1 Introduction

Ni-based alloys are the most widely used superalloys and are strengthened by a γ′ phase precipitated in the disordered fcc matrix phase (γ). Co-based alloys exhibit a better resistance to hot corrosion, but because of the lower strength compared with Ni-based superalloys, they have not found widespread usage.[1] However, Sato et al. discovered a new type of Co-based superalloy strengthened by a ternary compound, γ′ Co3(Al,W), which has great high-temperature strength.[2] Co-based alloys are thus regarded as the promising candidates for applications at high temperatures.[1,2] In adding elements to Co-based alloys, vanadium can play an important role in solution strengthening and precipitation strengthening. It enhances the matrix strength by forming a solid solution with Co[3] and by forming a carbide (or nitride) with a high hardness and high melting point in Co-based alloys.[4] In addition, it can refine crystal grains to improve the microhardness and wear resistance.[5] Aluminum is another important alloying element in Co-based alloys. Al can form a dense Al2O3 film which is able to improve the oxidation resistance and hot corrosion resistance of the alloy.[6] To understand the action mechanism and the related phase equilibrium of vanadium and aluminum in Co-based alloys at high temperature, the Co-Al-V isothermal section at 1073 K (800 °C) was determined experimentally in the present work.

2 Literature Data

In the Al-V binary system, two intermediate phases have been reported at 1073 K (800 °C): namely, Al3V and Al8V5, which are line compounds.[7] However, Richter and Ipser[8] have reinvestigated the system and found that Al3V and Al8V5 showed a small homogeneity range. Gong et al.[9] have carried out a thermodynamic reassessment of the system including the new results of Reference 7 in their optimization. In the Co-V system, three intermetallic compounds are present: Co3V, σ-CoV, and CoV3.[3] In 2003, a thermodynamic evaluation of the Co-V binary system was achieved by Bratberg and Sundman.[10] Many researchers have previously studied the Al-Co system.[11,12,13,14,15] Among the researchers, Massalski et al.[3] pointed out that there are five intermediate phases in the Al-Co system at 1073 K (800 °C): Al9Co2, Al3Co, Al13Co4, Al5Co2, and AlCo. Mihalkovič et al.[14] applied first-principles calculations to compare the cohesive energies of both proven and hypothetical structures. They confirmed the experimentally established phase diagram in every detail except near Al13Co4. However, the homogeneity range of the AlCo phase in the central part of the Al-Co phase diagram has not been well established. Stein et al.[15] established the Al-rich phase boundary of the AlCo phase and the Co-rich boundary of the Al5Co2 phase experimentally, and re-assessed the phase diagram based on the CALPHAD method. All crystallographic parameters for the binary compounds related to the present work are listed in Table I. However, the Co-Al-V ternary system has never been reported. The binary systems Al-V,[8] Co-V,[3] and Al-Co[15] are used as the boundary conditions for the present ternary system.

Table I Crystallographic Parameters for Binary Compounds Required in the Co-Al-V Ternary System
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3 Experimental Methods

In all, 25 alloys were prepared to determine the equilibrium phases in the Co-Al-V system. Their compositions are shown in Table II. The alloys with an aluminum content of less than 60 at. pct were prepared from high-purity Co chips (99.99 wt pct), Al grains (99.99 wt pct), and V grains (99.99 wt pct) by arc melting under an Ar atmosphere. Each alloy ingot was melted four times and turned over between melting to achieve uniformity. Each alloy was placed into a corundum crucible and then sealed in an evacuated quartz tube. To prevent excessive loss of aluminum in the arc, the other parts of the alloys with higher aluminum content used another method for smelting the alloy. These alloys were prepared from high-purity Co powders (99.99 wt pct), Al grains (99.99 wt pct), and V powders (99.99 wt pct) because the Co and V powders can be mixed more easily to uniformity than Co chips and V grains. The raw materials were mixed in a corundum crucible and then sealed in an evacuated quartz tube. To promote melting, the sealed specimens were heated to 1373 K (1100 °C) and held at that temperature for 72 hours, after which they were quenched in water. The quenched specimens were then resealed in an evacuated quartz tube. All specimens were annealed at 1073 K (800 °C) for 1440 hours, followed by quenching in water.

