Prediction of Phase Formation Sequence in Multicomponent NiCoW/Nb₃Al Interface

Abstract

The interdiffusion behavior between NiCoW alloy and Nb₃Al at 1000 °C was investigated. Based on the effective heat of formation model, the compounds at the interface are analyzed by calculating the effective heat of formation and establishing the map of favorable compound formation. It is concluded that the formation sequence of phases between the nickel-based alloy and Nb₃Al is Ni₆Nb₇ → Co₂Nb/Ni₆Nb₇ + Nb_ss → Ni₃Nb → W-rich phase. The results of this paper provide a reference for the phase formation sequence law of multicomponent interface between Nb₃Al and the matrix alloy.

1 Introduction

Nb₃Al has been widely studied as an ultra-high-temperature structural material (Ref 1,2,3,4). Besides high strength and creep resistance (Ref 2), Nb₃Al was shown to be characterized by high melting point (2233 K) (Ref 1), low density (7.29 g/cm³) (Ref 3), and high hardness (910 HV) (Ref 5). These properties of Nb₃Al exhibit obvious advantages compared with the strengthening phase γ′-Ni₃Al widely used in the nickel-based superalloys. Therefore, Nb₃Al exhibits the potential to be used as a strengthening phase to replace the γ′-Ni₃Al phase to obtain a new kind of nickel-based superalloy with stronger temperature-bearing capacity. The most important challenge of using δ-Nb₃Al as a strengthening phase of nickel-based superalloy is to maintain the stability of δ-Nb₃Al in the alloy. Different from the coarsening and instability of γ′-Ni₃Al during long-term aging, δ-Nb₃Al in the nickel-base superalloy may react with the matrix alloy to form new intermetallic compounds during the preparation and service process. At present, there are many reports about the interface reaction between intermetallic compounds, alloys, and metals (Ref 6,7,8,9,10). For example, McGregor et al. (Ref 6) found that the diffusion zone containing β-NiAl and Ni₂TaAl was formed between CMSX-4 and NiTaAl. Ren et al. (Ref 7) reported that diffusion bonding was achieved between Ti₂AlNb and nickel-based superalloy, while the thickness of diffusion zone was reduced by using Cu/Ti nano-laminated foil as the intermediate layer. Assari et al. (Ref 8) investigated the diffusion behavior at the interface between Ti and Al layers and found that the growth kinetics of the TiAl₃ diffusion layer formed at the interface accords with parabolic law. It is well known that the interface structure is directly related to the mechanical properties of materials (Ref 11, 12). Therefore, it is necessary to investigate the diffusion interface structure between the δ-Nb₃Al and matrix alloy.

A variety of intermetallic compounds may be formed between different elements at the interface (Ref 13,14,15,16,17). Meanwhile, the type and formation order of the intermetallic compounds determine the interface structure (Ref 16, 18,19,20,21). Pretorius et al. (Ref 13, 14) first put forward an EFH model to predict the formation sequences of intermetallic compounds in the binary system. Bhanumurthy et al. (Ref 17) modified the EFH model by introducing the congruent factor, which improved the prediction accuracy of the model. Mogilevsky et al. (Ref 22) extended the effective formation heat model to ternary system. The semiquantitative analysis results of the model are in good correlation with the experimental results observed in systems such as Ta-SiC. However, the current analysis for the formation order of intermetallic compounds at multicomponent interfaces is limited to qualitative analysis (Ref 23, 24).

In this paper, the interdiffusion behavior between NiCoW alloy and Nb₃Al was investigated. The phase formation sequence of the multielement NiCoW alloy/Nb₃Al interface was analyzed based on the alloy composition and the Mogilevsky model. The results of this paper provide a reference for the phase formation sequence law of multielement interface between Nb₃Al and the matrix alloy and contribute to evaluation the properties of novel nickel-based superalloy with δ-Nb₃Al as strengthening phase.

