Phase Equilibria in the Ni-VTa Ternary System

Two isothermal sections of the Ni-V-Ta ternary system at 1200 ◦C and 1000 ◦C have been experimentally established using X-ray diffraction, electron probe microanalysis, energy dispersive spectroscopy, and differential scanning calorimeter with equilibrated key alloys. The equilibrium composition of each phase is measured by electron probe microanalysis and energy dispersive spectroscopy. The results indicate that: (1) nine three-phase and eight three-phase regions were confirmed in the isothermal section at 1200 ◦C and 1000 ◦C, respectively; (2) the C14 phase with a large solubility of about 51.2 at. % Ni was observed at 1200 ◦C, while it forms a single-phase region at 1000 ◦C; (3) a small liquid region was confirmed at 1200 ◦C, but disappeared at 1000 ◦C.


Introduction
Nickel-based superalloys have become unique materials used in gas turbine engines, due to their excellent mechanical properties, oxidation, and creep resistance at elevated temperatures [1].With increasing industry requirements for better high-temperature strength materials, there is an urgent need to improve the high-temperature properties of these superalloys.Many studies have been carried out to confirm the fact that a small addition of refractory elements (e.g., Re, Ru and Ta) can improve the high-temperature strength [1][2][3].Besides, in 1981, Yang [4] found that a trace addition of 1 at.% Ta in complex nickel-based alloys improved the oxidation resistance significantly.Recently, Park et al. [5] have found that Ta exerted a beneficial effect on oxidation resistance in nickel-based superalloys.Additionally, the Ni 3 V (D0 22 ) phase is a critical phase in the dual two-phase intermetallic alloys composed of Ni 3 Al and Ni 3 V, which possessed outstanding mechanical and chemical properties [6][7][8].However, there is limited information on the Ni-V-Ta ternary system [9,10].In 1971, Moreen et al. [9] investigated the ordering reactions of Ni 8 X (X: V, Ta, Nb) by electrical resistivity and electron diffraction techniques.Then, Gupta [10] reviewed the experimental data of the ternary system.Unfortunately, no information of experimental phase diagram and thermodynamic database was found for this ternary system.Therefore, it is essential to establish the phase relation of Ni-V-Ta ternary system for developing thermodynamic database and practical application.
The Ni-V-Ta ternary system consists of three sub-systems: Ni-Ta, Ni-V, and V-Ta binary system, as shown in Figure 1.All the stable compounds and solid solutions are summarized in Table 1.
The Ni-V phase diagram was well studied by many investigators [24][25][26][27].There are two terminal solid solutions (fcc, bcc-(V)) and five intermetallic compounds (σ , Ni 8 V, Ni 3 V, Ni 2 V and NiV 3 ) in this system according to Smith et al. [24].Two eutectoids, a eutectic, a peritectic, a peritectoid, and three congruent transformations exist in the system.The Ni 3 V phase forms from a congruent transformation at 1045 • C. The information of the Ni-V system was thermodynamically analyzed by Watson et al. [26] and Kabanova et al. [27], which were in agreement with the experimental results.
As for the V-Ta binary system, a continuous solid solution bcc-(V, Ta) at elevated temperatures and an intermediate phase (V 2 Ta) at lower temperatures were identified according to the previous work [28][29][30][31][32].However, the focus lay on the phase transition of Laves phase at low temperatures.In 1972, Savieky et al. [28] confirmed a C15-type (MgCu 2 ) phase and C14-type (MgZn 2 ) phase two-phase field at the temperature range of 1125 • C to 1280 • C. Later, Danon et al. [30] and Pavl ů et al. [32] evaluated the V-Ta system by CALPHAD approach and first-principles calculation.
Thus, because of the lacking information of the Ni-V-Ta system, the goal of this work was to establish the isothermal sections of the Ni-V-Ta ternary system at 1000 Ni3Ta, Ni2Ta, Ni6Ta7 and6 NiTa2) coexist in this binary system according to Nash et al. [16] and Pan et al. [20].There are three peritectic reactions and two eutectic reactions in the binary system.The NiTa2 and Ni6Ta7 phases form from the peritectic reaction L + bcc-(Ta) ↔ NiTa2 at 1788 °C and L + NiTa2 ↔ Ni6Ta7 at 1570 °C , respectively.At 1330 °C , the eutectic reaction L ↔ Ni3Ta + fcc occur.Meanwhile, the thermodynamic information of this binary system was assessed by several investigators [16][17][18][19][20][21][22] with the Computer Coupling of Phase Diagrams and Thermochemistry (CALPHAD) approach and first-principles calculation.The Ni-Ta phase diagram adopted in present work was assessed by Nash et al. for its better agreement with the experimental results.The Ni-V phase diagram was well studied by many investigators [24][25][26][27].There are two terminal solid solutions (fcc, bcc-(V)) and five intermetallic compounds (σ', Ni8V, Ni3V, Ni2V and NiV3) in this system according to Smith et al. [24].Two eutectoids, a eutectic, a peritectic, a peritectoid, and three congruent transformations exist in the system.The Ni3V phase forms from a congruent transformation at 1045 °C .The information of the Ni-V system was thermodynamically analyzed by Watson et al. [26] and Kabanova et al. [27], which were in agreement with the experimental results.
As for the V-Ta binary system, a continuous solid solution bcc-(V, Ta) at elevated temperatures and an intermediate phase (V2Ta) at lower temperatures were identified according to the previous work [28][29][30][31][32].However, the focus lay on the phase transition of Laves phase at low temperatures.In 1972, Savieky et al. [28] confirmed a C15-type (MgCu2) phase and C14-type (MgZn2) phase two-phase field at the temperature range of 1125 °C to 1280 °C .Later, Danon et al. [30] and Pavlů et al. [32] evaluated the V-Ta system by CALPHAD approach and first-principles calculation.
Thus, because of the lacking information of the Ni-V-Ta system, the goal of this work was to establish the isothermal sections of the Ni-V-Ta ternary system at 1000 °C and 1200 °C .

