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

In this work, two isothermal sections of the Co-Ta-Si ternary system at 900 °C and 1100 °C are constructed in the whole composition range via phase equilibrium determination with the help of electron probe microanalysis (EPMA) and X-ray diffraction (XRD) techniques. Firstly, several reported ternary phases G (Co16Ta6Si7), G″ (Co4TaSi3), E (CoTaSi), L (Co3Ta2Si) and V (Co4Ta4Si7) are all re-confirmed again. The G″ phase is found to be a kind of high-temperature compound, which is unstable at less than 1100 °C. Additionally, the L phase with a large composition range (Co32-62Ta26-36Si10-30) crystallizes with a hexagonal crystal structure (space group: P63/mmc, C14), which is the same as that of the binary high-temperature λ1-Co2Ta phase. It can be reasonably speculated that the ternary L phase results from the stabilization toward low-temperature of the binary λ1-Co2Ta through adding Si. Secondly, the binary CoTa2 and SiTa2 phases are found to form a continuous solid solution phase (Co, Si)Ta2 with a body-centered tetragonal structure. Thirdly, the elemental Si shows a large solid solubility for Co-Ta binary compounds while the Ta and Co are hardly dissolved in Co-Si and Ta-Si binary phases, respectively.

1. Introduction

The γ′-Co3(Al, W) phase was found to be highly coherent orientation with the γ-Co matrix in the Co-based superalloys similar to the Ni-based superalloys, thus making Co-based superalloys expected to become the next generation of high-temperature structural materials such as aero-engine blades and turbine disk [1,2]. However, the novel Co-based superalloys have severe problems, such as high density and poor stability of γ′ phase at high temperatures, which limit their further development [3,4,5]. Previous studies have shown that the addition of alloying elements such as Ni, Si, Cr, V, Ta, Nb, and Ru can effectively improve the above-mentioned problems and enhance the overall mechanical properties [6,7,8,9,10,11,12]. The addition of Ta can improve the stability, volume fraction and solution temperature of the γ′ phase, and increase the stress required for the internal slip of the γ′ phase to improve the high-temperature mechanical properties of the cobalt-based superalloys [6,9,12]. The addition of Si can improve the oxidation resistance and reduce the density of the Co-based superalloys while maintaining the stability of γ′-phase and high-temperature mechanical properties of the superalloys [10,11]. However, interactions between alloying elements may also cause negative effects. For instance, the excessive addition of Si and Ta in Co-based superalloys promotes the formation of detrimental topological close-packed (TCP) phases that reduce the strength and ductility [13]. Therefore, theoretical and experimental research on the sub-systems of Co-based superalloy is needed to understand the interrelationship between composition and crystal structure, the related work has achieved considerable achievements [14,15,16,17,18,19,20,21]. Among them, the phase diagram is the theoretical fundamental to guiding the composition design and microstructure control of Co-based superalloys [22,23]. In order to better understand the relationship between the composition and microstructure of the critical Co-based superalloy system Co-Ni-Al-W-Ta-Ti-Hf-Cr-Si [24,25], the phase equilibria of the related sub-system Co-Ta-Si is of great importance.

The Co-Ta-Si ternary system is constituted by three sub-binary systems: Co-Si [26], Co-Ta [27] and Ta-Si [28], as shown in Figure 1. The crystal structure information of each equilibrium phases in the Co-Ta-Si ternary system and three sub-binary systems are shown in Table 1. Firstly, there are five intermediate phases in the Co-Si binary system, namely Co3Si, αCo2Si, βCo2Si, CoSi and CoSi2 and six intermediate phases in the Co-Ta binary system: Co7Ta2, Co6Ta7, CoTa2, λ1-Co2Ta, λ2-Co2Ta and λ3-Co2Ta. Among them, the λ1-Co2Ta is a high-temperature MnZn2-type Laves phase, which decomposes into the CoTa and λ2-Co2Ta phases by an eutectic reaction at 1294 °C. Additionally, the λ2-Co2Ta and λ3-Co2Ta phases are the MnCu2- and MgNi2-type Laves phases, respectively. The CoTa2 Laves phase is a kind of Al2Cu-type, which is the same as that of the Ta2Si. Besides, there are six intermediate compounds in the Ta-Si binary system, namely Ta3Si, Ta2Si, αTa5Si3, βTa5Si3, γTa5Si3, and TaSi2. The conversion among αTa5Si3, βTa5Si3 and γTa5Si3 are completed through crystal transformation.