Table II Composition of Alloys and Phases in the Co-Al-V Ternary System at 1073 K (800 °C) (At. Pct)
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All annealed alloys were prepared in the conventional way for metallographic examination. Microstructural observations and compositional analyses of various phases were carried out using an optical microscope and a JSM-6360LV scanning electron microscope (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) after etching by a 20 wt pct HF, 20 wt pct HNO3, and 60 wt pct H2O2 solution. The constituent phases of the alloys were further ascertained by analyzing X-ray diffraction (XRD) patterns generated by a Rigaku Ultima IV diffractometer, operating with Cu-Kα radiation. The compositions reported in this paper were obtained from the average value of at least five measured results.

4 Results and Discussion

All phases found in the alloys are listed in Table II, as are the chemical compositions of those phases. Please note that Al is in the liquid state at 1073 K (800 °C), so it was marked as “L-Al” in the present work.

According to the SEM–EDS analysis results of alloys A1, A2, A3, and A4 as shown in Figure 1, we have found four three-phase regions near the Al-rich side of the 1073 K (800 °C) isothermal section of the Co-Al-V system: Al9Co2+Al3V+L-Al, Al9Co2+Al3V+Al13Co4, Al13Co4+Al3V+Al3Co, and Al3Co+Al3V+Al5Co2 three-phase equilibrated regions. Figure 1(a) shows the microstructure of alloy A1. SEM–EDS analysis indicated that the L-Al phase is located around the Al9Co2 and Al3V grains. The Al3V phase is a bit darker than the Al9Co2 phase. The solubility of V in Al9Co2 reaches 0.3 at. pct. The microstructure of alloy A2 is shown in Figure 1(b). SEM–EDS analysis indicates that the L-Al phase is absent. Al3V, Al9Co2, and Al13Co4 are coexistent. The V solubility in Al9Co2 and Al13Co4 is 0.2 at. pct. Figure 1(c) shows the microstructure of alloy A3, indicating that Al3V, Al3Co, and Al13Co4 phases coexist in this alloy. Because of the low content of V in this alloy, the proportion of Al3V is much less than that of Al13Co4 and Al3Co. Figure 1(d) shows the microstructure of alloy A4 containing the following phases: Al3V, Al5Co2, and the gray Al3Co which contains 0.1 at. pct V. The Al3V phase is a bit darker than the Al5Co2 phase. The solubility of Co reaches 1.1 at. pct in Al3V. Figure 2 shows the XRD patterns of alloys A1, A2, A3, and A4, which confirms that these alloys are located in the Al9Co2+Al3V+L-Al, Al9Co2+Al3V+Al13Co4, Al13Co4+Al3V+Al3Co, and Al3Co+Al3V+Al5Co2 three-phase equilibrium regions, respectively. When cooled from 1073 K (800 °C) to room temperature, L-Al became α-Al phase, so the XRD patterns of alloy A1 contains α-Al phases.

Fig. 1
figure 1

SEM images of different alloys near the Al-rich side. (a) alloy A1, Co2Al9+Al3V+L-Al; (b) alloy A2, Al9Co2+Al3V+Al13Co4; (c) alloy A3, Al13Co4+Al3V+Al3Co; and (d) alloy A4, Al3Co+Al3V+Al5Co2