Experimental Details
The raw materials were the following high purity metals: nickel (99.9 wt.%), vanadium (99.9 wt.%) and tantalum (99.9 wt.%).All alloys were prepared in the form of atomic ratios (at.%).Bulk alloys with nominal composition were prepared by arc furnace melting in a high purity argon atmosphere with a non-consumable tungsten electrode on a water-cooled copper platform.The ingots, about 20 g, were re-melted at least five times to obtain homogeneous alloys with less than 0.5 wt.% weight loss.
All plate-shaped specimens were individually sealed in silica capsules backfilled with high purity argon, and annealed at 1000 • C for 55 days and 1200 • C for 40 days, respectively.Moreover, pure yttrium fillings were filled into the quartz capsules to prevent oxidation, and the alloys with liquid phase were wrapped in the pure tantalum slice to prevent contact reaction with quartz.
After heat treatment and metallographic preparation, the equilibrium compositions of all phases in specimens were determined by electron probe microanalysis (EPMA) with 20 kV accelerating voltage and 1.0 × 10 −8 A probe current.The measured values were calibrated by ZAF (Z: atomic number effect, A: absorption effect, F: fluorescence effect) correction, with pure elements were used as standard samples.Energy-dispersive spectrometry (EDS) was used to measure the alloys' nominal composition and liquid phase composition.In order to identify the crystal structure, we used powder X-ray diffraction (XRD), with a Phillips Panalytical X-pet diffractometer using Cu-Kα radiation at 40 kV and 40 mA.The data were collected in the range of 2θ from 20 to 90 • C with a step interval of 0.015308 • and a count time of 0.4 s per step.The melting points of the alloys were determined by differential scanning calorimeter (DSC) with a heating and cooling rate of 10 • C/min.

