Experimental Investigation of Phase Equilibria in the Co-Re-Ta Ternary System

Abstract

In this study, the isothermal sections of the Co-Re-Ta ternary system at 1100, 1200, and 1300 °C have been experimentally investigated by means of electron probe microanalysis and X-ray diffraction. The results indicated the following: (1) The solid solubilities of the λ₃, (εCo, Re), χ-Re₇Ta₃, and bcc-(Ta) phases were large and changed very little from 1100 to 1300 °C; (2) more interestingly, the λ₂ phase, with a very limited solubility of Re, was surrounded by the λ₃ phase; (3) the solubility of Re for the μ-Co₆Ta₇ phase increased slowly from 1100 to 1300 °C. These experimental results will be useful for Co-based high-temperature alloys, especially as a supplement for thermodynamic databases.

1. Introduction

It was first found by Sato et al. [1] that Co-based superalloys were strengthened by a stable ternary compound, γ’ Co₃(Al, W), with the L1₂ structure. Subsequently, the Co-based superalloys have been regarded as one of the promising high-temperature materials that exhibit better high-temperature strength than the conventional Ni-based superalloys [1]. To further improve high-temperature properties of Co-based superalloys, some refractory alloying elements, such as Re, Ta, W, and Mo, have been added to materials, which can improve high-temperature mechanical properties, creep properties, corrosion, and oxidation resistance [2,3,4,5,6,7,8,9,10,11]. Re does not randomly distribute in the alloy; it hinders dislocation movement by forming tiny clusters which act as obstacles during creep tests [2,3,4,5]. Thus, the addition of Re can effectively enhance the creep properties of superalloys. Meanwhile, as Re content increases, it refines the morphology and enhances the content of the alloy compound for Co-based superalloys [6]. Doping of Ta can maintain good microstructural stability and improve oxidation resistance [7,8]. The amount of Re and Ta additions is strictly restricted because excessive additions will cause the brittle and detrimental TCP (topologically close packed) phases to form at high stresses and temperatures [9,10,11]. Therefore, the knowledge of phase equilibria in the Co-Re-Ta ternary system is essential, which will provide significant basic data for the design of Co-based superalloys. However, there is no information on the experimental investigation of and thermodynamic data for the Co-Re-Ta ternary system. It is necessary to investigate the phase equilibria of the Co-Re-Ta ternary system.

The three binary systems, Co-Re, Re-Ta, and Co-Ta, that constitute the Co-Re-Ta ternary system are shown in Figure 1.

Elliott [12] has published the results for the Co-Re binary system. Later, Predel [13] reported the results for the Co-Re binary system based on experimental data. Recently, Liu et al. [14] and Guo et al. [15] estimated the Co-Re system and their findings were consistent with the experimental data. The newly assessed Co-Re phase diagram by Guo et al. [15] was applied in this work. The Co-Re system [15] is simple because there are two solid phases of (αCo) and (εCo, Re) and no intermediate phases. The (εCo, Re) phase has a wide homogeneity range.

The Re-Ta binary system was first studied by Greenfield and Beck [16]. They investigated alloys with Ta contents between 25 and 52 at. % and reported the composition range of the σ and χ phases. Cui and Jin [17] treated the σ phase as a stoichiometric phase and thermodynamically assessed the Re-Ta system. Afterwards, Liu and Chang [18] also evaluated the thermodynamic description of the Re-Ta system. Recently, Guo et al. [15] estimated the Re-Ta phase diagram with the latest thermodynamic description for pure Re, and this Re-Ta binary system was used in the paper. The two intermediate phases of χ-Re₇Ta₃ and σ-Re₃Ta₂ exist in the Re-Ta binary system [15]. The σ-Re₃Ta₂ phase forms from the peritectic reaction of liquid + χ-Re₇Ta₃ $\leftrightarrow$ σ-Re₃Ta₂. The melting point of the χ-Re₇Ta₃ phase is about 2832 °C.