Figure 1

The sub-binary phase diagrams of Co-Si [26], Co-Ta [27] and Ta-Si [28] systems.

Table 1

The stable solid phases in the Co-Ta-Si ternary systems.

System	Phase	Pearson Symbol	Prototype	Space Group	Struktur-bericht	Refs.
Co-Si	(αCo)	cF4	Cu	Fm-3m	A1	[26]
	(εCo)	hP2	Mg	P63/mmc	A3	[26]
	Co3Si	hP8	Mg3Cd	P63/mmc	-	[26]
	αCo2Si	oP12	Co2Si	Pnma	C23	[26]
	βCo2Si	-	-	-	-	[26]
	CoSi	cP8	FeSi	P213	B20	[26]
	CoSi2	cF12	CaF2	Fm-3m	C1	[26]
	(Si)	cF8	C(diamond)	Fd-3m	A4	[26]
Co-Ta	Co7Ta2	hR36	BaPb3	R-3m	-	[27]
	λ1-Co2Ta	hP12	Zn2Mg	P63/mmc	C14	[27]
	λ2-Co2Ta	cF24	Cu2Mg	Fd-3m	C15	[27]
	λ3-Co2Ta	hP24	MgNi2	P63/mmc	C36	[27]
	Co6Ta7	hR13	Fe7W6	R-3m	D85	[27]
	CoTa2	tI12	Al2Cu	I4/mcm	C16	[27]
	(Ta)	cI2	W	Im-3m	A2	[27]
Ta-Si	Ta3Si	tP32	Ti3P	P63/mcm	-	[28]
	Ta2Si	tI12	Al2Cu	I4/mcm	C16	[28]
	αTa5Si3	tI32	Cr5B3	I4/mcm	D81	[28]
	βTa5Si3	hP16	Mn5Si3	P63/mcm	D88	[28]
	γTa5Si3	tI32	W5Si3	I4/mcm	D8m	[28]
	TaSi2	hP9	CrSi2	P6222	C40	[28]
	(Si)	cF8	C(diamond)	Fd-3m	A4	[28]
Co-Ta-Si	CoTaSi (E)	oP12	TiNiSi	Pnma	C23	[34]
	Co16Ta6Si7 (G)	cF116	Mg6Cu16Si7	Fm3m	A1	[29]
	Co4TaSi3 (G″)	hP168	Y13Pd40Sn31	P6/mmm	-	[30]
	Co3Ta2Si (L)	hP12	MgZn2	P63/mmc	C14	[32]
	Co4Ta4Si7 (V)	tI60	Zr4Co4Ge7	-	-	[34]

Previous studies have reported five ternary compounds in the Co-Ta-Si ternary system: G (Co16Ta6Si7) [29], G″ (Co4TaSi3) [30,31], L (Co3Ta2Si) [32,33], E (CoTaSi) [29,34] and V (Co4Ta4Si7) [34,35,36]. The G phase was first reported in 1956 by Beattie [37] in A-286 alloy. It is a kind of ternary silicide with a cubic crystal system (space group: Fmm) [38], which was named because it is easy to precipitate at the grain boundaries. Later studies found that there is a special cube-on-cube orientation relationship between the G phase and ferrite, which results in low interfacial energy, and thus makes it a potential strengthening phase of ferritic steel [39]. Additionally, the L phase was detected as a ternary MnZn2-type Laves phase [32]. Mittal et al. [33] design an alloy with a composition of Co3Ta2Si, which detected L and an unknown phase. In addition, the E, G″ and V phases can be treated as stoichiometric compounds and these phases are also detected in Co-Ti-Si [40] and Co-Nb-Si [41] systems.