Phase Equilibria at 1000 • C
For the construction of isothermal section of the Ni-V-Ta ternary system at 1000 • C, 38 alloys were prepared and the equilibrium composition of each phase was summarized in Table 2. Microstructure and XRD results of the typical alloys after annealed at 1000 • C for 55 days are shown in Figures 2 and 3, respectively.Figure 2a-c shows the microstructure of the three-phase region of the alloys.In Figure 2a, the three-phase equilibrium of σ (black) + Ni 3 Ta (gray) + C14 (light gray) in the Ni 59 V 24 Ta 17 alloy was established.Figure 2b shows the microstructure of the Ni 61 V 30 Ta 9 alloy, in which the black phase σ , the white phase Ni 3 Ta and the gray matrix fcc were observed.As displayed in Figure 2c, three-phase region of NiTa 2 + Ni 6 Ta 7 + bcc-(Ta) in the Ni 19 V 11 Ta 70 alloy was identified.The light gray phase NiTa 2 precipitated in the matrix Ni 6 Ta 7 , along with the white phase bcc-(Ta).Figure 2d-f presents the microstructure of the two-phase section of the alloys.Figure 2d shows the two-phase section of bcc-(Ta) (gray) + C15 (black) in the Ni 5 V 49 Ta 46 alloy and the corresponding XRD result is shown in Figure 3a. Figure 2e is the microstructure of Ni 6 Ta 7 (gray) + C14 (black) in the Ni 33 V 25 Ta 42 alloy.In the Ni 13 V 46 Ta 41 alloy annealed at 1000 • C for 55 days, two-phase equilibrium of C14 + bcc-(Ta) was confirmed in Figure 2f.The corresponding crystal structure was identified by XRD pattern in Figure 3b.In order to better identify the crystal structure of the C14 phase, we obtained single-phase alloys and the XRD result is presented in Figure 3c.shows the two-phase section of bcc-(Ta) (gray) + C15 (black) in the Ni5V49Ta46 alloy and the corresponding XRD result is shown in Figure 3a. Figure 2e is the microstructure of Ni6Ta7 (gray) + C14 (black) in the Ni33V25Ta42 alloy.In the Ni13V46Ta41 alloy annealed at 1000 °C for 55 days, two-phase equilibrium of C14 + bcc-(Ta) was confirmed in Figure 2f.The corresponding crystal structure was identified by XRD pattern in Figure 3b.In order to better identify the crystal structure of the C14 phase, we obtained single-phase alloys and the XRD result is presented in Figure 3c.shows the two-phase section of bcc-(Ta) (gray) + C15 (black) in the Ni5V49Ta46 alloy and the corresponding XRD result is shown in Figure 3a. Figure 2e is the microstructure of Ni6Ta7 (gray) + C14 (black) in the Ni33V25Ta42 alloy.In the Ni13V46Ta41 alloy annealed at 1000 °C for 55 days, two-phase equilibrium of C14 + bcc-(Ta) was confirmed in Figure 2f.The corresponding crystal structure was identified by XRD pattern in Figure 3b.In order to better identify the crystal structure of the C14 phase, we obtained single-phase alloys and the XRD result is presented in Figure 3c.According to the microstructure and XRD results, the isothermal section of the Ni-V-Ta system at 1000 °C is shown in Figure 4. Eight three-phase sections, σ' + bcc-(V) + C14, fcc + σ' + Ni3Ta, C14 + σ' + Ni3Ta, fcc + Ni3Ta + Ni3V, C14 + Ni6Ta7 + Ni3Ta, Ni2Ta + Ni6Ta7 + Ni3Ta, NiTa2 + Ni6Ta7 + bcc-(Ta), and C14 + Ni6Ta7 + bcc-(Ta), were confirmed and marked by triangle with solid lines.Unfortunately, four three-phase equilibriums, fcc + Ni8Ta + Ni3Ta, fcc + Ni3Ta + Ni3V, C14 + C15 + bcc-(Ta) and C14 + C15 + bcc-(V) were not established and marked by triangle with dash lines.The determined twophase region was marked in black fine lines and single phase with small squares.The experimental data indicate that: (1) a single-phase region C14 with a large solubility of three elements, which was measured to be 23.3-52at.% V, 22.4-38.1 at.% Ta and 13.1-49 at.% Ni, exists at 1000 °C ; (2) although the C14 phase is not found in V-Ta binary system at 1000 °C , it is stabilized by addition of Ni confirmed by the presence of a single-phase C14; (3) the Ni3V phase was confirmed at 1000 °C near the Ni-V side, which did not form a continuous solid solution with the Ni3Ta phase; (4) the phases fcc, Ni3V and σ' dissolve up to 5.8 at.%, 7.1 at.% and 5.9 at.% Ta, respectively; (5) the solubility of V in the phases Ni3Ta, Ni2Ta and Ni6Ta7 was determined to be 17 at.%, 3.8 at.% and 31.9 at.%, respectively.

