Effect of Mo in Ni-30Cr filler metal on the eutectic phases of weld metal

The effects of Mo content on the microstructure and solidification path of gas tungsten arc welding deposited Nb-bearing nickel-based alloy welding wire were studied. The Mo entered the matrix γ phase as a solid solution and was precipitated in the form of the eutectic phases including Laves, σ, and MC carbides. In Ni-30Cr weld metal, Mo plays a leading role in the eutectic σ formation, while a relatively minor role in the eutectic MC carbides and Laves formation compared with Nb. The addition of Mo could reduce the content of Nb in the dendrite arm, interdendritic region, and Laves phase. With the increasing Mo content, the Laves phase changed from a rod-like shape to a blocky shape. When the Mo content exceeded 4%, σ + Laves + γ eutectic structure was formed. When the Mo content was about 5.0%, there were microcracks in the σ and Laves phases, and at the σ/MC interface. To avoid the formation of the σ phase due to the segregation of Mo, the Mo content had to be controlled below 4%.


Introduction
Ni-30Cr filler metal has been widely used in the manufacture of the key components of the main equipment nuclear islands such as nuclear power reactor pressure vessels and steam generators. ERNiCrFe-7A filler metal was developed based on ERNiCrFe-7 with Nb addition. The carbide formed in the final stage of crystallization is used to prevent grain boundary migration, forming a tortuous grain boundary, which improves the ductility dip cracking (DDC) resistance by pinning the grain boundary [1]. Nevertheless, the critical strain of the ERNiCrFe-7A weld measured by the STF (Strain-to-Fracture) test was only approximately 1-4% [2,3]. ERNiCrFe-13 was developed based on ERNiCrFe-7A by adding 2.5% Nb and 4% Mo [4,5]. Its DDC resistance has been greatly improved, and the critical strain can reach more than 10% [6].
The effect of Nb on the cracking susceptibility, microstructures, and mechanical properties of Ni-30Cr weld metals has been investigated [7][8][9][10][11][12][13][14]. Nb plays a vital role in both the DDC resistance improvement and the solidification cracking susceptibility increase due to its effect on MC and Laves eutectic phases. It has been indicated that Mo can enter the matrix as a solid solution, show dendritic microsegregation tendency during solidification, and increase the size and number of Laves phases. However, the role of Mo in the formation of eutectic phases, including MC carbide and Laves, has still not been clearly understood due to the disturbing influence from variable Nb content. Additionally, it has been reported that a very small amount of σ phase enriched with Mo and Cr may form in Ni-30Cr weld metals [15]. The σ phase is a typical brittle phase that has a tetragonal structure in the form of lamellae and needles [16,17]. Due to its high hardness, high strength, and high brittleness [18,19], the σ phase will seriously deteriorate mechanical properties [20,21]. The nucleation and growth of the σ phase can consume a large number of solid solution strengthening elements such as Mo, Cr, and Fe, as well as reducing creep resistance and reducing lifetimes. Furthermore, the σ phase hinders the movement of entanglement and dislocations at the γ/σ interface, leading to interface peeling and cracking [16,17,22,23]. Unlike stainless steel, the σ phase is often precipitated due to high-temperature heat treatment in the heat-affected zone (HAZ) [24][25][26][27].
In nickel-based alloys, the σ phase is often precipitated in eutectic form during solidification, and the Mo content will have a direct impact on Ni-Cr-Mo and Fe-Ni-Cr-Mo crystallization paths and structures. With an increase of Mo content, the formation of a γ/σ eutectic structure is promoted [28]. Even though the significant decrease in the ductility and impact toughness due to high Mo contents in Ni-30Cr weld metals is attributed to the large size of the Laves and the microcracks that form around the large Laves phase [11], there is a lack of sufficient attention being paid to the embrittlement problem caused by σ phase formation due to an increase in Mo content. The determination of the optimal range of Mo content in Ni-30Cr weld metals depends on not only improving the cracking resistance but also avoiding embrittlement. Therefore, in Nb-bearing Ni-30Cr weld metal, it is necessary to study the independent effect of the Mo content on the structure and performance without Nb variation.
Due to the interference of Nb content, the effect of Mo in the formation of MC carbide, Laves phase, and σ phase is still not clear. To elucidate the independent function of Mo in the formation of eutectic phases in Nb-bearing Ni-30Cr weld metals, the effects of Mo content on the microstructure and solidification mode were studied in this research, This research aimed at clarifying the distribution of Mo in different phases and the mechanism of Mo on the formation and growth of eutectic phases, so as to lay the foundation for investigating the effect of Mo content on the mechanical properties and determining the optimal range of Mo content between 3 and 5% based on the avoidance of embrittlement.