2 Experiment

Interdiffusion experiments of Nb₃Al block and nickel-based alloy block were carried out to simulate the interdiffusion behavior between δ-Nb₃Al strengthening phase and matrix alloy in the novel δ-Nb₃Al phase-strengthened nickel-based superalloy. The nickel-based alloy used in this study is Ni-25.3Co-15.8W (at.%) alloy prepared by electric arc furnace melting. Cylindrical samples with a diameter of 10 mm and a height of 10 mm were cut from the ingot by wire electrical discharge machining (WEDM). Cylindrical Nb₃Al blocks of the same size were prepared by spark plasma sintering (SPS) with Nb₃Al powder as raw material. The parameters of SPS were 1200 °C, 30 MPa and 5 min. Before the interdiffusion experiment, the nickel-based alloy and Nb₃Al bulk samples were both ground with 400-2000# SiC sandpaper and then ultrasonic cleaned with ethanol. The interdiffusion experiment was carried out in Gleeble-3500 thermal simulation machine. The experimental parameters were 1000 °C, 30 MPa, 1 h. After the interdiffusion, all samples were cooled to room temperature in the Gleeble-3500 thermal simulation machine. The assembly schematic diagram of the interdiffusion specimen is shown in Fig. 1. The metallographic sample was cut from the diffusion interface marked in Fig. 1 by WEDM, grounded and polished. X-ray diffraction (XRD) was used to identify the microstructure of diffusion interface. The microstructure of the samples was characterized by scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS).

Fig. 1

Assembly schematic diagram of the interdiffusion specimen

3 Results

Figure 2 displays the XRD pattern of the NiCoW/Nb₃Al interface. The Ni₃Nb, Ni₆Nb₇, and Co₂Nb are identified along with the FCC structure of NiCoW alloy and the δ-Nb₃Al. The morphology of diffusion zone formed between NiCoW and Nb₃Al is shown in Fig. 3. Five diffusion layers are visible according to the contrast difference, as named as I-V (shown in Fig. 3a). It can be seen that bright phases with granular and rod shape are distributed in the diffusion layer I. Meanwhile, according to the contrast, the diffusion layers II-IV are composed of single phase, while the diffusion layer V is composed of two phases. The total thickness of the diffusion zone is 23.45 μm. The thickness of the diffusion layer I-V is 8.56 μm, 4.71 μm, 3.85 μm, 2.39 μm, and 3.94 μm, respectively. Figure 3(b) shows the result of the line scan of the elemental distribution of the NiCoW/Nb₃Al interface. It is found that the element distribution in diffusion layer I close to NiCoW alloy is the same as that in NiCoW alloy, while that in the region close to diffusion layer II is consistent with that in diffusion layer II. The elements in diffusion layer II are evenly distributed, and there is no noticeable fluctuation. This indicates that the phase composition range in diffusion layer II is narrow. However, the distribution of elements in diffusion layer III fluctuates obviously. This indicates that the phase composition range in diffusion layer III is wide. Figure 3(c) shows the elements distribution map of the NiCoW/Nb₃Al interface. The results reveal that the contents of Ni, Co, and W gradually decrease and the content of Nb gradually increases along the direction from the NiCoW alloy to the Nb₃Al. Overall, the regions close to the NiCoW alloy show relatively low concentrations of Al, and it is noted that the diffusion zone I exhibits a slightly higher Al content than adjacent regions. The diffusion layer I is obviously rich in W, while Ni, Co, Nb, and Al are less. There is an obvious W-rich region in diffusion layer I, where Ni, Co, Nb, and Al are less.

Fig. 2

XRD pattern of the NiCoW/Nb₃Al interface

Fig. 3

Microstructure and elements distribution of interface region between the NiCoW alloy and Nb₃Al. (①-⑥ indicate the EDS detection position, and I-V indicate different areas at the interface) (a) Microstructure of NiCoW/Nb₃Al interface; (b) Line-scan result of NiCoW/Nb₃Al interface; and (c) Elements distribution of NiCoW/Nb₃Al interface