However, as far as we know, the phase equilibrium of the Co-Ta-Si ternary system has not been reported as far. To better understand the interaction between composition and crystal structure in Co-based superalloys, also to construct the thermodynamic database of the Co-V-Al-Ta-Ti-Ni-Cr-Si multisystem, we devote ourselves to systematically exploring the phase equilibrium relationships in ternary Co-Ta-Si alloys, a sub-system of Co-based superalloys.

2. Experimental Procedures

High-purity cobalt (>99.9 wt.%, bulk, Beijing Trillion Metals Co., Ltd., Beijing, China), tantalum (>99.9 wt.%, flake, Beijing Trillion Metals Co., Ltd., Beijing, China) and silicon (>99.9 wt.%, block, Beijing Trillion Metals Co., Ltd., Beijing, China) were adopted as starting materials. The samples were prepared by arc-melting using a water-cooled copper crucible with a non-consumable tungsten electrode under the high purity Ar atmosphere (DHL-1250, Sky Technology Development Co, Ltd., Shenyang, China). The weight of each sample was about 20 g and they were arc-melted at least five times to achieve the compositional uniformity. The overall weight loss after arc-melting was no more than 0.5 wt.%. Then, these samples were sealed in capsules via backfilling with Ar and annealed at 900 °C and 1100 °C, respectively. Considering the high melting point for the elemental Ta, the time of annealing was set as 90 days for 900 °C and 60 days for 1100 °C, respectively. To prevent contamination of samples, the capsules were inserted with Ti scrap and the samples were wrapped with a Ta sheet. After reaching the preset annealing time, the samples were quenched into ice water and prepared by standard metallographic methods.

The equilibrium composition of each phase was investigated by EPMA (electron probe microanalyzer) (JAX-8100R, JEOL, Tokyo, Japan) with WDS (wavelength dispersive X-ray spectroscopy) and BSE (backscattered electrons). Crystal structure analysis was carried out through XRD (X-ray diffractometer) (D8 Advance, Bruker, Karlsruhe, Germany) using Cu Kα radiation at 40.0 kV and 40 mA, and the data were collected in the range of 2θ from 10° to 90° at a step size of 0.0167°.

3. Results and Discussion

3.1. Microstructure and Phase Equilibrium

In Figure 1, the Co-Ta-Si ternary system is divided into four regions (zone 1–4), phase equilibrium from each region is carefully discussed below. The following composition of each phase and alloy is described by the atomic ratio (at.%). The representative micrograph images and corresponding XRD indexing results are given below to indicate the phase equilibria relationship of the alloy. The nominal composition, annealing time and equilibrium composition of each phase measured by WDS are all listed in Table 2 and Table 3.

Table 2

Equilibrium compositions of the Co-Ta-Si ternary system at 900 °C determined in the present work.

