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.


Introduction
The γ -Co 3 (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 Cobased 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  Figure 1. The sub-binary phase diagrams of Co-Si [26], Co-Ta [27] and Ta-Si [28] systems. Figure 1. The sub-binary phase diagrams of Co-Si [26], Co-Ta [27] and Ta-Si [28] systems.
Previous studies have reported five ternary compounds in the Co-Ta-Si ternary system: G (Co 16 Ta 6 Si 7 ) [29], G (Co 4 TaSi 3 ) [30,31], L (Co 3 Ta 2 Si) [32,33], E (CoTaSi) [29,34] and V (Co 4 Ta 4 Si 7 ) [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: Fm3m) [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 MnZn 2type Laves phase [32]. Mittal et al. [33] design an alloy with a composition of Co 3 Ta 2 Si, 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.

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 • .

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 Tables 2 and 3. 3.1.1. Equilibria at Zone 1 Zone 1 mainly contains the phase equilibria at the Si-rich corner (Si > 50 at.%). For the Co 34 Ta 26 Si 40 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 Co 26 Ta 7 Si 67 alloy is located in a three-phase equilibria region after being annealed at 1100 • C. The white and light gray phases correspond to the TaSi 2 and CoSi 2 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.

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.
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].   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)Ta 2 (white) + λ 1 -Co 2 Ta (black) was confirmed in the Co 30 Ta 43 Si 27 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 Co 11 Ta 75 Si 14 alloy formed a two-phase equilibrium microstructure of (Ta) + (Co, Si)Ta 2 after being annealed at 900 • C. Figure 4c depicts the three-phase equilibrium of (Co, Si)Ta 2 (white) + Co 6 Ta 7 (grey) + λ 1 -Co 2 Ta (black) in Co 41 Ta 47 Si 12 alloy after being annealed at 900 • C.

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 threephase 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.

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 threephase 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 sta-   21 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 Al 2 Cu-type CoTa 2 and Ta 2 Si phases form a continuous solid solution phase (Co, Si)Ta 2 . As far as we know, it is the first time to discover the infinite mutual solubility between CoTa 2 and Ta 2 Si phases. Similarly, the phenomenon of Ni and Si can be entirely substituted by each other in Al 2 Cu-type compounds was also found in the Ni-Ta-Si ternary system [42].

Equilibria at Zone 3
Zone 3 mainly discusses the phase equilibria around G phase. The alloy Co 46 Ta 13 Si 41 , Co 58 Ta 10 Si 32 and Co 48 Ta 22 Si 30 were designed to explore the existence of the G phase. For Co 46 Ta 13 Si 41 and Co 58 Ta 10 Si 32 alloys that being annealed at 1100 • C, two three-phase equilibrium CoSi + E + G and αCo 2 Si + 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, αCo 2 Si 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 Co 48 Ta 22 Si 30 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.

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].  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  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 λ 1 -Co 2 Ta phase for the Co 54 Ta 28 Si 18 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]. 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.  The alloy with a nominal composition of Co 59 Ta 31 Si 10 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 Zn 2 Mg-type compounds (P6 3 /mmc, C14) like the binary λ 1 -Co 2 Ta phase. Additionally, the λ 1 -Co 2 Ta 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 λ 1 -Co 2 Ta 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 Co 65 Ta 27 Si 8 was designed to determine the phase equilibria of the G and three Laves phases (λ 1 -Co 2 Ta, λ 2 -Co 2 Ta and λ 3 -Co 2 Ta). A single λ 3 -Co 2 Ta phase was detected in Co 65 Ta 27 Si 8 alloy annealed at 1100 • C (Figure 8c), while the λ 3 -Co 2 Ta + 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 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.

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.
The solid solubilities of Ta in the Co-Si binary phases (CoSi 2 , CoSi and αCo 2 Si) are negligible. The maximum solubility of Co in the TaSi 2 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 αTa 5 Si 3 phase increases from 3.5 at.% to 5.3 at.%. In addition, the maximum solubility of Si in the λ 3 -Co 2 Ta 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 λ 2 -Co 2 Ta phase at 900 • C. However, the solid solubility of Si in Co 6 Ta 7 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 λ 1 -Co 2 Ta (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 Co 46 Ta 13 Si 41 , Co 58 Ta 10 Si 32 and Co 48 Ta 22 Si 30 alloys after being annealed at 1100 • C, but it disappears at 900 • C.