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Alloy 3 is selected to specify the crystallization process. The process is described in order from 1 to 6 according to the enlarged portion in Fig. 3: 1′–2′, liquid → Pd3Zr; 2′–3′, liquid + Pd3Zr → PdZrM + liquid at 1,558.99 °C, and Pd3Zr also exists; 3′–4′, liquid + Pd3Zr → PdZrM + ZrAu2 in 1,403.77 °C, and the system exists three phases (liquid + PdZrM + ZrAu2); 4′–5′, liquid → Zr7Au10 and PdZrM + ZrAu2 → Pd3Zr + Zr7Au10 at 1,017.89 °C; 5–6, PdZrM + Pd3Zr + Zr7Au10 exist. Therefore, the whole solidification process of Alloy 3 is liquid → Pd3Zr + liquid → liquid + Pd3Zr + PdZrM → liquid + ZrAu2 + PdZrM → ZrAu2 + PdZrM + Zr7Au10.

3.2 XRD and EDS analysis for experiment phenomena

Verification experiments for Alloy 3 make the calculation more reasonable. As shown in Fig. 4a, the light gray color organizations are surrounded by deeper gray organizations in the circle section, and the point-like organizations in oval section appear after annealing for 72 h, which are similar to peritectic organization [12]. The precipitation phases show a transitional diffusion state from B to A region as shown in Fig. 4b, indicating that the unfinished reaction (PdZrM + ZrAu2 → Pd3Zr + Zr7Au10 at 1,017.89 °C) continues during annealing time, and the microstructure should be ZrAu2 + PdZrM + Zr7Au10 at room temperature.

Fig. 4
figure 4

OM images of Alloy 3 annealing for 72 h: a peritectic microstructure and b peritectic particle

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The EDS analysis for the point-like (oval section) structure for different annealing time shows that the amount of Pd and Au increases, whereas Zr decreases; meanwhile, the microstructure in circle section disappears and that in oval section appears largely and grows up with annealing time of 144 h, as shown in Fig. 5. The phenomenon can be interpreted that some unbalance phases (may be caused by peritectic reaction: liquid + Pd3Zr → PdZrM + ZrAu2 at 1,403.77 °C or PdZrM + ZrAu2 → Pd3Zr + Zr7Au10 at 1,017.89 °C) disappear with long annealing time. The three phases ZrAu2, PdZrM, and Zr7Au10 were confirmed by XRD, as shown in Fig. 6. However, there are three stronger diffraction peaks (located at about 30°, 40°, 60°, etc.), and corresponding phases cannot be found according to PDF data card; therefore, it maybe ternary compounds.

Fig. 5
figure 5

SEM images of Alloy 3 annealing for a 72 h and b 144 h

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Fig. 6
figure 6

XRD pattern of annealed Alloy 3

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3.3 Speculated possible ternary compounds

The amount of Zr is higher than those of Pd and Au, thus Zr is considered to be the matrix by EDS analysis in Table 2, and the possible ternary compounds are Zr2AuPd, Pd2AuZr, AuPdZr, Zr2Au2Pd, and Zr4Pd2Au by the ratio of Au:Pd:Zr from EDS analysis. The lattice parameters of possible ternary compounds are listed in Table 3.

Table 2 EDS results of marked zones in Fig. 5a and b
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Table 3 Lattice parameters of ternary compounds
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The crystal structures of above five phases were created by materials studio (MS) software. The crystal structure model of Zr2AuPd is shown in Fig. 7, the occupation ratio of Pd atoms and Au atom is 50 % at the (0, 0, 0) position, taken 3 × 3 × 3 super-cell structure. The XRD simulation results show that diffraction peaks for AuPdZr2 appear at 2θ = 28°, 60°, and the strongest diffraction peak also appears at 2θ = 41° or 42° at the same time, as shown in Fig. 8. It is consistent with the experimental XRD result of Alloy 3 in Fig. 6. The simulation results suggest that there is a Zr2AuPd ternary compound phase.

Fig. 7
figure 7

Simulation model of Zr2AuPd

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Fig. 8
figure 8

Simulated XRD pattern for Zr2AuPd

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4 Conclusion

In this article, the method to successfully calculate phase diagram of Au–Pd–Zr ternary system and predication process were reported. The solidification sequence of the alloy 32 % Au–32 % Pd–36 % Zr is liquid → Pd3Zr + liquid → liquid + Pd3Zr + PdZrM → liquid + ZrAu2 + PdZrM→ZrAu2 + PdZrM + Zr7Au10, and a peritectic reaction liquid + Pd3Zr → PdZrM + ZrAu2 occurs at 1,403.77 °C. The non-corresponding phase to PDF card for 2θ = 30°, 60° and other position in XRD should be Zr2AuPd. The well agreements for calculation with experiment show that other solidification processes are correct. Our results indicate that the thermal data and the Panda phase calculation software are suitable for directing alloying design and heat treatment.