Isothermal Sections of the Ni-Cr-Ta Ternary System at 1200 ◦ C and 1300 ◦ C

: Two isothermal sections of the Ni-Cr-Ta ternary system at 1200 ◦ C and 1300 ◦ C have been determined by using electron probe microanalysis, energy dispersive spectroscopy and di ﬀ erential scanning calorimeter. A Laves phase (Ni, Cr) 2 Ta(HT)(C14 structure) with large solid solubility stabilized by the Ni addition was determined in both two isothermal sections. The composition range of this phase was about 25.8–66.0 at.% Cr, 2.5-44.3 at.% Ni, and 24.0-40.0 at.% Ta at 1200 ◦ C, which increased with raising temperature. The melting point of the Ni-Cr alloys decreased with the addition of Ta. No ternary compound was found in both these two isothermal sections. The present work could be signiﬁcant for practical application of nickel-based alloys and future thermodynamics assessment of the Ni-Cr-Ta ternary system.


Introduction
Nickel-based superalloys have been applied in the aerospace field due to their excellent high-temperature properties, oxidation and corrosion resistance in the extreme harsh environment [1,2]. However, with higher industrial requirements in the structure materials for high-temperature applications in aviation field, materials capable of better mechanical strength, oxidation and corrosion resistance are required. In order to improve the properties of Ni-based alloys, an excellent alternative is to alloy refractory elements [3][4][5][6]. Technologically, Cr addition could significantly improve the oxidation and hot-corrosion resistance for the nickel-based alloys by forming a stable oxidation protective layer Cr 2 O 3 at elevated temperatures [4,[7][8][9]. Meanwhile, as a solid solution strengthening element, the alloying of Ta also improves the hot-corrosion and oxidation resistance [10,11]. However, the stabilization of topologically close packed (TCP) phase will deteriorate the mechanical properties of the superalloys for excessive addition of Cr and Ta elements [12,13]. Therefore, it is of significant necessity to investigate the phase diagram of the ternary Ni-Cr-Ta system, not only for the future thermodynamics assessment, but also enhancing the potential practical applications.
The Ni-Cr-Ta ternary system consisting of three binary subsystems, Ni-Cr, Ni-Ta, Cr-Ta, is illustrated in Figure 1. In 1986, Nash [14] reviewed the Ni-Cr system with a eutectic reaction at 1345 • C, where an extensive Ni terminal solid solution (face centered cubic) region and a less extensive Cr terminal solid solution (body centered cubic) region were identified. Additionally, Lee [15] and Zhu et al. [16] re-evaluated the Ni-Cr binary system, which is in agreement with the experimental results with the work of Nash.  [14,30,31].
In 1987, Venkatraman et al. [32] reviewed the Cr-Ta system, in which two terminal solid solutions bcc-(Cr), bcc-(Ta) and intermediate phase Cr 2 Ta formed from eutectic reaction occurred at 1760 and 1965 • C, respectively. The intermediate compound Cr 2 Ta exhibits two Laves phase modifications. The high-temperature form, Cr 2 Ta (HT), has the hexagonal MgZn 2 -type (C14) structure, while the low-temperature form, Cr 2 Ta (LT), has the cubic MgCu 2 -type (C15) structure. In 1996, Okamoto [33] had redrawn this binary phase diagram based on the Venkatraman's work with an adjustment in the form of Cr 2 Ta solidus complying with the Gibbs-Konovalov rule. In 1991, Kaufman et al. [23] assessed the Cr-Ta system with CALPHAD approach, then, Dupin et al. [34] re-evaluated the thermodynamic information based on the experimental results and the assessment of Kaufman. Recently, Pavlů et al. [31] re-modeled the Laves phases in the system using first-principles calculation and re-optimized the phase diagram with CALPHAD method.
As for the Ni-Cr-Ta ternary system, Nash et al. [35] investigated the phase equilibria in the Ni-rich portion of this system at 1000 • C and 1250 • C to establish a ternary Laves phase NiCrTa (the lattice parameter, a = 4.844 Å, c = 7.89 Å, annealed at 1250 • C, and a = 4.885 Å, c = 7.888 Å, annealed at 1000 • C) with hexagonal MgZn 2 -type (C14) structure using electron microprobe and X-ray diffraction analysis. In 1985, Schittny et al. [36] reconfirmed the ternary compound NiCrTa in the partial isothermal section at 1000 • C with concentration range of 0-40 at. % Ta. However, there is no ternary compound except for a Laves phase Cr 2 Ta (hexagonal, MgZn 2 -type, a = 4.844 Å, c = 7.9091 Å) in the isothermal section at 1100 • C, according to Nikolaev's et al. experimental phase diagram [37]. Additionally, Dupin et al. assessed the thermodynamic database of the Ni-Cr-Ta system [38]. The stable phases in the ternary Ni-Cr-Ta system are listed in the Table 1.