The Co-Ta binary system has been investigated by many researchers [19,20,21,22,23,24]. Itoh et al. [19] investigated the homogeneity ranges, crystal structures, and magnetic properties of the three Laves phases in the Co-Ta system. Okamoto [20] assessed the Co-Ta phase diagram and treated C14 as a line compound. Liu and Chang [21] thermodynamically assessed the Co-Ta binary system. Shinagawa et al. [24] studied the Co-Ta binary system and revised the λ₁ phase as an intermetallic compound with a narrow composition range, and the phase diagram [24] was adopted in this work. There are three Laves phases of λ₁ (C14), λ₂ (C15), and λ₃ (C36), each with different polytypes. The other intermediate phases of CoTa₂, μ-Co₆Ta₇, and Co₇Ta₂, and solution phases of bcc-(Ta), (εCo), and (αCo) also exist in the Co-Ta binary system. The λ₁ and λ₃ phases are obtained from peritectic reactions of liquid + λ₂ $\leftrightarrow$ λ₁ and liquid + λ₂ $\leftrightarrow$ λ₃, respectively. The λ₁ phase exists at ~1293–1587 °C and the λ₃ phase exists at ~947–1456 °C. The Co₇Ta₂ phase with the BaPb₃ [23] crystal structure is obtained from a peritectoid reaction: (αCo) + λ₃ $\leftrightarrow$ Co₇Ta₂. The information for the stable solid phases and their crystal structures in the three binary systems is listed in

Table 1. Crystal structures of each phase in the Co-Re-Ta ternary system [15,23,24].

System	Phase	Pearson Symbol	Space Group	Prototype	Structure Type	References
Re-Ta	(Re)	hP2	P63/mmc	Mg	A3	[15]
	χ-Re7Ta3	cI58	I-43m	αMn	A12	[15]
	σ-Re3Ta2	tP30	P42/mnm	σCrFe	D8b	[15]
	bcc-(Ta)	cI2	Im-3m	W	A2	[15]
Co-Re	(αCo)	cF4	Fm-3m	Cu	A1	[15]
	(εCo, Re)	hP2	P63/mmc	Mg	A3	[15]
Co-Ta	(αCo)	cF4	Fm-3m	Cu	A1	[24]
	(εCo)	hP2	P63/mmc	Mg	A3	[24]
	λ3-Co2Ta	hP24	P63/mmc	Ni2Mg	C36	[24]
	λ2-Co2Ta	cF24	Fd-3m	Cu2Mg	C15	[24]
	λ1-Co2Ta	hP12	P63/mmc	Zn2Mg	C14	[24]
	μ-Co6Ta7	hR13	R-3m	Fe7W6	D8b	[24]
	CoTa2	tI12	I4/mcm	Al2Cu	C16	[24]
	bcc-(Ta)	cI2	Im-3m	W	A2	[24]
	Co7Ta2	hR36	R-3m	BaPb3 [23]	-	[23,24]

.

The temperatures of 1100, 1200, and 1300 °C were selected because the Co-based superalloys are widely used in high-temperature areas such as aircraft engines and turbine blades. Thus, it is more meaningful to investigate the phase equilibria at high temperatures. The present work aimed to experimentally investigate the phase equilibria of the Co-Re-Ta ternary system at 1100, 1200, and 1300 °C using electron probe microanalysis and X-ray diffraction techniques in order to understand the microstructures of the Co-Re-Ta ternary system and provide useful information for the development of Co-based high-temperature alloys.

2. Experimental Procedure

High-purity rhenium (99.9 wt %), tantalum (99.9 wt %), and cobalt (99.9 wt %) were used as raw materials. The required weights of the elements (with a total weight of about 20 g) were measured with a semi-micro analytical balance with an accuracy of at least 0.5 mg. During the whole sample preparation procedure, the mass loss was usually less than 1%. Therefore, the mass loss was assumed to make no significant effect on the sample composition.

All the bulk alloys were prepared in the form of atomic ratios (at. %). The bulk alloys with nominal compositions were melted by arc-melting in an argon atmosphere using a non-consumable tungsten electrode on a water-cooled plate. Titanium was used as a getter material. The buttons were re-melted at least five times to ensure that the ingots were homogeneous.