Materials and experimental procedure
In this research, four types of Φ1.2 mm welding wires with different Mo contents were designed and prepared. The chemical composition of the welding wires is shown in Table 1. The deposited metal was prepared in a single V-groove butt joint for a Q235 steel plate with a size of 350 mm × 150 mm × 25 mm. To ensure the purity of the deposited metal, three layers of weld were made on the groove surface in advance as a buttering layer. Subsequently, cold wire GTAW was used for butt groove welding. The GTAW process parameters are shown in Table 2. Deposited metal preparation is shown in Fig. 1.
The four types of deposited metal samples for structure analysis were mechanically polished. The etching process was as follows: The corrosion solution was a 10% chromic acid CrO3 solution, the corrosion voltage was 6 V, and the corrosion time was 10 s. The metallographic structure was observed with an OLYMPUS GX51 optical microscope, and the microstructure and fracture morphology of the deposited metal were analyzed using a ZEISS EVO18 scanning electron microscope (SEM), which was combined with an OX-FORD INCA X-ray energy-dispersive spectrometer (XEDS) to carry out microscopic composition analysis. A HELIOS NanoLab 600i focused ion beam (FIB) SEM was used to extract the eutectic microstructure from the bulk sample as a site-specific TEM sample preparation method. A Talos 200 X-type in situ multifunctional transmission electron microscope (TEM) was used to analyze the morphology and structure of the eutectic phases. JMatPro software and a nickel alloy database were used for the thermodynamic and solidification calculations. Figure 2 shows the metallographic structure of deposited metal with different Mo contents. The deposited metals with different Mo contents were mainly columnar dendrites, but there were significant differences in the sizes and distributions of the interdendritic phases. As shown in Fig. 2a and b, the eutectic phases between the FM1 (0Mo) and FM2 (3.5Mo) dendrites were similar. The eutectic phases between  the FM3 (4.0Mo) dendrites in Fig. 2c were clustered. The precipitated phases between the FM4 (5.0Mo) dendrites in Fig. 2d exhibited an increase in size and were more densely distributed, and long chain-like structures appeared locally along the interdendritic regions. Figure 3 shows the SEM results of the morphology of the precipitates in the deposited metal with different Mo contents. Table 3 lists the energy-dispersive spectroscopy (EDS) composition analysis results of each precipitated phase shown in Fig. 3. In a previous investigation on Ni-30Cr weld metals with Nb and Mo additions, it has been indicated that there were three main eutectic phases for the TEM and EDS results: MC carbides enriched with Nb and Ti, Laves enriched with Nb and Mo, and σ phase enriched with Mo and Cr. In this research, the eutectic phases in the SEM images were identified mainly based on their EDS compositions. The precipitated phases in the FM1 (0Mo) deposited metal shown in Fig. 3a were mainly composed of MC and Laves. The MC exhibited a round shape with a size less than or equal to 1 μm, and the Laves had irregular blocky and thin rod-like structures with sizes of approximately 1-2 μm. Compared to the nominal composition, the MC phase was abundant in Nb and Ti, while the Laves was rich in Nb.