Alloy(at.%)	Annealed Time	Phase Equilibria	Composition (at.%)
		Phase 1/Phase 2/Phase 3	Phase 1		Phase 2		Phase 3
			Ta	Si	Ta	Si	Ta	Si
Co10Ta10Si80	90 days	TaSi2 / CoSi2 / (Si)	30.3	68.6	0.1	70.3	0.1	99.7
Co33Ta14Si53	90 days	V / CoSi	24.8	48.4	0.1	51.6
Co30Ta43Si27	90 days	(Co, Si)Ta2 / λ1-Co2Ta	62.6	30.8	32.2	29.5
Co54Ta28Si18	90 days	λ1-Co2Ta / G	28.2	18.9	19.2	25.1
Co16Ta18Si66	90 days	TaSi2 / CoSi / CoSi2	30.4	67.8	0.2	51.8	0.2	67.2
Co22Ta27Si51	90 days	TaSi2 / V / CoSi	30.6	67.8	25.3	48.2	0.5	51.6
Co27Ta60Si13	90 days	(Co, Si)Ta2 / Co6Ta7	63.7	22.8	49.6	6.7
Co34Ta26Si40	90 days	E / V / CoSi	31.6	34.8	22.7	46.5	0.2	50.5
Co25Ta38Si37	90 days	αTa5Si3 / E / V	58.4	39.0	32.1	35.2	25.2	48.1
Co64.5Ta22.5Si13	90 days	λ3-Co2Ta / G	23.5	11.5	19.9	25.4
Co36Ta54Si10	90 days	(Co, Si)Ta2 / Co6Ta7	64.0	20.6	49.7	9.2
Co2Ta75Si23	90 days	(Ta) / (Co, Si)Ta2 / Ta3Si	92.3	5.4	63.8	28.6	71.0	28.6
Co26Ta7Si67	90 days	CoSi2 / TaSi2	0.1	67.5	29.5	67.1
Co46Ta13Si41	90 days	G / CoSi	18.2	25.8	0.1	51.2
Co58Ta10Si32	90 days	Co2Si / CoSi / G	0.1	34.6	0.1	50.6	18.0	26.2
Co48Ta22Si30	90 days	E / G / CoSi	30.2	35.1	18.3	26.1	0.1	51.1
Co36Ta33Si31	90 days	E / λ1-Co2Ta	30.9	35.2	31.1	30.9
Co59.5Ta22.5Si18	90 days	λ3-Co2Ta / G	22.1	11.0	18.6	24.6
Co59Ta31Si10	90 days	λ1-Co2Ta / λ2-Co2Ta	29.7	13.0	28.8	8.5
Co11Ta75Si14	90 days	(Ta) / (Co, Si)Ta2	91.2	3.0	63.7	28.1
Co38Ta58Si4	90 days	Co6Ta7 / (Co, Si)Ta2	63.3	12.4	52.1	5.9
Co25Ta33Si42	90 days	αTa5Si3 / E / V	57.2	39.6	32.4	35.6	24.4	48.9
Co24Ta44Si31	90 days	αTa5Si3 / E	58.1	39.4	31.6	34.8
Co65Ta27Si8	90 days	G / λ3-Co2Ta	19.1	24.1	25.3	9.2
Co56Ta9Si35	90 days	G / αCo2Si / CoSi	19.0	25.0	0.4	33.6	0.3	49.2
Co44Ta25Si31	90 days	E / G / CoSi	30.7	33.6	19.3	24.6	0.7	47.4
Co10Ta59Si31	90 days	αTa5Si3 / E / (Co, Si)Ta2	58.8	39.4	32.0	29.7	63.4	33.2
Co61Ta11Si28	90 days	G / αCo2Si	17.7	24.8	0.1	32.6
Co70Ta9Si21	90 days	G / (εCo) / αCo2Si	57.7	24.3	0.1	14.4	0.2	29.7
Co37Ta42Si21	90 days	(Co, Si)Ta2 / λ1-Co2Ta	62.0	24.7	44.3	22.2
Co71Ta13Si16	90 days	G / (αCo)	18.8	24.4	0.5	6.7
Co78Ta10Si12	90 days	λ3-Co2Ta / G / (αCo)	20.6	8.5	18.7	23.7	0.7	4.0
Co51Ta37Si12	90 days	Co6Ta7 / λ1-Co2Ta	46.5	6.2	32.4	16.2
Co41Ta47Si12	90 days	(Co, Si)Ta2 / Co6Ta7 / λ1-Co2Ta	62.9	22.3	47.6	8.9	36.8	13.5
Co78Ta17Si5	90 days	λ3-Co2Ta / (αCo)	21.0	6.4	0.9	1.9
Co57Ta38Si5	90 days	Co6Ta7 / λ2-Co2Ta	43.2	4.1	35.5	6.1
Co27Ta68Si5	90 days	(Ta) / (Co, Si)Ta2	91.7	2.9	63.5	6.2

Table 3

Equilibrium compositions of the Co-Ta-Si ternary system at 1100 °C determined in the present work.