Figure 4 displays the high magnification images of diffusion layer I and diffusion layer V, respectively. Figure 4(a) confirms that there are two phases in the diffusion layer I besides the granular-like and rod-like bright phase. Figure 4(b) displays that the light phase and dark phase in the diffusion layer V are distributed alternately. It can be seen that the elements contents, especially the content of Nb, are quite different between the two phases in the diffusion layer V. Combined with the EDS results and phase composition in the phase diagram, it is shown from Fig. 3(a) that the bright phase in diffusion zone I is W-rich phase and the dark phase is Ni₃Nb. The EDS results of positions marked in Fig. 3 and 4 are listed in Table 1. Combining the EDS and XRD results, the bright phase in diffusion zone I is identified as W-rich phase. The gray phase is FCC-structured phase of the NiCoW alloy and the dark phase adjacent to the diffusion layer II is identified as Ni₃Nb. Meanwhile, the phase in diffusion layer II is identified as Ni₃Nb, the phase in diffusion layer III is identified as Co₂Nb, and the phase in diffusion layer IV is identified as Ni₆Nb₇. The phases in diffusion layer V are Ni₆Nb₇ (dark phase) and Nb_ss (bright phase, the BCC phase of Nb with slight solid solutions of Ni and Co). The formation process of each phase at the interface is analyzed in the discussion section. To summarize, the diffusion interface structure between the nickel-based alloy and Nb₃Al is composed of NiCoW/W-rich phase/Ni₃Nb/Co₂Nb/Ni₆Nb₇/Ni₆Nb₇ + Nb_ss/Nb₃Al.

Fig. 4

High magnification images of (a) diffusion layer I and (b) diffusion layer V

Table 1 EDS results (at.%) of the marked locations in Fig. 3 and 4

4 Discussion

At the initial stage of diffusion between the NiCoW alloy and the Nb₃Al intermetallic compound, atoms from the two materials form a mixed interface and new compounds under the driving force of chemical potential gradient (Ref 23). According to phase diagrams (Ref 24), Ni, Co, and W atoms in NiCoW are anticipated to react with Nb and Al atoms in Nb₃Al to form various compounds at 1000 °C. The formation order of compounds determines the interface structure (Ref 25). Here, the formation order of different compounds can be evaluated by the effective heat of formation $\Delta H{\prime}$ (Ref 26), which is defined as:

$$\Delta H{\prime} = {{\left( {\Delta H^{0} + \Delta H^{f} } \right)C_{{\text{e}}} } \mathord{\left/ {\vphantom {{\left( {\Delta H^{0} + \Delta H^{f} } \right)C_{{\text{e}}} } {C_{1} }}} \right. \kern-0pt} {C_{1} }}$$

(1)

where $\Delta H^{0}$ is the standard heat of formation, $C_{{\text{e}}}$ is the effective concentration of the controlling element at the interface, $C_{1}$ is the limiting element concentration in the compound, and $\Delta H^{f}$ represents the congruency factor, which is zero for non-congruent compounds but proportional to the melting point T_m of congruent compounds as (Ref 16):

$$\Delta H^{f} = 8.13T_{{\text{m}}}$$

(2)