Experimental Details
High purity metals nickel (99.9 wt. %), chromium (99.9 wt. %) and Tantalum (99.9 wt. %) were used as our raw material to obtain alloys. All the metals were well cleaned to avoid the input of impurity surface oxidation before melting. All alloys were displayed in the form of atomic ratios (at. %). The ingots, around 20 g, were re-melted at least four times to get the uniformity with less than 0.5 wt. % weight loss. The alloys were melted in a high purity argon atmosphere arc furnace with a non-consumable tungsten electrode on a water-cooled copper platform. Then, all specimens were individually sealed in silica capsules with high purity argon, annealed at 1200 • C for 35 days and 1300 • C for 15 days, respectively. Additionally, in order to prevent oxidation, we put some pure yttrium fillings in the quartz capsules. Some alloys with liquid phase at 1300 • C were wrapped in the pure tantalum foil to avoid contact reaction with quartz.
All alloys were water quenched after heat treatment and well prepared for metallographic analysis. The equilibrium compositions of phases in the specimens were determined by electron probe microanalysis (EPMA) with 20 kV accelerating voltage and 1.0 × 10 −8 A probe current. Additionally, the equilibrium compositions of liquid phases in some alloys annealed at 1300 • C were measured by energy dispersive spectroscopy (DSC) with 20 kV accelerating voltage and 2.0 × 10 −9 A probe current. The crystal structure was identified by a Phillips Panlytical X-pert diffractometer using Cu-Kα radiation with 40 kV voltage and 40 mA current. The results were measured in the range of 2θ from 20 • to 90 • with a step interval of 0.015308 • and a count time of 0.3 s per step. The melting points of some alloys were determined by differential scanning calorimeter (DSC) with a heating and cooling rate of 10 • C/min.

