Solidification processes of as-cast alloys and phase equilibria at 1 300 °C of the Nb–Si–V ternary system

The liquidus projection and the isothermal section at 1300 8C of the Nb–Si–V ternary system were experimentally studied. Using scanning electron microscopy, electron probe microanalysis and X-ray diffraction, the primary solidification phases and the precipitation paths in each region of the liquidus projection as well as the equilibrium relations of the phases in the isothermal section were determined. Ten primary solidification regions were found in the liquidus projection and eight three-phase equilibrium regions were observed in the isothermal section at 1300 8C. The compounds bNb5Si3 with V5Si3 and NbSi2 with VSi2 are the phases of the same structure but different compositions and form two linear compounds bNb(V)5Si3 or V(Nb)5Si3 and Nb(V)Si2 or V(Nb)Si2, respectively The ternary linear compound (Nb,V)2Si with the stoichiometry about 2 :1 of (Nb + V) :Si was found in both the liquidus projection and the isothermal section at 1300 8C.


Introduction
In the multi-component Nb-Si-based alloy systems, the balance of the mechanical properties such as room temperature toughness, high temperature creep resistance and oxidation resistance can be achieved by adding alloying elements to stabilize intermetallic compounds and by forming Laves phase to improve the oxidation resistance [1 -3]. The addition of V can promote the transformation of Nb 3 Si ? bcc + Nb 5 Si 3 , increase the volume fraction of Nbss/Nb 5 Si 3 in-situ composites. The bcc solid solution phase Nbss provides room temperature toughness and ductility, and the Nb 5 Si 3 intermetallic compound provides high temperature strength. Nbss/Nb 5 Si 3 insitu composites exhibit well-balanced mechanical propertie [4,5]. Kim et al. [6] found that V not only reduced the stability of Nb 3 Si and promoted its decomposition, but also directly formed the phase of aNb 5 Si 3 . At the same time, due to the solid solution strengthening effect and the elastic interaction caused by the size difference between Nb and V atoms, adding V to Nb-Si-based compounds can improve the room temperature strength of the alloy [7,8]. In summary, V is an important alloying element in optimizing the properties of the Nb-Si based superalloys [9]. Therefore, the study of the phase equilibria of the Nb-Si-V ternary system is of great practical significance. Geng et al. [10] made a comprehensive evaluation of the experimental phase equilibria, crystal structures and thermochemical properties and therefore assessed the Gibbs free energies of all the phases of the system. The Nb-Si binary system includes the solid solution phases bcc(Nb) and Diamond (Si), the intermetallic compounds Nb 3 Si, aNb 5 Si 3 , bNb 5 Si 3 ,N b S i 2 and the liquid phase. The V-Si binary system was optimized by Zhang et al. [11]. The V-Si binary system includes the solid solution phases bcc(V) and Diamond (Si), four stable intermetallic compounds V 3 Si, V 5 Si 3 ,V 6 Si 5 [12], VSi 2 , and the liquid phase. Kumar et al. [13] [14] observed the existence of a miscibility gap and spinodal decomposition in the low temperature region (T < 804 8C), and re-optimized the system accordingly. There are few early reports of the phase equilibria of the Nb-Si-V ternary system [15,16]. Li et al. [17] measured the phase equilibrium relations of the Nb-Si-V ternary system at three temperatures of 1 100, 1200 and 13008C and found a new linear-stoichiometric compound (Nb,V) 2 Si, There are relatively few studies on the liquidus projection of the system. The isothermal section of the Nb-Si-V ternary system at 1 300 8C determined by Li et al. [17] and binary phase diagrams [10,11,14] constituting the ternary system are shown in Fig. 1.
In order to assess the Nb-Si-V ternary system thermodynamically, the experimental liquidus projection is essential for determining the phase relations between the liquid phase and the primary solidified phase as well as for analyzing the solidification paths of the as-cast alloys. In addition, the newly found linear stoichiometric compound (Nb,V) 2 Si needs to be further confirmed. Therefore, the present study targets the solidification processes of the as-cast alloys and the phase equilibria at 1 300 8C of the Nb-Si-V ternary system.