Effect of Mo on interdendritic phases
As shown in Fig. 3b, similar to the precipitated phases in FM1, the precipitated phases in the FM2 (3.5Mo) deposited metal were mainly composed of MC and Laves. The MC showed a round shape, and the size seemed to be smaller. The Laves had a thin rod-like structure with a size of approximately 1-2 μm. Compared with the nominal composition, the MC was rich in Nb and Ti, and the Mo content did not exhibit a significant difference, while the Laves phase was abundant in Nb and Mo.
As shown in Fig. 3c, compared with FM1 and FM2, the distribution of the eutectic phases between dendrites in the FM3 deposited metal was denser, and it had a larger size. The precipitated phases were mainly composed of MC and Laves, and the MC still showed mostly a round shape with a diameter of ~ 1 μm. The Laves phase was dominated by blocky and script-type morphology, and the size was in the range of 2-5 μm. Figure 3d shows a local (Laves + σ) coexisting structure and the Laves phase located in the edge of the lamellar σ phase. The σ phase was in the lamellar structure, and the size was about 2 μm. Compared with the composition shown in Table 3, the MC composition was similar to the MC compositions in FM1 and FM2, while the Mo content in the Laves phase increased greatly. Compared with the nominal composition, the σ phase was rich in Mo and Cr.  As shown in Fig. 3e and f, the interdendritic phase in the FM4 (5.0Mo) deposited metal had a (σ + Laves + γ) coexisting structure and MC. Additionally, the size of the (σ + Laves + γ) coexisting structure was between 5 and 10 μm, and the σ phase was located in the core of the coexisting structure. The Laves phases with blocky and round shapes were located on the edge of the coexisting structure. It can be seen from the composition in Table 3 that the Mo content of the Laves phase in the FM4 further increased, the Nb content decreased, and the Mo and Cr contents in the σ phase were higher compared with those in FM1, FM2, and FM3.
In order to further identify the eutectic phases in the FM3 (4.0Mo) weld metals, TEM, EDS, and SAED were performed. The results are shown in Fig. 4. Figure 4a shows that the shape of the Laves phase in FM3 (4.0Mo) was polygonal, and the Laves phase had three structures: C 14 (MgZn 2 ), C 15 (MgCu 2 ), and C 36 (MgNi 2 ) [29]. It can be seen from Fig. 4b that in this research, the Laves had a C 15 type structure and a face-centered cubic structure. In combination with the surface distribution results in Fig. 4c, the chemical formula of the Laves phase had to be (Ni,Cr,Fe) 2 (Nb, Mo). With the increasing Mo content, the size and quantity of the Laves phase increased, and the shapes changed from a dot shape and a rod shape to a blocky shape and a script shape. The Mo content in the Laves phase increased and the Nb content decreased, indicating that Mo reduced the solubility of the Nb.
For further detailed TEM investigation of the eutectic phases in the FM4 (5.0Mo) weld metals, a site-specific TEM sample preparation process with FIB was conducted. HAADF-STEM imaging and the corresponding element  Fig. 5. The upper gray eutectic phase in the HAADF-STEM image is enriched with Nb and Mo. The lower bright phase is enriched with Cr and Mo, which is similar to the results by Li et al. [15].
The SAED pattern in Fig. 6 further confirmed that the gray eutectic phase was C 15 Laves with a cubic structure, which was along the [112] zone axis. In combination with the analysis results for the mapping results shown in Fig. 5, it could be seen that the Laves phase was (Ni,Cr,Fe) 2 (Nb, Mo). The diffraction pattern shown in Fig. 6c indicated that the bright phase enriched with Cr and Mo was a σ phase with a tetragonal structure. Compared with the matrix, the σ phase was rich with Cr and Mo, and contained more Fe. As shown in Fig. 5, the matrix γ was enriched with Ni with a suborbicular morphology distributed between the Laves and σ phases. The matrix γ associated with the σ and Laves phases further confirmed that its formation was related to the eutectic reaction during solidification.
Effect of Mo on microstructures can be summarized as follows: (1) In Nb-bearing Ni-30Cr weld metals, Mo distributed in γ phase as a solid solution and also entered into eutectic phases including MC carbides, Laves, and σ; (2) with the increasing Mo content, the Laves phase changed from a thin rod-like shape to a bulky shape; (3) when the Mo content exceeded 4%, the formation of the σ phase was promoted, forming an (σ + Laves + γ) coexisting eutectic structure in the core of the segregation zone between dendrites.

Microcracks
In addition, as shown in Fig. 7, some microcracks were found in the deposited metal of FM4 (5.0Mo) in the bulk σ phase, σ/MC interface, bulk Laves phase, and Laves/γ interface. As shown in Fig. 7a, the cracks passed through the Laves and σ phases at the same time, with a length of 1-2 μm. As shown in Fig. 7b, the cracks were mainly located in the σ phase and at the σ/MC interface.