Phase Equilibria at 1200 °C
Based on the experimental results, eight intermetallic compounds (σ', Ni8Ta, Ni3Ta, Ni2Ta, Ni6Ta7, NiTa2, C14, C15) and two solid solutions-fcc and bcc-were confirmed at 1200 °C , as shown in Table 1.The isothermal section of the Ni-V-Ta ternary system at 1200 °C was established by 38 alloys summarized in Table 3. Table 3. Equilibrium compositions of the phase in the Ni-V-Ta ternary alloys annealed at 1200 °C for 40 days except the starred ones.

Phase Equilibria at 1200 • C
Based on the experimental results, eight intermetallic compounds (σ , Ni 8 Ta, Ni 3 Ta, Ni 2 Ta, Ni 6 Ta 7 , NiTa 2 , C14, C15) and two solid solutions-fcc and bcc-were confirmed at 1200 • C, as shown in Table 1.The isothermal section of the Ni-V-Ta ternary system at 1200 • C was established by 38 alloys summarized in Table 3.The microstructure and the corresponding XRD analysis are presented in the Figures 5a-f and  6a-c, respectively.Figure 5a shows the three-phase equilibrium of C14 + σ + bcc-(V) in the Ni 24 V 67 Ta 9 alloy annealed at 1200 • C for 40 days.Figure 5b features the microstructure of the Ni 48 V 18 Ta 34 alloy.The small black phase Ni 3 Ta and the white phase Ni 6 Ta 7 were evenly distributed in the gray matrix phase C14.The corresponding phase relationship was identified by XRD result, in which the characteristic peaks of the C14, Ni 3 Ta and Ni 6 Ta 7 phases were labelled by different symbols, as presented in Figure 6a.In the Ni 14 V 36 Ta 50 alloy quenched from 1200 • C, the three-phase region of C14 (dark gray) + Ni 6 Ta 7 (light gray) + bcc-(Ta) (white) was confirmed in Figure 5c and the corresponding XRD pattern is displayed in Figure 6b. Figure 5d features the microstructure of the Ni 19 V 11 Ta 70 alloy.The small light gray phase NiTa 2 and white block phase bcc-(Ta), along with the dark gray matrix phase Ni 6 Ta 7 were discovered.As presented in Figure 5e, a three-phase section of dark gray matrix phase Ni6Ta7 were discovered.As presented in Figure 5e, a three-phase section of Ni2Ta (light gray) + Ni6Ta7 (white) + Ni3Ta (black) was identified in the Ni64V5Ta31 alloy annealed at 1200 °C for 40 days, and its XRD result is shown in Figure 6c. Figure 5f is the microstructure of threephase region of fcc + Ni3Ta + L in the Ni61V30Ta9 alloy annealed at 1200 °C for 3 h.Based on the experimental data mentioned above, the isothermal section of the Ni-V-Ta system at 1200 °C was established, as shown in Figure 7. Nine three-phase sections, σ' + bcc-(V) + C14, σ' + dark gray matrix phase Ni6Ta7 were discovered.As presented in Figure 5e, a three-phase section of Ni2Ta (light gray) + Ni6Ta7 (white) + Ni3Ta (black) was identified in the Ni64V5Ta31 alloy annealed at 1200 °C for 40 days, and its XRD result is shown in Figure 6c. Figure 5f is the microstructure of threephase region of fcc + Ni3Ta + L in the Ni61V30Ta9 alloy annealed at 1200 °C for 3 h.Based on the experimental data mentioned above, the isothermal section of the Ni-V-Ta system at 1200 °C was established, as shown in Figure 7. Nine three-phase sections, σ' + bcc-(V) + C14, σ' + Based on the experimental data mentioned above, the isothermal section of the Ni-V-Ta system at 1200 • C was established, as shown in Figure 7 (2) due to the congruent transformation fcc ↔ Ni3V at 1045 °C , the Ni3V phase disappeared at 1200 °C ; (3) a small liquid region was detected; (4) the solubility of V in the phases Ni3Ta, Ni2Ta and Ni6Ta7 was measured to be around 17.2 at.%, 3.9 at.% and 33.8 at.%, respectively, which increases slightly as the temperature increases from 1000 to 1200 °C ; (5) the phases fcc, σ' and bcc-(V) dissolve about 6.2 at.%, 7.3 at.% and 10.1 at.% Ta, respectively.