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Fig. 2
figure 2

XRD patterns of alloys near the Al-rich side

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A new ternary compound, named the ‘T’ phase (Al2CoV), containing 21.2-27.6 at. pct V and 47.9-53.7 at. pct Al, was found in this study. Alloys A5, A6, A7, A8, and A9 contain the T phase. Based on the SEM–EDS analysis, the T phase is confirmed to be in equilibrium with Al3V, Al8V5, α-V, Al5Co2, and AlCo. As shown in Figure 3(a), the microstructure of alloy A5 clearly indicates the existence of three phases: the bright bulk T phase, convex gray Al8V5, and the Al3V matrix. The solubility of Co in Al8V5 is 2.5 at. pct. Figure 3(b) shows the backscattered electron (BSE) micrograph of alloy A6, which indicates that alloy A6 is located in the T+Al5Co2+Al3V three-phase equilibrium region. The Al5Co2 phase contains 2.1 at. pct V, and the T phase contains 21.7 at. pct V, and 53.7 at. pct Al. Figure 3(c) shows the BSE micrograph of alloy A7, which indicates that the AlCo, T, and Al5Co2 phases coexist. The solubility of V and Al in T is 21.2 and 53.2 at. pct, respectively. Figure 4(d) shows the microstructure of alloy A8 with the following phases: the gray bulk α-V, dark-gray Al8V5, and light-gray T. The microstructure of alloy A9 is shown in Figure 3(e). SEM–EDS analysis indicates that the AlCo matrix contains α-V and T phases. Because the α-V phase is very similar in contrast to the T phase, it is not easy to identify. In this alloy, the T phase contains 26.7 at. pct V and 47.9 at. pct Al. Figure 4 shows the XRD patterns of alloys A5, A6, A7, A8, and A9. These patterns clearly indicate that these alloys are in the T+Al3V+Al8V5, T+Al3V+Al5Co2, T+AlCo+Al5Co2, T+α-V+Al8V, and T+AlCo+α-V three-phase equilibrium states, respectively. It is remarkable that the diffraction peaks of the α-V phase of alloy A8 move to the left side of the standard α-V phase XRD pattern. This is because Al atoms occupy the lattice points of V atoms without changing the crystal structure of the α-V phase and thus form a solid solution.

Fig. 3
figure 3

The typical microstructures of different alloys around the T phase. (a) alloy A5, T+Al3V+Al8V5; (b) alloy A6, T+Al3V+Al5Co2; (c) alloy A7, T+AlCo+Al5Co2; (d) alloy A8, T+α-V+Al8V; and (e) alloy A9, T+AlCo+α-V

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Fig. 4
figure 4

XRD patterns of alloys near the T phase

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To identify the crystal structure of the T phase, alloy S1 with nominal composition Co25Al50V25 was prepared with the intent of obtaining a pure T phase. The alloy was annealed at 1073 K (800 °C) for 1440 h. Figure 5 presents the XRD patterns of alloy S1, which shows that the major phase is the T phase. From Figure 5, we can see there are four lower peaks of the AlCo phase and three of the α-V phase. The XRD result shows the real composition of alloy S1 located in the T+AlCo+α-V three-phase-equilibrated field, which results from the loss of aluminum during the smelting process. By means of Jade 6, the refinement results of the XRD confirmed that the T phase belongs to a fcc structure with a lattice parameter of 11.7224 Å. The index of the T phase is labeled in Figure 5.

Fig. 5
figure 5

XRD patterns of alloy S1 (Co25Al50V25)