Afterwards, the specimens were cut into small pieces by a wire-cutting machine for heat treatment. The samples were cleaned by an ultrasonic cleaner and then encapsulated in quartz ampoules which were evacuated and flushed several times with purified argon. Heat treatments were performed at 1100, 1200, and 1300 °C, respectively. The time of the heat treatments ranged from 15 days to 65 days to reach phase equilibria based on the different temperatures and compositions of the samples. The samples containing over 20 at. % Re were heat-treated for a relatively long time. Subsequently, the specimens were quenched, mounted, grinded, and polished.

The microstructural observation and equilibrium composition analysis of specimens was characterized by electron probe microanalysis (EPMA) (JXA-8100R, JEOL, Tokyo, Japan). Pure elements were used as standards and the measurements were carried out at a voltage of 20 kV and a current of 1.0 × 10⁻⁸ A. To identify the crystal structures, powder X-ray diffraction (XRD) measurements were performed on a Philips Panalytical X-pert diffractometer (Bruker Daltonic Inc., Billerica, MA, USA) with Cu Kα radiation at 40 kV and 40 mA. The scanning range of 2θ was from 20° to 90° at a step size of 0.0167°.

3. Results and Discussion

3.1. Microstructure

The typical back-scattered electron (BSE) images of ternary Co-Re-Ta alloys annealed at 1100, 1200, or 1300 °C for different times are shown in Figure 2; there are three-phase equilibrium microstructures shown in Figure 2a–d and two-phase equilibrium microstructures shown in Figure 2e–j. The corresponding results of the XRD are presented in Figure 3.

For the Co₇₄Re₁₆Ta₁₀ alloy, the light grey phase ((εCo, Re)), dark grey phase (λ₃), and black phase ((αCo)) were observed after annealing at 1100 °C for 50 days, as shown in Figure 2a. Figure 2b shows the three-phase equilibrium of the μ-Co₆Ta₇ phase, λ₃ phase, and bcc-(Ta) phase in the 1200 °C/35 days-annealed Co₄₄Re₁₄Ta₄₂ alloy. The white phase was bcc-(Ta), the light grey phase was μ-Co₆Ta₇, and the dark grey phase was λ₃. Figure 3a shows that the corresponding XRD pattern, and the μ-Co₆Ta₇ phase, λ₃ phase, and bcc-(Ta) phase were clearly distinguished by the different symbols. The Co₃₁Re₃₅Ta₃₄ alloy annealed at 1300 °C for 25 days contained the three phases of χ-Re₇Ta₃ (light grey), bcc-(Ta) (dark grey), and λ₃ (black), which are shown in Figure 2c. The λ₃ phase was the matrix while the bcc-(Ta) phase was on the edge of the χ-Re₇Ta₃ phase. XRD identification, as shown in Figure 3b, confirmed the existence of the three phases of χ-Re₇Ta₃, bcc-(Ta), and λ₃. Figure 2d shows the BSE image of the Co₄₅Re₃₀Ta₂₅ alloy annealed at 1300 °C for 25 days. There were three phases of λ₃, χ-Re₇Ta₃, and (εCo, Re) existing in an equilibrium. Figure 2e shows a two-phase microstructure constituted by a white (εCo, Re) phase and a black λ₃ phase in the Co₅₄Re₂₅Ta₂₁ alloy quenched from 1100 °C. The two-phase equilibrium of bcc-(Ta) and CoTa₂ was identified in the annealed Co₂₂Re₅Ta₇₃ alloy (1100 °C/50 days), as shown in Figure 2f. In the Co₅₈Re₃Ta₃₉ alloy annealed at 1200 °C for 35 days, the white μ-Co₆Ta₇ phase and the dark grey λ₃ phase were observed in Figure 2g while their crystal structure was confirmed by the XRD pattern in Figure 3c. Figure 2h shows the BSE image of the bcc-(Ta) phase and the μ-Co₆Ta₇ phase in the Co₂₅Re₂₀Ta₅₅ alloy annealed at 1200 °C for 50 days. The white bcc-(Ta) phase was homogeneously distributed in the grey μ-Co₆Ta₇ phase. Figure 2i shows that the two phases of λ₃ and (αCo) were found in the Co₈₀Re₁₁Ta₉ alloy annealed at 1300 °C for 15 days. The light grey phase was λ₃ and the dark grey phase was (αCo). The two-phase microstructure of the λ₂ phase (dark grey) and μ-Co₆Ta₇ phase (light grey) was identified in the Co₆₀Re₁Ta₃₉ alloy annealed at 1300 °C for 15 days, as shown in Figure 2j. The corresponding XRD pattern is displayed in Figure 3d.