Liquid Region
It is noticeable that a small liquid section was confirmed near the Ni-V side.As shown in Figure 2f, the bar-shaped white phase Ni3Ta and the black phase fcc uniformly distribute in the liquid matrix.According to the eutectic reaction, L ↔ fcc + σ' at 1202 °C in the Ni-V binary system, the eutectic microstructure is suggested to be consistent with the fcc and σ' phase.In order to identify the temperature of phase transition, we conducted DSC analysis.With the DSC result shown in Figure 8, we observed that the σ' phase transformed to the liquid phase when the temperature increased from 1100 to 1200 °C in the Ni59V24Ta17 alloy.The melting point of the σ' phase in the alloy was confirmed to be about 1137 °C .The corresponding results indicate that a third element addition of Ta in Ni-V alloys reduces the melting point of the alloys.

Liquid Region
It is noticeable that a small liquid section was confirmed near the Ni-V side.As shown in Figure 2f, the bar-shaped white phase Ni 3 Ta and the black phase fcc uniformly distribute in the liquid matrix.According to the eutectic reaction, L ↔ fcc + σ at 1202 • C in the Ni-V binary system, the eutectic microstructure is suggested to be consistent with the fcc and σ phase.In order to identify the temperature of phase transition, we conducted DSC analysis.With the DSC result shown in Figure 8, we observed that the σ phase transformed to the liquid phase when the temperature increased from 1100 to 1200

Conclusions
The results are shown as follows: (1) two isothermal sections of the Ni-V-Ta ternary system at 1000 °C and 1200 °C were experimentally established by the means of equilibrated alloy method; (2) the C15 phase dissolves less Ni, while the solubility of Ni in the C14 phase increases when the temperature raises from 1000 to 1200 °C ; (3) the C14 phase was stabilized by addition of Ni against low temperature confirmed by the presence of a single-phase region at 1000 °C ; (4) a small liquid phase was determined, while the Ni3V phase just disappeared at 1200 °C ; (5) the solubility of Ta in the phases fcc and σ' increases as the temperature increases from 1000 to 1200 °C , while the solubility of Ni in the bcc-(Ta) phase shows the opposite tend.This research is of great significance to support the thermodynamic assessment of the Ni-V-Ta system.

Figure 4 .
Figure 4. Experimentally determined isothermal section of the Ni-V-Ta system at 1000 °C .

Figure 4 .
Figure 4. Experimentally determined isothermal section of the Ni-V-Ta system at 1000 • C.

Ni 2
Ta (light gray) + Ni 6 Ta 7 (white) + Ni 3 Ta (black) was identified in the Ni 64 V 5 Ta 31 alloy annealed at 1200 • C for 40 days, and its XRD result is shown in Figure 6c. Figure 5f is the microstructure of three-phase region of fcc + Ni 3 Ta + L in the Ni 61 V 30 Ta 9 alloy annealed at 1200 • C for 3 h.Metals 2018, 8, x FOR PEER REVIEW 8 of 12

Figure 7 .
Figure 7. Experimentally determined isothermal section of the Ni-V-Ta system at 1200 °C .

Figure 7 .
Figure 7. Experimentally determined isothermal section of the Ni-V-Ta system at 1200 • C.

12 Figure 8 .
Figure 8. Heating curve of the Ni59V24Ta17 alloy.The microstructure of the Ni59V24Ta17 alloy annealed at (a) 1100 °C for 30 days; (b) 1200 °C for 3 h.

Table 2 .
Equilibrium compositions of the phase in the Ni-V-Ta ternary alloys annealed at 1000 • C for 55 days.

Table 3 .
Equilibrium compositions of the phase in the Ni-V-Ta ternary alloys annealed at 1200 • C for 40 days except the starred ones.
* The alloys indicated with (*) were annealed at 1200 • C for 3 h.