Phase Equilibria in the Ni-V-Ta Ternary System

Abstract

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.

1. 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 Ni₃V (D0₂₂) phase is a critical phase in the dual two-phase intermetallic alloys composed of Ni₃Al and Ni₃V, 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 Ni₈X (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 stable solid phases in the three binary systems. Data from [16,24,30].

System	Phase	Pearson Symbol	Prototye	Space Group	Strukturbericht	References
Ni-Ta	fcc	cF4	Cu	Fm-3m	A1	[16]
	Ni8Ta	tI36	Ni8Nb	I4/mmm		[16]
	Ni3Ta	tI8	TiAl3	I4/mmm	D022	[16]
		mP16	NbPt3	P21/m		[16]
		oP8	Cu3Ti	Pmmn	D0a	[16]
	Ni2Ta	tI6	MoSi2	I4/mmm	C11b	[16]
	Ni6Ta7	hR39	Fe7W6	R-3mh	D85	[16]
	NiTa2	tI12	Al2Cu	I4/mcm	C16	[16]
	bcc-(Ta)	cI2	W	Im-3m	A2	[16]
Ni-V	fcc	cF4	Cu	Fm-3m	A1	[24]
	Ni8V	tI18	Ni8Nb	I4/mmm		[24]
	Ni2V	oI6	MoPt2	Immm		[24]
	Ni3V	tI8	Al3Ti	I4/mmm	D022	[24]
	σ′	tP30	σ	P42/mnm		[24]
	NiV3	cP8	Cr3Si	Pm3n		[24]
	bcc-(V)	cI2	W	Im-3m	A2	[24]
V-Ta	bcc-(V, Ta)	cI2	W	Im-3m	A2	[30]
	C14	hP12	MgZn2	P63/mmc	C14	[30]
	C15	cF4	MgCu2	Fd-3m	C15	[30]

.

The Ni-Ta system was systematically investigated by several researchers [11,12,13,14,15,16,17,18,19,20,21,22,23]. Solid solution face-centered cubic (fcc), body-centered cubic (bcc-(Ta)), and five intermetallic compounds (Ni₈Ta, 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.

2. 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.

3. Results and Discussion

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

Nominal Alloys (at. %)	Phase Equilibrium	Composition (at. %)
	Phase 1/Phase 2/Phase 3	Phase 1		Phase 2		Phase 3
		V	Ta	V	Ta	V	Ta
Ni24V67Ta9	bcc-(V)/σ′/C14	90	3.1	70.6	5.2	47.1	27.1
Ni13V65Ta22	bcc-(V)/C14	90.5	5.3	49.8	30.1
Ni41V49Ta10	σ′/C14	54.2	4.4	31.8	21.8
Ni9V41Ta50	bcc-(Ta)/C14	37	58.5	47.3	36.2
Ni61V30Ta9	fcc/Ni3Ta/σ′	36.4	3.2	13.2	15	51.6	2.7
Ni59V24Ta17	σ′/C14/Ni3Ta	53	4.7	31	22.1	6.8	19.8
Ni48V18Ta34	Ni6Ta7/C14/Ni3Ta	17.9	47	23.5	29	2.5	24.1
Ni77V14Ta9	fcc/Ni3V/Ni3Ta	13.1	4.6	17	6.7	12	12.4
Ni19V11Ta70	bcc-(Ta)/Ni6Ta7/NiTa2	9.6	88.3	14.5	51.5	2.2	65.9
Ni17V61Ta22	bcc-(V)/C14	90.3	4.8	46.5	29.5
Ni52V45Ta3	fcc/σ′	37.9	2.8	51.8	2.7
Ni26V34Ta40	Ni6Ta7/C14	26.1	45	37.3	37.9
Ni68V15Ta17	Ni3Ta/σ′	8.1	18.6	52.5	3.8
Ni64V5Ta31	Ni2Ta/Ni3Ta/Ni6Ta7	3.4	30.6	1.1	25.2	15.8	47.5
Ni82V7Ta11	Ni8Ta/Ni3Ta	8.9	7.1	6.5	17
Ni18V23Ta59	Ni6Ta7/bcc-(Ta)	23.1	49	22	74.9
Ni8V28Ta64	Ni6Ta7/bcc-(Ta)	30	46.9	26.6	70
Ni5V49Ta46	C15/bcc-(Ta)	55.5	35.3	41.1	56.3
Ni6V71Ta23	C15/bcc-(V)	58.4	30.5	90.7	7.5
Ni35V50Ta15	σ′/C14	62.5	5.9	37.5	25
Ni71V19Ta10	fcc/Ni3Ta	33.7	2.5	17.1	11
Ni79V10Ta11	fcc/Ni3Ta	12.2	5.3	10.2	14.3
Ni50V6Ta44	Ni2Ta/Ni6Ta7	2.1	34	6.7	48.8
Ni39V4Ta57	NiTa2/Ni6Ta7	0.6	64.1	6.5	52.9
Ni9V2Ta89	NiTa2/bcc-(Ta)	1	69.1	3.1	94.5
Ni7V13Ta80	Ni6Ta7/bcc-(Ta)	18	50.2	15.1	81.9
Ni59V17Ta24	Ni3Ta/C14	3.5	22.6	25.2	26.4
Ni37V35Ta28	C14	35	28
Ni34V33Ta33	C14	33	33
Ni44V26Ta30	C14	26	30
Ni4V68Ta28	C15/bcc-(V)	61.2	31.5	90.1	8.5
Ni5V54Ta41	C15/bcc-(Ta)	56.5	37	39.8	48.9
Ni13V46Ta41	bcc-(Ta)/C14	39.1	57.8	51	35
Ni14V36Ta50	Ni6Ta7/C14/bcc-(Ta)	31.6	45.2	44.1	36.4	28.7	66.4
Ni18V55Ta27	C14/bcc-(V)	50.2	31	90.2	5.4
Ni81V1Ta18	Ni8Ta/Ni3Ta	1.5	11.5	0.7	22.5
Ni12V62Ta26	C14/bcc-(V)	52	31.3	91.3	4.8
Ni33V25Ta42	Ni6Ta7/C14	21.4	45.9	29.8	36.1