In order to figure out the phase boundary between the λ₃ and λ₂ phases, several alloys were prepared. Unfortunately, all the compositions were located at single field region, which meant that the λ₂/λ₃ two-phase field was extremely narrow. This was consistent with that of the Co-Ta binary. The phase boundaries were then plotted with approximations based on the microstructure observation results of these single-phased compositions. Figure 4a,b shows the typical XRD patterns of the Co₆₆Re₅Ta₂₉ and Co₆₈Re₃Ta₂₉ alloys annealed at 1300 °C for 15 days, which were confirmed to be λ₃ and λ₂ single phases, respectively. The corresponding microstructure was consistent with the results.

3.2. Isothermal Sections

All the equilibrium compositions of the Co-Re-Ta ternary system at 1100, 1200, and 1300 °C are listed in

Table 2. Equilibrium compositions of the Co-Re-Ta ternary system at 1100 °С as determined in the present work.

Nominal Alloys (at. %)	Annealed Time (days)	Phase Equilibrium	Composition (at. %)
		Phase 1/Phase 2/Phase 3	Phase 1		Phase 2		Phase 3
			Re	Ta	Re	Ta	Re	Ta
Co31Re35Ta34	65	λ3/bcc-(Ta)/χ-Re7Ta3	19.7	26.5	46.5	45.5	52.7	34.8
Co45Re30Ta25	65	λ3/(εCo, Re)/χ-Re7Ta3	20.1	25.5	30.4	22.0	60.1	26.0
Co60Re20Ta20	65	(εCo, Re)/λ3	27.3	16.6	16.5	20.5
Co45Re20Ta35	65	bcc-(Ta)/λ3	41.4	48.4	13.1	30.9
Co25Re20Ta55	65	μ-Co6Ta7/bcc-(Ta)	2.3	50.6	34.1	58.3
Co54Re25Ta21	65	(εCo, Re)/λ3	28.7	19.1	18.4	22.9
Co78Re5Ta17	50	(αCo)/λ3	2.7	3.3	4.7	21.8
Co44Re14Ta42	50	λ3/μ-Co6Ta7/bcc-(Ta)	2.7	33.8	3.8	43.7	39.2	51.2
Co22Re5Ta73	50	CoTa2/bcc-(Ta)	0.7	65.8	8.9	87.5
Co58Re3Ta39	50	λ3/μ-Co6Ta7	1.9	33.6	3.2	43.4
Co80Re2Ta18	50	(αCo)/λ3	0.9	3.5	1.8	22.7
Co80Re5Ta15	50	(αCo)/λ3	3.5	3.6	5.7	20.8
Co80Re11Ta9	50	(αCo)/λ3	9.2	2.7	13.8	17.2
Co74Re16Ta10	50	(αCo)/λ3/(εCo, Re)	10.1	2.4	15.2	17.3	25.0	14.2
Co54Re18Ta28	50	λ3	18.2	28.3
Co89Re5Ta6	50	(αCo)/λ3	4.6	3.7	7.5	19.6
Co33Re5Ta62	50	CoTa2/bcc-(Ta)	1.7	59.2	18.0	77.3
Co30Re10Ta60	50	μ-Co6Ta7/bcc-(Ta)	1.7	54.2	24.0	71.3
Co69Re9Ta22	50	λ3	9.3	22.3
Co70Re5Ta25	50	λ3	4.5	24.9
Co71Re1Ta28	50	λ2	0.8	28.2
Co68Re3Ta29	50	λ2	2.7	28.7
Co66Re5Ta29	50	λ3	4.7	28.5
Co64Re7Ta29	50	λ3	7.3	28.9
Co60Re1Ta39	50	λ2/μ-Co6Ta7	0.7	32.3	1.2	43.7
Co28Re6Ta66	50	CoTa2/bcc-(Ta)	1.4	60.2	14.2	80.7
Co43Re1Ta56	50	CoTa2/μ-Co6Ta7	0.9	57.8	1.1	54.8

,

Table 3. Equilibrium compositions of the Co-Re-Ta ternary system at 1200 °С as determined in the present work.