Alloy(at.%)	Annealed Time	Phase Equilibria	Composition (at.%)
		Phase 1/Phase 2/Phase 3	Phase 1		Phase 2		Phase 3
			Ta	Si	Ta	Si	Ta	Si
Co10Ta10Si80	60 days	TaSi2 / CoSi2 / (Si)	31.0	67.3	0.7	68.0	0.1	99.0
Co33Ta14Si53	60 days	V / CoSi	25.4	48.2	0.1	50.9
Co10Ta38Si52	60 days	TaSi2 / αTa5Si3 / V	31.4	68.1	58.5	39.3	25.6	48.8
Co46Ta28Si26	60 days	E / G	30.8	34.8	19.7	25.5
Co30Ta43Si27	60 days	(Co, Si)Ta2 / λ1-Co2Ta	63.6	30.3	32.9	26.4
Co54Ta28Si18	60 days	λ1-Co2Ta / G	28.2	19.5	20.1	25.4
Co16Ta18Si66	60 days	CoSi / TaSi2 / V	30.8	67.6	0.1	51.8	24.5	48.4
Co22Ta27Si51	60 days	TaSi2 / V	31.4	68.4	25.3	48.9
Co27Ta60Si13	60 days	(Co, Si)Ta2 / Co6Ta7	63.8	21.1	50.7	7.5
Co35Ta5Si60	60 days	V / CoSi	23.8	48.6	0.1	51.2
Co34Ta26Si40	60 days	E / V / CoSi	31.8	35.4	25.2	46.8	0.1	51.9
Co25Ta38Si37	60 days	E / αTa5Si3 / V	57.7	39.6	32.0	35.6	25.7	48.3
Co64.5Ta22.5Si13	60 days	λ3-Co2Ta / G	22.1	12.6	19.5	23.8
Co2Ta75Si23	60 days	Ta / (Co, Si)Ta2 / Ta3Si	94.8	4.0	64.0	30.0	27.9	71.6
Co26Ta7Si67	60 days	TaSi2 / CoSi2 / (Si)	30.4	68.1	0.1	67.8	0.1	99.5
Co46Ta13Si41	60 days	E / G″ / CoSi	30.2	35.0	13.0	37.4	0.1	50.5
Co58Ta10Si32	60 days	αCo2Si / G / G″	0.1	34.7	18.6	25.8	13.6	37.1
Co48Ta22Si30	60 days	G / G″ / E	18.7	25.9	13.4	37.3	30.7	34.8
Co36Ta33Si31	60 days	E / λ1-Co2Ta	30.7	35.4	30.5	26.8
Co59.5Ta22.5Si18	60 days	G / λ3-Co2Ta	18.8	25.1	24.2	15.3
Co59Ta31Si10	60 days	λ1-Co2Ta	28.7	11.3
Co11Ta75Si14	60 days	(Ta) / (Co, Si)Ta2	95.0	3.1	63.3	27.9
Co38Ta58Si4	60 days	Co6Ta7 / (Co, Si)Ta2	63.5	12.0	52.1	3.4
Co25Ta33Si42	60 days	E / V	31.4	35.9	24.6	48.8
Co24Ta44Si31	60 days	αTa5Si3 / E	58.6	39.4	30.8	35.5
Co65Ta27Si8	60 days	λ3-Co2Ta	25.1	9.1
Co42Ta15Si43	60 days	CoSi / E	0.3	50.8	30.0	35.0
Co26Ta33Si41	60 days	V / E	24.9	48.5	31.3	35.1
Co56Ta9Si35	60 days	G" / αCo2Si / CoSi	13.6	36.5	0.1	33.9	0.1	49.3
Co44Ta25Si31	60 days	G / G"	18.9	25.6	13.9	36.4
Co10Ta59Si31	60 days	αTa5Si3 / Ta2Si / E	58.7	39.1	63.4	34.1	32.9	34.4
Co61Ta11Si28	60 days	G / αCo2Si	18.1	24.9	0.2	32.7
Co70Ta9Si21	60 days	G / (εCo) / αCo2Si	17.9	24.6	0.2	17.4	0.1	32.0
Co54Ta25Si21	60 days	λ1-Co2Ta / G	26.8	18.5	19.2	25.6
Co54Ta25Si21	60 days	λ1-Co2Ta	31.2	24.1
Co37Ta42Si21	60 days	(Co, Si)Ta2 / λ1-Co2Ta	62.4	26.6	32.5	23.5
Co26Ta53Si21	60 days	(Co, Si)Ta2 / λ1-Co2Ta	62.7	26.9	33.9	19.9
Co60Ta24Si16	60 days	λ3-Co2Ta / G	21.3	13.9	18.9	24.5
Co78Ta10Si12	60 days	λ3-Co2Ta / G / (αCo)	20.6	13.8	18.8	24.2	1.3	8.9
Co51Ta37Si12	60 days	Co6Ta7 / λ1-Co2Ta	45.8	5.5	32.6	10.9
Co41Ta47Si12	60 days	(Co, Si)Ta2 / Co6Ta7 / λ1-Co2Ta	62.6	21.5	45.9	10.1	33.6	16.5
Co21Ta67Si12	60 days	(Co, Si)Ta2 / (Ta)	94.1	3.0	63.6	24.5
Co78Ta17Si5	60 days	λ3-Co2Ta / (αCo)	22.4	7.1	2.6	3.5
Co57Ta38Si5	60 days	Co6Ta7 / λ2-Co2Ta	45.3	3.6	32.5	6.8