Experimental procedure
Niobium (99.95 wt.%), silicon (99.99 wt.%) and vanadium (99.99 wt.%) from Trillion Metals Co. Ltd. were used as the raw materials, and then were melted into 5 g button-like specimens by vacuum arc furnace. Each alloy specimen was turned over at least 3 times during the melting process to ensure a sufficiently homogeneous composition. In the meanwhile, some sponge titanium was used to absorb oxygen. The weight losses of the prepared alloys were less than 1 %. The as-cast alloys were ground and then polished directly to study the liquidus projection. Specimens sealed Fig. 1. Binary phase diagrams constituting the Nb-Si-V ternary system [10,11,14] and the isothermal section at 1 300 8C determined by Li et al. [17]. J. Gao et al.: Solidification processes of as-cast alloys and phase equilibria at 1 300 8C of the Nb-Si-V ternary system in argon filled quartz tubes were isothermally treated at 1 300 8C for 240 h, and were then used to study the isothermal section. The microstructural observation, the phase identification and the composition determination were performed by using SEM (scanning electron microscopy, JSM-6480LV), XRD (X-ray diffraction, STOFDARM-STADT-STOE/2, 3 KW) and EPMA (electron probe microanalysis, GEOLJXA-8230), respectively.

Experimental results of the liquidus projection
The information of all the stable binary solid phases in the Nb-Si, the V-Si and the Nb-V binary systems constituting the Nb-Si-V ternary system is summarized in Table 1 [18 -20]. From Table 1, it can be seen that bNb 5 Si 3 and V 5 Si 3 ,just like NbSi 2 and VSi 2 , are phases of the same structure but different compositions, while Nb 3 Si and V 3 Si are the phases of the same stoichiometry but different structures. Therefore, in the Nb-Si-V ternary system, bNb 5 Si 3 with V 5 Si 3 and NbSi 2 with VSi 2 form two linear compounds, bNb(V) 5 Si 3 or V(Nb) 5 Si 3 and Nb(V)Si 2 or V(Nb)Si 2 .
In the present work, a series of as-cast alloys have been studied. The primary solidification phases, the compositions of the constituent phases and the precipitation paths of the alloys are listed in Table 2. There is no report on the structural refinement of the newly observed linear-stoichiometric compound (Nb,V) 2 Si. The constructed liquidus projection for the Nb-Si-V ternary system is shown in Fig. 2.
In general, the phase with the largest morphology or the dendritic structure in the SEM/BSE image is the primary phase precipitated directly from the liquid, which is determined by combining XRD analysis and EPMA determination. The secondary solidified phase and the related reaction type are determined according to the morphology next to the primary phase. If a phase adjoining the primary phase is formed, a peritectic reaction is occurring concurrently, (primary phase) + liquid ? (secondary phase). Here after, the secondary phase precipitates directly from the liquid. While if a microstructure with the two-phase eutectic feature, usually with a finer microstructure than that of the primary phase, is formed near the primary phase, a eutectic reaction occurs, liquid ? (primary phase) + (secondary phase). Sometimes, the peritectic reactions occur successively, one after another. Occasionally, a three-phase eutectic microstructure is formed with the finest morphology after the two-phase eutectic. In such a way, the solidification process continues until the liquid phase is exhausted. Typical SEM/ BSE micrographs and the X-ray diffractograms are analyzed as follows.
3.1.3. Primary solidification region of V(Nb) 5 Si 3 As is shown in Fig. 5, the experimental results of the alloys A4# Nb-55Si-42V and A5# Nb-40Si-57V, indicate that   3 Si their microstructures are composed of the white V(Nb) 5 Si 3 phase and the gray V(Nb)Si 2 phase. The solidification path of each alloy is as follows: the primary phase V(Nb) 5 Si 3 first precipitated from liquid L ? V(Nb) 5 Si 3 , then the liquid composition reached the univariant line of the eutectic reaction L ? V(Nb)Si 2 + V(Nb) 5 Si 3 , and the liquid composition moved forward along the univariant line until the solidification process was over.
As is shown in Fig. 6, the SEM/BSE micrograph and the X-ray diffractogram of the as-cast alloy A6# Nb-55Si-30V indicate that its microstructure is composed of the white V(Nb) 5 Si 3 phase, the light gray Nb(V)Si 2 phase and the havy gray V(Nb)Si 2 phase. The solidification path of the alloy is as follows: the primary phase V(Nb) 5 Si 3 first solidified from liquid L ? V(Nb) 5 Si 3 , then V(Nb) 5 Si 3 and Nb(V)Si 2 solidified eutectically from liquid L ? V(Nb) 5 Si 3 + Nb(V)Si 2 , and then the liquid composition moved forward along the univariant line until reached the invariant reaction L + Nb(V)Si 2 ? V(Nb) 5 Si 3 + V(Nb)Si 2 .
As is shown in Fig. 7, the SEM/BSE micrograph and the X-ray diffractogram of the as-cast alloy A10# Nb-30Si-60V indicate that the microstructure is composed of the light gray V(Nb) 5 Si 3 phase, the white bNb(V) 5 Si 3 phase and the heavy gray V(Nb) 3 Si phase. Different from the alloy A6#, the solidification path of the alloy A10# is as follows: the primary phase V(Nb) 5 Si 3 first solidified from liquid L ? V(Nb) 5 Si 3 , then the liquid composition reached the univariant line, L ? bNb(V) 5 Si 3 + V(Nb) 5 Si 3 . and finally the liquid composition moved forward along the univariant line until reached the eutectic invariant reaction L ? bNb(V) 5 Si 3 + V(Nb) 5 Si 3 + V(Nb) 3 Si. 5 Si 3 SEM/BSE micrographs and the X-ray diffractograms of the alloys A12# Nb-50Si-15V and A13# Nb-50Si-22V are shown in Fig. 8, indicating that the microstructures of both alloys are  J. Gao et al.: Solidification processes of as-cast alloys and phase equilibria at 1 300 8C of the Nb-Si-V ternary system composed of the white bNb(V) 5 Si 3 phase, the gray V(Nb) 5 Si 3 phase and the dark Nb(V)Si 2 . The solidification path is as follows: the white primary phase bNb(V) 5 Si 3 first solidified from liquid L ? bNb(V) 5 Si 3 , then the composition of liquid phase moved to L ? bNb(V) 5 Si 3 +Nb(V) Si 2 , and along the the univariant line the liquid composition reached the invariant reaction L + bNb(V) 5 Si 3 ? V(Nb) 5 Si 3 +Nb(V)Si 2 .