Nominal Alloys (at. %)	Annealed Time (days)	Phase Equilibrium	Composition (at. %)
		Phase 1/Phase 2/Phase 3	Phase 1		Phase 2		Phase 3
			Re	Ta	Re	Ta	Re	Ta
Co31Re35Ta34	50	λ3/bcc-(Ta)/χ-Re7Ta3	21.7	26.1	50.0	47.6	56.6	31.4
Co45Re30Ta25	50	λ3/(εCo, Re)/χ-Re7Ta3	21.7	25.4	31.3	24.3	59.9	26.8
Co60Re20Ta20	50	(εCo, Re)/λ3	27.2	18.7	17.2	20.9
Co45Re20Ta35	50	bcc-(Ta)/λ3	41.1	48.6	14.0	31.9
Co25Re20Ta55	50	μ-Co6Ta7/bcc-(Ta)	2.8	51.1	35.3	58.7
Co54Re25Ta21	50	(εCo, Re)/λ3	28.0	20.9	20.1	22.9
Co78Re5Ta17	35	(αCo)/λ3	2.7	4.4	4.3	21.9
Co44Re14Ta42	35	λ3/μ-Co6Ta7/bcc-(Ta)	3.5	35.3	4.0	44.2	39.4	50.4
Co22Re5Ta73	35	CoTa2/bcc-(Ta)	0.8	65.2	8.9	86.5
Co58Re3Ta39	35	λ3/μ-Co6Ta7	2.4	34.4	3.3	43.9
Co80Re2Ta18	35	(αCo)/λ3	1.4	4.9	1.8	22.5
Co80Re5Ta15	35	(αCo)/λ3	3.2	4.2	5.5	21.1
Co80Re11Ta9	35	(αCo)/λ3	9.7	3.0	13.6	18.4
Co74Re16Ta10	35	(αCo)/λ3/(εCo, Re)	11.2	2.8	15.5	17.6	24.9	15.3
Co54Re18Ta28	35	λ3	18.3	28.2
Co89Re5Ta6	35	(αCo)/λ3	4.3	4.5	7.1	20.6
Co33Re5Ta62	35	CoTa2/bcc-(Ta)	1.5	59.6	19.5	76.8
Co30Re10Ta60	35	μ-Co6Ta7/bcc-(Ta)	2.0	55.1	23.5	72.2
Co69Re9Ta22	35	λ3	8.4	22.0
Co70Re5Ta25	35	λ3	4.5	24.8
Co71Re1Ta28	35	λ2	0.8	28.0
Co68Re3Ta29	35	λ2	2.6	28.8
Co66Re5Ta29	35	λ3	4.8	29.0
Co64Re7Ta29	35	λ3	7.0	28.8
Co60Re1Ta39	35	λ2/μ-Co6Ta7	0.5	32.4	1.0	44.1
Co28Re6Ta66	35	CoTa2/bcc-(Ta)	1.3	60.2	15.9	79.6
Co43Re1Ta56	35	CoTa2/μ-Co6Ta7	0.6	60.5	1.3	54.8

and

Table 4. Equilibrium compositions of the Co-Re-Ta ternary system at 1300 °С as determined in the present work.