3.1.1. Equilibria at Zone 1

Zone 1 mainly contains the phase equilibria at the Si-rich corner (Si > 50 at.%). For the Co34Ta26Si40 alloy that was annealed at 1100 °C, a distinct three-phase equilibrium of E + V + CoSi was observed in Figure 2a and their crystal structures could be confirmed by the corresponding XRD pattern in Figure 3a. As shown in Figure 2b, the Co26Ta7Si67 alloy is located in a three-phase equilibria region after being annealed at 1100 °C. The white and light gray phases correspond to the TaSi2 and CoSi2 phases, respectively, while the dark gray phase is presumed to be the solid solution phase of Si (signed as (Si)). The corresponding XRD pattern indexing result in Figure 3b further confirmed this speculation.

Figure 2

Typical ternary micrograph images obtained of (a) Co34Ta26Si40 alloy annealed at 1100 °C for 60 days and (b) Co26Ta7Si67 alloy annealed at 1100 °C for 60 days.

Figure 3

XRD patterns obtained of (a) Co34Ta26Si40 alloy annealed at 1100 °C for 60 days and (b) Co26Ta7Si67 alloy annealed at 1100 °C for 60 days.

3.1.2. Equilibria at Zone 2

Zone 2 corresponds to the phase equilibria at the Ta-rich corner (Ta > 50 at.%). In Figure 4a, a two-phase equilibrium of (Co, Si)Ta2 (white) + λ1-Co2Ta (black) was confirmed in the Co30Ta43Si27 alloy quenching from 1100 °C based on the support of the corresponding X-ray diffraction pattern in Figure 5. As shown in Figure 4b, the Co11Ta75Si14 alloy formed a two-phase equilibrium microstructure of (Ta) + (Co, Si)Ta2 after being annealed at 900 °C. Figure 4c depicts the three-phase equilibrium of (Co, Si)Ta2 (white) + Co6Ta7 (grey) + λ1-Co2Ta (black) in Co41Ta47Si12 alloy after being annealed at 900 °C.

Figure 4

Typical ternary micrograph images obtained of (a) Co30Ta43Si27 alloy annealed at 1100 °C for 60 days; (b) Co11Ta75Si14 alloy annealed at 900 °C for 90 days and (c) Co41Ta47Si12 alloy annealed at 900 °C for 90 days.

Figure 5

XRD patterns obtained of Co30Ta43Si27 alloy annealed at 1100 °C for 60 days.

Compositional analysis of the Co30Ta43Si27, Co27Ta60Si13, Co36Ta54Si10, Co2Ta75Si23, Co11Ta75Si14, Co38Ta58Si4, Co10Ta59Si31, Co37Ta42Si21, Co41Ta47Si12, Co27Ta68Si5 and Co26Ta53Si21 alloys indicate that these alloys contain a phase with about 63 at.% Ta after being annealed at 900 °C and 1100 °C. The X-ray diffraction analysis results of these alloys strongly suggest that the binary Al2Cu-type CoTa2 and Ta2Si phases form a continuous solid solution phase (Co, Si)Ta2. As far as we know, it is the first time to discover the infinite mutual solubility between CoTa2 and Ta2Si phases. Similarly, the phenomenon of Ni and Si can be entirely substituted by each other in Al2Cu-type compounds was also found in the Ni-Ta-Si ternary system [42].