The effective concentration $C_{{\text{e}}}$ is mainly influenced by element content and diffusion coefficient (Ref 21). Then, the effective concentration at the interface can be compared according to these two parameters. Because the diffusion coefficient of W in the nickel-based alloy is two orders of magnitude lower than that of Ni and Co (Ref 26), the relatively low content of W in the mixed reaction layer at the interface can be ignored. Meanwhile, the diffusion coefficients of Ni and Co in Ni-based alloy are close, but the Ni concentration is obviously higher than that of Co. Therefore, the mixed reaction layer formed at the initial stage of reaction between NiCoW and Nb₃Al is approximated as a Ni-Nb₃Al system. For such a ternary system, the formation sequence of phases at the interface can be analyzed according to the model proposed by Mogilevsky (Ref 21).This is determined by the effective heat of formation obtained on the vertical section of the Ni-Nb₃Al and the ${{X_{{{\text{Nb}}}} } \mathord{\left/ {\vphantom {{X_{{{\text{Nb}}}} } {X_{{{\text{Al}}}} }}} \right. \kern-0pt} {X_{{{\text{Al}}}} }}$ for the most favorable compound formation map (Ref 21). $X_{{{\text{Nb}}}}$ and $X_{{{\text{Al}}}}$ refer to the effective concentrations of Nb and Al, respectively. ${{X_{{{\text{Nb}}}} } \mathord{\left/ {\vphantom {{X_{{{\text{Nb}}}} } {X_{{{\text{Al}}}} }}} \right. \kern-0pt} {X_{{{\text{Al}}}} }}$ is used to determine the compound region in the most favorable compound formation map. The effective heat of formation of the compounds obtained on the vertical section of the Ni-Nb₃Al is listed in Table 2, and the values of ${{X_{{{\text{Nb}}}} } \mathord{\left/ {\vphantom {{X_{{{\text{Nb}}}} } {X_{{{\text{Al}}}} }}} \right. \kern-0pt} {X_{{{\text{Al}}}} }}$ are listed in Table 3. The map of favorable compound formation and the effective heat of formation diagram for Ni-Nb₃Al are depicted in Fig. 5. It is noteworthy that the effective heats of formation for Ni₂Al₃, NiAl, and Ni₃Al in the Ni-Al system surpass those of Ni₆Nb₇ and Ni₃Nb in the Ni-Nb system. Following the criteria of the model, since the favorable formation regions of compounds in the Ni-Al system are situated farther from the Ni-Nb₃Al line compared to those in the Ni-Nb system (as illustrated in Fig. 4a), compounds in the Ni-Nb system are formed prior to those in the Ni-Al system (Ref 21). Furthermore, the effective heat of formation of Ni₃Nb is smaller than that of Ni₆Nb₇, as indicated in Fig. 7(b). Hence, Ni₆Nb₇ is initially formed at the initial interface of NiCoW/Nb₃Al, leading to the appearance of the interface structure of NiCoW/Ni₆Nb₇/Nb₃Al.

Table 2 Free energy of formation, $\Delta H^{0}$, and the effective free heat of formation on the vertical section of the Ni-Nb₃Al system, $\Delta H^{\prime }$

Table 3 Data necessary for constructing projections of effective free energy of formation diagrams

Fig. 5

Analysis basis for compound formation sequence in Ni-Nb₃Al system. (a) Map of favorable compound formation; (b) Vertical section of the effective heat of formation diagram

After the formation of the interface structure of NiCoW/Ni₆Nb₇/Nb₃Al, the phases at the new interfaces of Ni₆Nb₇/Nb₃Al and NiCoW/Ni₆Nb₇ were analyzed, respectively. Since the diffusion rates of Ni and Co in Ni₆Nb₇ (Ref 27) are similar but the content of Ni in NiCoW alloy is much higher than that of Co, the content of Ni diffused from NiCoW to the Ni₆Nb₇/Nb₃Al interface is higher than that of Co. Therefore, the content of Ni is higher than that of Co at the interface of Ni₆Nb₇/Nb₃Al. The mixed reaction layer at the interface of Ni₆Nb₇/Nb₃Al can be considered as Ni-Nb₃Al system. Considering that Ni and Co in the NiCoW diffuse through Ni₆Nb₇ to the interface of Ni₆Nb₇/Nb₃Al, the contents of Ni and Co at the interface are lower than those at the initial interface of NiCoW/Nb₃Al. Therefore, the composition at the interface of Ni₆Nb₇/Nb₃Al proceeds in the direction favorable to the formation of Ni₆Nb₇ (the direction marked by arrows in Fig. 5) and Ni₆Nb₇ continues to be generated at the interface of Ni₆Nb₇/Nb₃Al. Given that Nb consumption through formation of Ni₆Nb₇ phase and diffusion to NiCoW alloy is slower than Nb generation from Nb₃Al decomposition, Nb undergoes local enrichment at the Ni₆Nb₇/Nb₃Al interface. These Nb atoms then precipitate as a solid solution (Nb_ss) and intermix with Ni₆Nb₇, resulting in the formation of the two-phase structure in layer V. At the NiCoW/Ni₆Nb₇ interface, the mixed reaction layer can be considered as Co-Ni₆Nb₇ ternary system. Therefore, the formation sequence of phase can be analyzed according to the model proposed by Mogilevsky (Ref 21). In addition, the reaction in Eq 1 also occurs at the interface according to the phase diagram of Ni-Nb (Ref 28).