Microstructure
The phase relationship of the Ni-Cr-Ta ternary system at 1200 • C was established from 33 alloys annealed for 35 days. The nominal compositions of alloys and compositions of different phases at equilibrium are displayed in the Table 2. Meanwhile, the microstructure and XRD results of typical alloys annealed at 1200 • C for 35 days are presented in Figures 2 and 3, respectively.   As presented in Figure 2a-c, the microstructure of three-phase regions was detected in these alloys. Figure 2a showed the three-phase equilibrium, two terminal solid solutions fcc, bcc-(Cr) and a compound Ni 3 Ta, bright regions in the microstructure of the Ni 53 Cr 37 Ta 10 alloy. Figure 2b showed the three-phase equilibrium of the Ni 64 Cr 7 Ta 29 alloy, in which the white precipitated (Ni, Cr) 2 Ta(HT) phase was uniformly distributed in the Ni 3 Ta and Ni 2 Ta phase. Moreover, the three-phase equilibrium state was identified by the XRD result in Figure 3a. Three-phase region, Ni 2 Ta, Ni 6 Ta 7 and (Ni, Cr) 2 Ta(HT) was found in the Ni 53 Cr 8 Ta 39 alloy after annealed at 1200 • C for 35 days. Additionally, the XRD analysis in Figure 3b just confirmed the microstructure. As can be seen from Figure 2d-f, three two-phase regions were identified in these three alloys. Figure 2d showed the equilibrium of the gray matrix fcc and white Ni 3 Ta phase in the Ni 59 Cr 33 Ta 8 alloy annealed at 1200 • C for 35 days. The phase relation of the Ni 27 Cr 53 Ta 20 alloy, a terminal solid solution bcc-(Cr) and a compound (Ni, Cr) 2 Ta(HT), was described in Figure 2e. Furthermore, there was a two-phase section of white bcc-(Ta) and gray Ni 6 Ta 7 phase in the Ni 12 Cr 33 Ta 55 alloy as illustrated in Figure 2f. Additionally, the crystal structure of the Ni 12 Cr 33 Ta 55 alloy was identified by the XRD result displayed in Figure 3c. Figure 3d showed the XRD result of a single (Ni, Cr) 2 Ta(HT) phase in the Ni 13 Cr 53 Ta 34 alloy and the microstructure was displayed in the Figure 3e.
In the experiment, several alloys were designed to investigate phase relation of the Ni-Cr-Ta system at 1300 • C. Table 3 listed the alloys compositions and phase equilibrium compositions of the alloys annealed at 1300 • C. In addition, the microstructure and XRD patterns of typical alloys were presented in the Figures 4 and 5, respectively. Figure 4a,c showed two three-phase equilibriums with liquid phase. As presented in Figure 4a, the microstructure of three-phase region, gray liquid phase, black fcc phase and white Ni 3 Ta phase, was determined in the Ni 59 Cr 33 Ta 8 alloy annealed at 1300 • C for 3 h. Figure 4c displayed the three-phase microstructure of liquid phase, oval-shaped bcc-(Cr) and (Ni, Cr) 2 Ta(HT) phase in the Ni 35 Cr 53 Ta 12 alloy annealed at 1300 • C for 3 h. There was a three-phase section, black matrix Ni 3 Ta, white precipitated Ni 2 Ta and gray (Ni, Cr) 2 Ta(HT) identified in the Ni 65 Cr 5 Ta 30 alloy as shown in Figure 4b. Figure 4d,e illustrated two two-phase equilibrium. The Ni 6 Ta 7 and (Ni, Cr) 2 Ta(HT) phase were determined in the Ni 16 Cr 38 Ta 46 alloy described in Figure 4d, and the two-phase equilibrium was supported by the XRD result in Figure 5a. The microstructure of the Ni 12 Cr 33 Ta 55 alloy in Figure 4e was confirmed as bcc-(Ta) and Ni 6 Ta 7 by the XRD pattern presented in Figure 5b. Figure 4f showed the three-phase region, bcc-(Ta), Ni 6 Ta 7 and (Ni, Cr) 2 Ta(HT), of the Ni 5 Cr 40 Ta 55 alloy, and the result was confirmed by the XRD pattern in Figure 5c. As shown in Figure 4d-e, the cracks and holes were observed in the brittle Laves phase Ni 6 Ta 7 .

The Liquid Region
As can be observed from the microstructure in Figure 4c and isothermal section at 1300 • C, a liquid phase was confirmed at 1300 • C. However, according to the experimental results in the three subsystems, no liquid phase exists at 1300 • C. In order to confirm the experimental results in the present work, the DSC analysis was conducted to obtain the melting point of the related alloys. On the basis of DSC result and microstructure in Figure 8, the bcc-(Cr) phase transformed to liquid phase as the temperature increased from 1200 • C to 1285 • C and the melting point of the bcc-(Cr) phase in the Ni 51 Cr 28 Ta 21 alloy was measured to be about 1237 • C. Owing to the liquid phase that appeared near the Ni-Cr side, we supposed that the Ta addition in Ni-Cr alloys decreases the temperature of the eutectic reaction, L → fcc + bcc-(Cr). The corresponding results indicate that the Ta addition reduces the melting point of the Ni-Cr alloys.

Conclusions
In the present work, the isothermal sections of the Ni-Cr-Ta ternary system at 1200 • C and 1300 • C were experimentally established. The corresponding results are shown as follows: (1) The solubility of Cr in Ni 6 Ta 7 phase was about 41.6 at. % at 1300 • C, and no ternary compound was found at two sections. (2) The high temperature (Ni, Cr) 2 Ta(HT) (MgZn 2 -type) phase with a large composition range was determined at both two temperatures, which was stabilized by the Ni addition to Cr-Ta alloys against low temperature, and its solubility increased as temperature raise from 1200 • C to 1300 • C. (3) A small liquid region was confirmed at 1300 • C, while it disappeared at 1200 • C. The results indicate that the addition of Ta reduced the melting point of the Ni-Cr alloys.