3.1.3. Equilibria at Zone 3

Zone 3 mainly discusses the phase equilibria around G″ phase. The alloy Co46Ta13Si41, Co58Ta10Si32 and Co48Ta22Si30 were designed to explore the existence of the G″ phase. For Co46Ta13Si41 and Co58Ta10Si32 alloys that being annealed at 1100 °C, two three-phase equilibrium CoSi + E + G″ and αCo2Si + G + G″ were detected as shown in Figure 6a,b. The corresponding XRD patterns in Figure 7a,b indicate that the diffraction peaks belonging to the phases (CoSi, E, αCo2Si and G) are in good agreement with the standard patterns. However, the diffraction peak of the G″ phase was not interpreted due to the lack of crystallographic information. For the Co48Ta22Si30 alloy annealed at 1100 °C, a clearly three-phase equilibria E (white) + G (light gray) + G″ (dark gray) was detected, as shown in Figure 6c. However, the EMPA micrographs and WDS analysis of the same alloy (see Figure 6d) quenching from 900 °C show that it is a three-phase of E + G + CoSi. This result implies that the G″ phase is a high-temperature compound, which is not stable at 900 °C.

Figure 6

Typical ternary micrograph images obtained of (a) Co46Ta13Si41 alloy annealed at 1100 °C for 60 days; (b) Co58Ta10Si32 alloy annealed at 1100 °C for 60 days; (c) Co48Ta22Si30 alloy annealed at 1100 °C for 60 days and (d) Co48Ta22Si30 alloy annealed at 900 °C for 90 days.

Figure 7

XRD patterns obtained of (a) Co46Ta13Si41 alloy annealed at 1100 °C for 60 days and (b) Co58Ta10Si32 alloy annealed at 1100 °C for 60 days.

3.1.4. Equilibria at Zone 4

Zone 4 describes the phase equilibria of other ternary phases including G and L. As shown in Figure 8a, the dark grey G phase was observed at the grain boundaries of the light grey λ1-Co2Ta phase for the Co54Ta28Si18 alloy after being annealed at 1100 °C. This morphology is extremely similar to that of the G phase precipitation on the C14 Laves phase [42].

Figure 8

Typical ternary micrograph images obtained of (a) Co54Ta28Si18 alloy annealed at 1100 °C for 60 days; (b) Co59Ta31Si10 alloy annealed at 1100 °C for 60 days; (c) Co65Ta27Si8 alloy annealed at 1100 °C for 60 days and (d) Co65Ta27Si8 alloy annealed at 900 °C for 90 days.

The alloy with a nominal composition of Co59Ta31Si10 was designed to detect the L phase as shown in Figure 8b. The interpretation of the corresponding XRD pattern in Figure 9a indicates that this ternary L phase crystalizes the same as the Zn2Mg-type compounds (P63/mmc, C14) like the binary λ1-Co2Ta phase. Additionally, the λ1-Co2Ta phase is a high-temperature phase that only exists above 1294 °C [27]. However, the detected ternary L phase is stable at 900 °C and 1100 °C, suggesting that the addition of Si stabilizes the λ1-Co2Ta phase toward low temperature. Besides, this Co-Ta-Si ternary L phase shows an extremely similar compositional range compared with the Ni-Nb-Si system [43]. The alloy Co65Ta27Si8 was designed to determine the phase equilibria of the G and three Laves phases (λ1-Co2Ta, λ2-Co2Ta and λ3-Co2Ta). A single λ3-Co2Ta phase was detected in Co65Ta27Si8 alloy annealed at 1100 °C (Figure 8c), while the λ3-Co2Ta + G was observed for the same alloy being annealed at 900 °C (Figure 8d). The corresponding XRD patterns in Figure 9b,c confirm the above equilibrium relationship.