.

Microstructure and XRD results of the typical alloys after annealed at 1000 °C for 55 days are shown in Figure 2 and Figure 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) + Ni₃Ta (gray) + C14 (light gray) in the Ni₅₉V₂₄Ta₁₇ alloy was established. Figure 2b shows the microstructure of the Ni₆₁V₃₀Ta₉ alloy, in which the black phase σ′, the white phase Ni₃Ta and the gray matrix fcc were observed. As displayed in Figure 2c, three-phase region of NiTa₂ + Ni₆Ta₇ + bcc-(Ta) in the Ni₁₉V₁₁Ta₇₀ alloy was identified. The light gray phase NiTa₂ precipitated in the matrix Ni₆Ta₇, 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 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 two-phase 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 23.3–52 at. % 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.

3.2. 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. Equilibrium compositions of the phase in the Ni-V-Ta ternary alloys annealed at 1200 °C for 40 days except the starred ones.

Alloys (at. %)	Phase Equilibrium	Composition (at. %)
	Phase 1/Phase 2/Phase 3	Phase 1		Phase 2		Phase 3
		V	Ta	V	Ta	V	Ta
Ni24V67Ta9	bcc-(V)/σ′/C14	85.1	4.1	66.2	6.5	43.6	24.6
Ni13V65Ta22	bcc-(V)/C14	88.8	7.5	49.5	29
Ni41V49Ta10 *	σ′/L/C14	55.6	5.5	42.2	11.1	34.5	22.2
Ni9V41Ta50	bcc-(Ta)/C14	33.2	66.4	45.6	37.3
Ni61V30Ta9 *	fcc/Ni3Ta/L	34.2	4	9.1	27.5	37.3	11.8
Ni75V18Ta7	fcc/Ni3Ta	20.9	6.5	16.8	9.1
Ni59V24Ta17 *	Ni3Ta/L/C14	6.4	20.2	37.5	12.2	26.3	23.7
Ni48V18Ta34	Ni3Ta/Ni6Ta7/C14	2.3	23.1	18.6	46	21	28.6
Ni77V14Ta9	fcc/Ni3Ta	15.1	6.2	10.8	14.1
Ni19V11Ta70	bcc-(Ta)/Ni6Ta7/NiTa2	9.3	89.1	12.5	52.7	0.9	66.1
Ni17V61Ta22	bcc-(V)/C14	88.3	6.6	48.3	28.1
Ni52V45Ta3 *	fcc/σ′/L	41.6	2	53.8	3	45.9	5
Ni26V34Ta40	Ni6Ta7/C14	28.3	44.9	38	36.8
Ni68V15Ta17 *	Ni3Ta/L	7.9	18.7	37.3	12
Ni64V5Ta31	Ni2Ta/Ni3Ta/Ni6Ta7	3.8	29.9	1.7	24.5	16	46.5
Ni82V7Ta11	Ni8Ta/Ni3Ta	8	7.9	5.5	18.2
Ni18V23Ta59	Ni6Ta7/bcc-(Ta)	23.8	50	20.9	78.9
Ni8V28Ta64	Ni6Ta7/bcc-(Ta)	31.5	47.5	24.8	75
Ni5V49Ta46	C14/bcc-(Ta)	55	36.2	44.3	55.6
Ni6V71Ta23	C14/bcc-(V)	56.5	30.6	89.8	9.5
Ni35V50Ta15	σ′/C14	60.5	7.2	39.4	23.1
Ni71V19Ta10	fcc/Ni3Ta	26.8	5.1	14.7	12.1
Ni79V10Ta11	fcc/Ni3Ta	10.8	7.5	6.6	17.2
Ni50V6Ta44	Ni2Ta/Ni6Ta7	2.1	32.5	8.1	48.5
Ni39V4Ta57	NiTa2/Ni6Ta7	0.6	64.8	5.7	53.5