Primary solidification region of bNb(V)
As is shown in Fig. 9a, the microstructure of the alloy A14# Nb-40Si-15V is composed of the white bNb(V) 5 Si 3 phase and the gray Nb(V)Si 2 phase. Both the phases bNb(V) 5    J. Gao et al.: Solidification processes of as-cast alloys and phase equilibria at 1 300 8C of the Nb-Si-V ternary system structure. Therefore, the solidification path of the alloy A14# is as follows: the primary phase bNb(V) 5 Si 3 first solidified from liquid L ? bNb(V) 5 Si 3 , and then the liquid composition reached the univariant line L ? bNb(V) 5 Si 3 + Nb(V)Si 2 . The microstructure of the alloy A16# Nb-40Si-35V, as shown in Fig. 9b, is composed of the white bNb(V) 5 Si 3 phase, the light gray V(Nb) 5 Si 3 phase and the heavy gray Nb(V)Si 2 phase. Different from that of the alloy A14#, the solidification path of the alloy A16# is as follows: the primary phase bNb(V) 5 Si 3 first solidified from liquid L ? bNb(V) 5 Si 3 , then the liquid composition moved to the univariant line, L ? bNb(V) 5 Si 3 +V(Nb) 5 Si 3 , along which the liquid composition reached the invariant reaction L + bNb(V) 5 Si 3 ? V(Nb) 5 Si 3 + Nb(V)Si 2 .
SEM/BSE micrographs and the X-ray diffractograms of the alloys A17# Nb-30Si-18V and A18# Nb-30Si-28V are shown  J. Gao et al.: Solidification processes of as-cast alloys and phase equilibria at 1 300 8C of the Nb-Si-V ternary system in Fig. 10. The major constituent phase of the alloy A17# is bNb(V) 5 Si 3 , as shown in Fig. 10a. The microstructure of the alloy A18# is composed of the white primary phase bNb(V) 5 Si 3 and the eutectic structure formed by the two phases bNb(V) 5 Si 3 (the white phase) and V(Nb) 3 Si (the gray phase).
3.1.5. Primary solidification region of V(Nb) 3 Si As is shown in Fig. 11, the microstructure of A20# Nb-20Si-65V is composed of the dark gray primary phase V(Nb) 3 Si and the eutectic structure formed by the two phases V(Nb) 3 Si (the dark gray phase) and Bcc(Nb,Si,V) (the white phase).