Figure 9

XRD patterns obtained of (a) Co59Ta31Si10 alloy annealed at 1100 °C for 60 days; (b) Co65Ta27Si8 alloy annealed at 1100 °C for 60 days and (c) Co65Ta27Si8 alloy annealed at 900 °C for 90 days.

3.2. Isothermal Sections

Based on the phase equilibrium information discussed in Section 3.1, the isothermal sections of the Co-Ta-Si ternary system at 900 °C and 1100 °C are constructed in the whole composition range (see Figure 10). Nineteen and twenty-one three-phase regions were observed at 900 °C and 1100 °C, respectively. Few undetected three-phase regions are indicated by the dashed line.

Figure 10

Experimental determined isothermal sections of the Co-Ta-Si system at (a) 900 °C and (b) 1100 °C.

The solid solubilities of Ta in the Co-Si binary phases (CoSi2, CoSi and αCo2Si) are negligible. The maximum solubility of Co in the TaSi2 phase is 1.7 at.% at 1100 °C, and it increases to 3.4 at.% at 900 °C. When the temperature decreases from 1100 °C to 900 °C, the solid solubility of Co in the αTa5Si3 phase increases from 3.5 at.% to 5.3 at.%. In addition, the maximum solubility of Si in the λ3-Co2Ta phase was measured to be 15.3 at.% at 1100 °C and decreased to 11.5 at.% at 900 °C. At least 8.5 at.% of Si can be dissolved in the λ2-Co2Ta phase at 900 °C. However, the solid solubility of Si in Co6Ta7 hardly varies with temperature, maintaining 12 at.% at both 900 °C and 1100 °C.

In the Co-Ta-Si ternary system, there are four ternary compound phases G, G″, E and V and a binary high-temperature phase λ1-Co2Ta (L) stabilized by Si. The G, E, V and G″ phases are stoichiometric compounds exhibiting only small homogeneity ranges. Meanwhile, the G, E, V and L phases exist at 900 °C and 1100 °C. The G″ phase has also been reported in similar ternary systems like the Co-Nb-Si [41] and Co-Ti-Si [40]. The G″ phase was detected in the Co46Ta13Si41, Co58Ta10Si32 and Co48Ta22Si30 alloys after being annealed at 1100 °C, but it disappears at 900 °C.

4. Conclusions

The phase equilibrium of the Co-Ta-Si ternary system at 900 °C and 1100 °C is systematically studied by equilibrated alloy and the following conclusions can be drawn:

The four known ternary phases, G (Co16Ta6Si7), E (CoTaSi), G″ (Co4TaSi3) and V (Co4Ta4Si7) have almost no ternary solid solubilities, which could be treated as stoichiometric compounds.

The high-temperature phase G″ (Co4TaSi3) is only stable at 1100 °C and it disappears when the temperature decreases to 900 °C.

The addition of Si increases the thermal stability of the binary λ1-Co2Ta (C14 Laves) phase, resulting in the formation of the ternary L phase with the composition of Co32.3–58.8Ta28.2–36.8Si13.0–30.9 at 900 °C and Co39.3–62.3Ta26.8–33.9Si10.9–26.8 at 1100 °C.

Both the binary CoTa2 and SiTa2 phases crystallize with the same body-centered tetragonal structure (space group: I4/mcm, C16) and they form a continuous solid solution phase (Co, Si)Ta2.

The maximum solid solubility of Si for the λ3-Co2Ta phase is ~15.3 at.% at 1100 °C and it slightly decreases to be ~11.5 at.% at 900 °C. The solid solubility of Si for the Co6Ta7 phase is always ~12 at.% and does not change with temperature.

The elemental Ta is hardly dissolved in the CoSi2, CoSi and α-Co2Si phases. Similarly, the elemental Co has negligible solubilities in the TaSi2 and α-Ta5Si3 phases.

The results obtained from the present work make up the phase diagram information of the Co-Ta-Si ternary system, also provide the key experimental data for the establishment of Co-based superalloys thermodynamic database.