Effect of Mo on solidification path
The Scheil-Gulliver solidification calculation module in the JMatPro software was used for calculation according to the composition shown in Table 1. The calculation results of the solidification paths with different Mo contents are shown in Fig. 8. It could be seen that for different Mo contents, the MC was precipitated by the eutectic reaction first, and the Laves phase was precipitated at the end of solidification. The addition of Mo could lower the formation temperature of the MC and increase the formation temperature of the Laves phase. After the Mo content increased from 4 to 5%, the σ phase could be precipitated at the temperature between the MC and the Laves formation. This was essentially consistent with the results of the microstructure analysis. It could be concluded that the addition of Mo promoted the precipitation of the σ phase.
Effect of Mo on eutectics phases can be concluded that Mo plays a leading role in the eutectic σ formation, while a relatively minor role in the eutectic MC carbides and Laves formation compared with Nb in Ni-30Cr weld metals.

Avoidance of eutectic σ phase
To avoid the formation of a eutectic σ phase, we calculated the eutectic σ formation boundary based on the Mo isopleth in the Ni(balance element)-30%Cr-9%Fe-2.5%Nb-Mo(variation element) multicomponent alloy phase diagram with JMatPro and we predicted the Mo content in the liquid during solidification based on the Scheil model. Nishimoto [24] and Popov [25] used the Johnson-Mehl equation to predict the isothermal precipitation of the σ phase in stainless steel, while the σ phase in this research was formed as a eutectic phase during solidification due to Mo segregation. According to the Scheil model [30], the solid phase composition during solidification was where k is the partition coefficient, C 0 is the nominal composition of the solute, f s is the solid phase fraction, and C s is the solid phase composition. In the model, it was assumed that the liquid phase could diffuse sufficiently, but there was no diffusion in the solid phase. For the situation in this research, reference [30] showed that Mo diffusion in the solid phase was negligible under similar conditions. The composition in the liquid phase was C L 1 − f s was the liquid phase ratio fraction, and the composition of the solute element in the liquid phase was C L = C 0 f L k−1 . In this research, the 0.7 value measured in reference [28] was used as the partition coefficient.
The trend of the Mo content in the liquid phase with temperature for different nominal Mo contents is shown in Fig. 9. The black dot dash line represents the phase field boundary, which was extracted from the Mo concentration isopleth phase diagram. It could be determined that only the line of Mo content in the liquid with a nominal content of FM4(5.0Mo) crossed the boundary line of the L → (γ + σ) eutectic reaction, while the FM3(4.0Mo) did not. Therefore, the critical nominal Mo content of the L → (γ + σ) eutectic reaction was between 4.0 and 5.0%. As shown in Fig. 3, the σ phase was also found locally in FM3 (4.0Mo), which was inconsistent with the predicted result. This may have been due to the unevenness of the local composition in the weld and the influence of other elements. It is worth noting that Fig. 9 only predicted the L → (γ + σ) eutectic reaction based on the Mo content, while many factors affected the L → (γ + σ) eutectic reaction, such as the contents of Cr, Fe, and other alloying elements.
According to the prediction results of Fig. 9 combined with the results of the microstructures characterization, it was indicated that controlling the Mo content less than 4.0% could essentially avoid the formation of the eutectic σ phase.

Conclusion
Conclusions are summarized as follows: 1. Mo distributed in the matrix γ phase as a solid solution and eutectic phases including MC carbides, Laves, and σ in Nb-bearing Ni-30Cr weld metals. 2. Mo plays a leading role in the eutectic σ formation, while a relatively minor role in the eutectic MC carbides and Laves formation compared with Nb in Ni-30Cr weld metals. 3. When the Mo content reached 5%, the solidification path was L → L + γ → L + γ + MC → L + γ + MC + σ → L + γ + MC + σ + Laves → γ + MC + σ + Laves. To avoid the formation of the σ phase, the Mo content should be controlled below 4%.

Conflict of interest The authors declare no competing interests.
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