$${\text{Ni}} + {\text{Ni}}_{6} {\text{Nb}}_{7} \to {\text{Ni}}_{3} {\text{Nb}}$$

(3)

The heat of formation in Eq 3 is − 8.73 kJ/(mol. at.) (Ref 28). Due to the formation of ideal solid solution between Ni and Co (Ref 26), no compounds are formed between them, the map of favorable formation of compounds at the Co-Ni₆Nb₇ interface cannot be distinguished. Meanwhile, the effective heat of formation of compounds on the section of Co-Ni₆Nb₇ is calculated and shown in Table 4 and Fig. 6. It is revealed that Co₂Nb has the largest effective heat of formation in the compounds of Co-Ni₆Nb₇ ternary system, while the effective heat of formation of Co₂Nb is also larger than that of Ni₃Nb. Therefore, Co₂Nb is formed at the interface of NiCoW/Ni₆Nb₇ and the interface structure composed of NiCoW/Co₂Nb/Ni₆Nb₇/Ni₆Nb₇ + Nb_ss/Nb₃Al forms.

Table 4 Free energy of formation, $\Delta H^{0}$ (Ref 27), and the effective free heat of formation on the vertical section of the Co-Ni₆Nb₇ system, $\Delta H{\prime}$

Fig. 6

Vertical section of the effective heat of formation diagram for Co-Ni₆Nb₇ system

The formation of the interface structure of NiCoW/Co₂Nb/Ni₆Nb₇/Ni₆Nb₇ + Nb_ss/Nb₃Al gives rise to new interfaces, including Ni₆Nb₇ + Nb_ss/Nb₃Al, Co₂Nb/Ni₆Nb_7, and NiCoW/Co₂Nb. In comparison with the Ni₆Nb₇/Nb₃Al interface, there is a further reduction in the Ni and Co contents at the interface of Ni₆Nb₇ + Nb_ss/Nb₃Al. Consequently, the composition of the mixed reaction layer continues to proceed in favor of Ni₆Nb₇ formation, as indicated by the arrows in Fig. 7. Consequently, Ni₆Nb₇ and Nb_ss continue to be generated at the interface. Additionally, since Ni and Co do not form compounds, the mixed reaction layer at the interface of Co₂Nb/Ni₆Nb₇ can still be considered as Co-Ni₆Nb₇ ternary system. Unlike the NiCoW/Ni₆Nb₇ interface, the Ni and Co contents at the Co₂Nb/Ni₆Nb₇ interface are reduced. Hence, the composition proceeds in favor of Co₂Nb formation, as indicated by the arrows in Fig. 6, and Co₂Nb continues to form at the interface of Co₂Nb/Ni₆Nb₇. Moreover, the NiCoW/Co₂Nb interface can be viewed as a Ni-Co₂Nb ternary system, and the phase formation sequence is analyzed according to the model proposed by Mogilevsky (Ref 21). The reaction in Eq 2 also occurs at the interface according to the phase diagram of Co-Nb (Ref 25).

$${\text{Co}} + {\text{Co}}_{2} {\text{Nb}} \to {\text{Co}}_{3} {\text{Nb}}$$

(4)

Fig. 7

Vertical section of the effective heat of formation diagram for Ni-Co₂Nb system

The heat of formation in Eq 4 is − 7.87 kJ/(mol. at.) (Ref 25). Figure 7 and Table 5 present the map of favorite compound formation and the vertical section of the effective heat of formation diagram for Ni-Co₂Nb, respectively. Similar to the Co-Ni₆Nb₇ system, the lack of Ni-Co compounds prevents the acquisition of a favorite formation diagram for the Ni-Co₂Nb system. The effective heat of formation of each compound on the Ni-Co₂Nb section is displayed in Table 5 and Fig. 7. Notably, Ni₃Nb exhibits the highest effective heat of formation among the compounds in the Ni-Co₂Nb ternary system, surpassing that of the reaction between Co and Co₂Nb to produce Co₃Nb. Consequently, Ni₃Nb is formed at the NiCoW/Co₂Nb interface, resulting in the interface structure of NiCoW/Ni₃Nb/Co₂Nb/Ni₆Nb₇/Ni₆Nb₇ + Nb_ss/Nb₃Al (Table 6).