Nominal Alloys (at. %)	Annealed Time (days)	Phase Equilibrium	Composition (at. %)
		Phase 1/Phase 2/Phase 3	Phase 1		Phase 2		Phase 3
			Re	Ta	Re	Ta	Re	Ta
Co31Re35Ta34	25	λ3/bcc-(Ta)/χ-Re7Ta3	22.2	27.1	49.0	48.5	56.6	32.6
Co45Re30Ta25	25	λ3/(εCo, Re)/χ-Re7Ta3	22.6	26.1	32.0	23.2	58.7	25.8
Co60Re20Ta20	25	(εCo, Re)/λ3	27.5	17.0	17.3	20.6
Co45Re20Ta35	25	bcc-(Ta)/λ3	42.8	48.9	14.2	32.8
Co25Re20Ta55	25	μ-Co6Ta7/bcc-(Ta)	3.9	51.2	35.5	58.9
Co54Re25Ta21	25	(εCo, Re)/λ3	28.8	18.9	19.7	22.1
Co78Re5Ta17	15	(αCo)/λ3	2.8	5.5	5.0	21.3
Co44Re14Ta42	15	λ3/μ-Co6Ta7/bcc-(Ta)	5.5	36.4	6.1	43.2	41.8	50.4
Co22Re5Ta73	15	CoTa2/bcc-(Ta)	0.8	65.2	8.7	87.6
Co58Re3Ta39	15	λ3/μ-Co6Ta7	2.7	35.4	3.6	44.0
Co80Re2Ta18	15	(αCo)/λ3	1.3	5.6	1.9	22.3
Co80Re5Ta15	15	(αCo)/λ3	3.5	5.0	5.9	20.8
Co80Re11Ta9	15	(αCo)/λ3	10.0	3.8	13.8	17.8
Co74Re16Ta10	15	(αCo)/λ3/(εCo, Re)	13.0	2.6	16.2	17.8	26.0	14.2
Co54Re18Ta28	15	λ3	18.2	28.5
Co89Re5Ta6	15	(αCo)/λ3	4.8	5.0	7.2	22.4
Co33Re5Ta62	15	CoTa2/bcc-(Ta)	2.1	59.2	19.2	76.2
Co30Re10Ta60	15	μ-Co6Ta7/bcc-(Ta)	2.4	54.1	24.2	71.4
Co69Re9Ta22	15	λ3	8.9	22.0
Co70Re5Ta25	15	λ3	4.5	24.5
Co71Re1Ta28	15	λ2	0.7	27.8
Co68Re3Ta29	15	λ2	2.7	28.6
Co66Re5Ta29	15	λ3	4.6	28.4
Co64Re7Ta29	15	λ3	7.0	28.6
Co60Re1Ta39	15	λ2/μ-Co6Ta7	0.8	32.3	1.3	44.2
Co28Re6Ta66	15	CoTa2/bcc-(Ta)	1.8	58.3	14.9	81.1
Co43Re1Ta56	15	CoTa2/μ-Co6Ta7	0.6	59.7	1.3	54.9

, respectively. Figure 5a–c shows the isothermal sections at 1100, 1200, and 1300 °C based on the experimental data, respectively. The λ₂ single phase, λ₃ single phase, two-phase equilibrium, and three-phase equilibrium are characterized by different symbols. The solid triangle represents the determined three-phase equilibrium while the dashed triangle represents the undetermined three-phase equilibrium.

Figure 5a shows the 1100 °C isothermal section of the Co-Re-Ta ternary system. There were three solid solution phases of (αCo), (εCo, Re), and bcc-(Ta), two Laves phases of λ₂ and λ₃, and the intermetallic compounds of the μ-Co₆Ta₇, CoTa₂, and χ-Re₇Ta₃ phases. Investigations of the Co₇₄Re₁₆Ta₁₀, Co₄₄Re₁₄Ta₄₂, Co₃₁Re₃₅Ta₃₄, and Co₄₅Re₃₀Ta₂₅ alloys were used to determine the three-phase equilibria of (αCo) + (εCo, Re) + λ₃, λ₃ + bcc-(Ta) + μ-Co₆Ta₇, λ₃ + χ-Re₇Ta₃ + bcc-(Ta), and (εCo, Re) + λ₃ + χ-Re₇Ta₃, respectively. Seven alloys were confirmed to be single phases, two alloys (Co₇₁Re₁Ta₂₈ and Co₆₈Re₃Ta₂₉) were located in the λ₂ single-phase region, and five alloys (Co₅₄Re₁₈Ta₂₈, Co₆₉Re₉Ta₂₂, Co₇₀Re₅Ta₂₅, Co₆₆Re₅Ta₂₉ and Co₆₄Re₇Ta₂₉) were located in the λ₃ single-phase region. The solubility of Re in the λ₃ phase was measured to be about 20.1 at. % while the solubility of Re in the λ₂ phase was measured to be about 3.8 at. %. The λ₃ phase extended from the left side to the right side of the λ₂ phase and was wrapped around the λ₂ phase. The solubility of Re in the μ-Co₆Ta₇ and CoTa₂ phases was quite small at roughly 3.3 at. % and 1.5 at. %, respectively. The solubility of Co in the χ-Re₇Ta₃ phase was about 16.8 at. %. The (εCo, Re) phase extended from the Re-rich side to the Co-rich side, and the solubility of Ta in (εCo, Re) phase was about 21.7 at. %.