Ni9V2Ta89	NiTa2/bcc-(Ta)	0.6	66.9	2.6	97.1
Ni7V13Ta80	Ni6Ta7/bcc-(Ta)	18	51.5	12.4	87.5
Ni59V17Ta24	Ni3Ta/C14	4.7	21.5	24.4	25.1
Ni37V35Ta28	C14	35	28
Ni34V33Ta33	C14	33	33
Ni44V26Ta30	C14	26	30
Ni3V57Ta40	C14/C15	47	35.9	46.8	42.1
Ni13V46Ta41	C14/bcc-(Ta)	48.8	36.8	41.5	58
Ni14V36Ta50	Ni6Ta7/C14/bcc-(Ta)	34	44.5	40.9	37	30.1	68.8
Ni18V55Ta27	C14/bcc-(V)	49.6	30.5	89.1	8.2
Ni81V1Ta18	Ni8Ta/Ni3Ta	1.5	12.1	0.9	22.1
Ni12V62Ta26	C14/bcc-(V)	54.1	30.4	89.3	9.1
Ni33V25Ta42	Ni6Ta7/C14	22.5	45	30.2	34.7

* The alloys indicated with (*) were annealed at 1200 °C for 3 h.

.

The microstructure and the corresponding XRD analysis are presented in the Figure 5a–f and Figure 6a–c, respectively. Figure 5a shows the three-phase equilibrium of C14 + σ′ + bcc-(V) in the Ni₂₄V₆₇Ta₉ alloy annealed at 1200 °C for 40 days. Figure 5b features the microstructure of the Ni₄₈V₁₈Ta₃₄ alloy. The small black phase Ni₃Ta and the white phase Ni₆Ta₇ 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, Ni₃Ta and Ni₆Ta₇ phases were labelled by different symbols, as presented in Figure 6a. In the Ni₁₄V₃₆Ta₅₀ alloy quenched from 1200 °C, the three-phase region of C14 (dark gray) + Ni₆Ta₇ (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 Ni₁₉V₁₁Ta₇₀ alloy. The small light gray phase NiTa₂ and white block phase bcc-(Ta), along with the 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 three-phase 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′ + C14 + L, σ′ + fcc + L, fcc + L + Ni₃Ta, C14 + L + Ni₃Ta, C14 + Ni₆Ta₇ + Ni₃Ta, Ni₂Ta + Ni₆Ta₇ + Ni₃Ta, NiTa₂ + Ni₆Ta₇ + bcc-(Ta) and C14 + Ni₆Ta₇ + bcc-(Ta), were experimentally determined and presented as a triangle with solid lines. The other two three-phase equilibriums C15 + C14 + bcc-(Ta) and fcc + Ni₃Ta + Ni₈Ta were not confirmed and were displayed as a triangle with dashed lines. The corresponding results indicate that: (1) the C14 phase extends from the V-Ta side to the center and dissolves more Ni at 1200 °C, while the solubility of Ni in the C15 phase decreases than that at 1000 °C; (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.

3.3. 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.

4. 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.