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On the basis of the SEM–EDS analysis of alloys A10, A11, A12, and A13 as shown in Figure 6, we have also found that four three-phase regions exist near the Co-V-rich side of the 1073 K (800 °C) isothermal section in the Co-Al-V system. They are CoV3+α-V+σ-CoV, AlCo+α-V+σ-CoV, Co3V+AlCo+σ-CoV, and Co3V+α-Co+AlCo three-phase equilibrated regions. The microstructure of alloy A10 is shown in Figure 6(a). SEM–EDS analysis indicates that the α-V and CoV3 phases coexist in the σ-CoV matrix. The CoV3 phase is very similar in contrast to σ-CoV, but after etching, the σ-CoV phase has been more deeply corroded. In this alloy, the Al solubility in CoV3 is 1.9 at. pct and the Co solubility in α-V is 23.7 at. pct. Figure 6(b) reveals the microstructure of alloy A11. Due to the fine microstructure, the EDS results are not very accurate. SEM–EDS analysis indicates that the σ-CoV phase, which contains about 6.3 at. pct Al, shows the best corrosion resistance. The sunken microstructure is AlCo, and the remaining phase is α-V, which contains about 24.9 at. pct Co. Figure 6(c) shows the BSE micrograph of alloy A12, wherein three different phases can be observed. The gray whiskers appertain to the Co3V phase and the bright regions represent σ-CoV phases. The matrix is in the AlCo phase. The solubility of V in AlCo is 31.9 at. pct. Figure 6(d) shows the microstructure of alloy A13, containing three phases: the gray granular Co3V, which contains 5.6 at. pct Al, the convex gray α-Co, and the sunken AlCo. The solubility of V reaches 16.4 at. pct in α-Co. Figure 7 shows the XRD patterns of alloys A10, A11, A12, and A13, which confirms that these alloys are located in the CoV3+V+σ-CoV, AlCo+V+σ-CoV, Co3V+AlCo+σ-CoV, and Co3V+α-Co+AlCo three-phase equilibrium regions, respectively. It is worth noting that the diffraction peaks of the α-V phase of alloy A10 move to the right side, which can be attributed to the fact that Co atoms occupy the lattice points of V atoms without changing the crystal structure of the α-V phase and form a solid solution.

Fig. 6
figure 6

Microstructures of different alloys near the Co-V-rich side. (a) alloy A10, CoV3+α-V+σ-CoV; (b) alloy A11, AlCo+α-V+σ-CoV; (c) alloy A12, Co3V+AlCo+σ-CoV; and (d) alloy A13, Co3V+α-Co+AlCo

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Fig. 7
figure 7

XRD patterns of alloys near the Co-V-rich side

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The alloys mentioned above are in a three-phase equilibrium state. The alloys in a two-phase equilibrium state are not detailed herein, but their compositional details are listed in Table II. Based on the above results and binary systems in the relevant literature,[3,8,15] the 1073 K (800 °C) isothermal section of the Co-Al-V ternary system is established, as shown in Figure 8. There are thirteen three-phase regions which have been confirmed experimentally. A new ternary compound T phase was found, which is in equilibrium with Al3V, Al8V5, α-V, Al5Co2, and AlCo. The AlCo phase regions are extraordinarily large and can be in equilibrium with all of the phases near the Co-V-rich side, except for CoV3.

Fig. 8
figure 8

The 1073 K (800 °C) isothermal section of the Co-Al-V ternary system

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5 Conclusions

  1. 1.

    The isothermal section of the Co-Al-V system at 1073 K (800 °C) has been determined by means of SEM/EDS and XRD techniques.

  2. 2.

    A new ternary compound T phase (Al2CoV) is confirmed which possesses a fcc structure with a lattice parameter of 11.7224 Å. The T phase can be in equilibrium with Al3V, Al8V5, α-V, Al5Co2, and AlCo.

  3. 3.

    The AlCo phase regions are extraordinarily large and can be in equilibrium with all of the phases near the Co-V-rich side except for the CoV3 phase.

  4. 4.

    There are thirteen three-phase regions: Al9Co2+Al3V+ L-Al, Al9Co2+Al3V+Al13Co4, Al13Co4+Al3V+Al3Co, Al3Co+Al3V+Al5Co2, T+Al3V+Al8V5, T+Al3V+Al5Co2, T+AlCo+Al5Co2, T+α-V+Al8V, T+AlCo+α-V, CoV3+α-V+σ-CoV, AlCo+α-V+σ-CoV, Co3V+AlCo+σ-CoV, and Co3V+α-Co+AlCo in the Co-Al-V isothermal section at 1073 K (800 °C).

  5. 5.

    The maximum solubility of Al in Co3V, σ-CoV, and CoV3 is 5.6, 6.3, and 4 at. pct, respectively. The maximum solubility of Co in Al3V, Al8V5, and α-V is 1.1, 2.5, and 24.9 at. pct, respectively. The maximum solubility of V in Al9Co2, Al13Co4, Al3Co, Al5Co2, AlCo, and α-Co is 0.3, 0.2, 0.1, 2.1, 35.0, and 16.4 at. pct, respectively.