Figure 5b shows the isothermal section at 1200 °C. Four three-phase regions of the (αCo) + (εCo, Re) + λ₃, λ₃ + bcc-(Ta) + μ-Co₆Ta₇, λ₃ + χ-Re₇Ta₃ + bcc-(Ta), and (εCo, Re) + λ₃ + χ-Re₇Ta₃ were experimentally determined. The 1200 °C isothermal section was similar to the 1100 °C isothermal section. The solubility of Re in the λ₃ phase increased a little from 20.1 at. % at 1100 °C to 21.5 at. % at 1200 °C. The solubility of Re in the λ₂ phase was about 3.5 at. %, almost the same as at 1100 °C. The μ-Co₆Ta₇ and CoTa₂ phases dissolved about 3.8 at. % and 1.4 at. % Re, respectively. The solubility of Ta in the (εCo, Re) phase was about 24.1 at. %. The solubility of Co in the χ-Re₇Ta₃ phase was measured to decrease from 16.8 at. % to 13.4 at. %. The three-phase region of λ₃ + χ-Re₇Ta₃ + bcc-(Ta) at the 1200 °C isothermal section was larger than that of the 1100 °C isothermal section, while the (εCo, Re) + λ₃ + χ-Re₇Ta₃ three-phase region was smaller.

The isothermal section at 1300 °C is shown in Figure 5c. Compared to the 1200 and 1300 °C isothermal sections, another Laves phase of λ₁, which existed at 1293 °C~1587 °C in the Co-Ta binary system, appeared in the 1100 °C isothermal section. However, the solubility of Re was so small that it was not identified. The 1300 °C isothermal section had one more three-phase region (λ₁ + λ₂ + μ-Co₆Ta₇) than in the 1100 and 1200 °C isothermal sections. The solubility of Re in the λ₃ and λ₂ phases was about 22.3 at. %, 3.6 at. %, respectively. The solubility of Re in μ-Co₆Ta₇ was found to be about 6.1 at. %, occupying the most out of all the investigated isothermal sections. The CoTa₂ phase dissolved about 1.8 at. % Re. The solubility of Ta in the (εCo, Re) phase was about 23.2 at. %. There were seven three-phase regions of (αCo) + (εCo, Re) + λ₃, λ₃ + bcc-(Ta) + μ-Co₆Ta₇, λ₃ + χ-Re₇Ta₃ + bcc-(Ta), (εCo, Re) + λ₃ + χ-Re₇Ta₃, λ₃ + λ₂ + μ-Co₆Ta₇, λ₁ + λ₂ + μ-Co₆Ta₇, and CoTa₂ + μ-Co₆Ta₇ + bcc-(Ta) existing at 1300 °C. The former four three-phase regions were experimentally confirmed, and the last three three-phase regions were not experimentally evidenced. The three-phase equilibrium of λ₃ + bcc-(Ta) + μ-Co₆Ta₇ was smaller than those in the 1100 and 1200 °C isothermal sections.

4. Conclusions

The isothermal sections of the Co-Re-Ta ternary system at 1100, 1200, and 1300 °C were experimentally investigated. The results were as follows: (1) There were six three-phase regions at the 1100 and 1200 °C isothermal sections and seven three-phase regions at the 1300 °C isothermal section; (2) the (εCo, Re) phase had a large solubility of Ta and extended from the Re-rich side to the Co-rich side; (3) the λ₃ phase, with a large solubility of Re, surrounded the λ₂ phase which dissolved a little Re; (4) the solubility of Re in the CoTa₂ phase changed little while the solubility of Re in the μ-Co₆Ta₇ phase increased with the temperature increase from 1100 to 1300 °C; (5) no ternary compound was found.