Table 5 Free energy of formation, $\Delta H^{0}$ (Ref 27), and the effective free heat of formation on the vertical section of the Ni-Co₂Nb system, $\Delta H^{\prime }$

Table 6 Free energy of formation, $\Delta H^{0}$ (Ref 27), and the effective free heat of formation on the vertical section of the Nb-Ni_2.32Co system, $\Delta H^{\prime }$

With the formation of the interface structure of NiCoW/Ni₃Nb/Co₂Nb/Ni₆Nb₇/Ni₆Nb₇ + Nb_ss/Nb₃Al, new interfaces of Ni₃Nb/Co₂Nb and NiCoW/Ni₃Nb emerge. Due to the reduction of the content of Ni and Co at the Ni₃Nb/Co₂Nb interface compared with that of the NiCoW/Co₂Nb interface, the composition of the mixed reaction layer continues to proceed in the direction favorable to the formation of Ni₃Nb (as indicated by the arrows in Fig. 7). Consequently, Ni₃Nb continue to be generated at the interface. Meanwhile, the mixed reaction layer at the interface of NiCoW/Ni₃Nb can be considered as Nb-Ni_2.32Co ternary system, and the phase formation sequence is analyzed following the model proposed by Mogilevsky (Ref 21). The map of favorite compound formation and vertical section of the effective heat of formation diagram for Nb-Ni_2.32Co, along with the necessary data for drawing, are presented in Fig. 8, Tables 6 and 7, respectively. It is evident that Ni₃Nb exhibits the highest effective heat of formation along the compounds in the Nb-Ni_2.32Co ternary system, confirming its formation at the interface of Nb-Ni_2.32Co. Additionally, as Ni and Co diffuse within the NiCoW alloy toward the Nb₃Al, W precipitates in the NiCoW alloy close to the interface of NiCoW/Ni₃Nb, resulting in the formation of a W-rich phase. In summary, the interface structure of NiCoW/W-rich phase/Ni₃Nb/Co₂Nb/Ni₆Nb₇/Ni₆Nb₇ + Nb_ss/Nb₃Al is established. Figure 9 illustrates the schematic diagram of the formation sequence of compounds at the NiCoW/Nb₃Al interface, where the sequence proceeds as follows: Ni₆Nb₇ → Co₂Nb/Ni₆Nb₇ + Nb_ss → Ni₃Nb → W-rich phase (Table 7).

Fig. 8

Analysis basis for compound formation sequence in Nb-Ni_2.32Co system. (a) Map of favorable compound formation; (b) Vertical section of the effective heat of formation diagram for Nb-Ni_2.32Co system

Fig. 9

Schematic diagram of the formation sequence of compounds at the NiCoW/Nb₃Al interface

Table 7 Data necessary for constructing projections of effective free energy of formation diagrams for Nb-Ni_2.3Co system

5 Conclusions

The diffusion interface of NiCoW/W-rich phase/Ni₃Nb/Co₂Nb/Ni₆Nb₇/Ni₆Nb₇ + Nb_ss/Nb₃Al was formed between the NiCoW alloy and Nb₃Al at 1000 °C for 1 h.

Based on the effective heat of formation (EHF) model, the compounds at the interface are analyzed by calculating the effective heat of formation and establishing the map of favorable compound formation. The formation sequence of compounds at the NiCoW/Nb₃Al interface is confirmed to be Ni₆Nb₇ → Co₂Nb/Ni₆Nb₇ + Nb_ss → Ni₃Nb → W-rich phase.