Primary solidification region of (Nb,V) 2 Si
The SEM/BSE micrographs of A21# Nb-17Si-30V and A22# Nb-20SiV-45V are shown in Fig. 12. The microstructure of the alloy A21# is composed of the gray primary phase (Nb,V) 2 Si and the eutectic structure formed by the two phases (Nb,V) 2 Si (the gray phase) and bcc(Nb,Si,V) (the white phase). It is very important to note that the newly discovered ternary linear compound (Nb,V) 2 Si has been reported in the previous study of Li et al. [17] and has been found in both the as-cast and the isothermal alloys in the present work. This compound has a stoichiometric ratio of 2 : 1 for (Nb,V) : Si, while its refined crystal structure has not been reported yet. The microstructure of the alloy A22# is composed of the primary white phase (Nb,V) 2 Si and the eutectic structure formed by the two phases (Nb,V) 2 Si (the white phase) and bcc(Nb,Si,V) (the gray phase).

Primary solidification region of bcc(Nb,Si,V)
SEM/BSE morphologies and X-ray diffractograms for the alloys A25# Nb-15Si-20V and A26# Nb-17Si-12V are shown in Fig. 13. The microstructures of both the alloys A25# and A26# are composed of the white primary phase bcc(Nb,Si,V), and the eutectic structure formed by the two phases bcc(Nb,Si,V) (the white phase) and bNb(V) 5 Si 3 .( t h eg r a y phase). The microstructure of the alloy A23# Nb-18Si-5V, as  J. Gao et al.: Solidification processes of as-cast alloys and phase equilibria at 1 300 8C of the Nb-Si-V ternary system shown in Fig. 14, is composed of the fine binary eutectic structure formed by the two phases Bcc(Nb,Si,V) (the white phase) and bNb(V) 5 Si 3 (the gray phase), and the even more fine ternary eutectic structure formed by the three phases bcc(Nb,Si,V) (the white phase), bNb(V) 5 Si 3 (the gray phase) and Nb(V) 3 Si (the phase indicated by XRD diffractogram).
The microstructure of the alloy A24# Nb-10Si-35V is composed of the white primary phase bcc(Nb,Si,V) and the black phase (Nb,V) 2 Si. The precipitation path of the alloy A24# is that the white phase bcc(Nb,Si,V) is first precipitated from liquid. Then the liquid composition reached the univariant line, L ? (Nb,V) 2 Si + bcc(Nb,Si,V).
The X-ray diffractograms and BSE micrographs of Nb-10Si-(50,80)V (A27#,A29#) alloys are shown in Fig. 15. The microstructures of the alloys are all gray-white lamellar eutectic structures of bcc(Nb,Si,V)/V(Nb) 3 Si. The nominal compositions of the alloys are all very close to the univariant line of bcc(Nb,Si,V)/V(Nb) 3 Si.

Conclusion
The liquid phase projection and the isothermal section at 1 300 8C of the Nb-Si-V ternary system have been experimentally studied by means of SEM/BSE, XRD and EPMA for observing the microstructures, identifying the constituent phases and measuring the phase compositions, respectively. The microstructures and the precipitation paths of the as-cast alloys were analyzed for constructing the liquid phase projection. The constituent phases and the related equilibrium compositions are characterized for determining the isothermal section. The existence of the linear compound (Nb,V) 2 Si was confirmed in studying both the liquid phase projection and the isothermal section at 1 300 8C. Together with the literature reports, the present work outlines the phase relations in the temperature-composition space of the Nb-Si-V ternary system.