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Experimental study on phase relationships in the Si-rich portion of the Nb–Si–W ternary system
CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry 41 (2013) 60–70
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Experimental study on phase relationships in the Si-rich portion of the Nb–Si–W ternary system Yan Li a, Changrong Li a,n, Zhenmin Du a, Cuiping Guo a, Jingbo Li b a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2012 Received in revised form 9 January 2013 Accepted 12 January 2013 Available online 4 February 2013
This paper describes the experimental study on liquidus projection and the isothermal section at 1873 K of the Nb–Si–W ternary system in the Si-rich portion. The microstructures and solidification paths of the as-cast alloys were analysed. The constituent phases and their equilibrium compositions of the as-cast þ heat-treated alloys were determined. The microstructure observation, the phase identification and the composition measurement were performed using scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron probe microanalysis (EPMA), respectively. No ternary compound was found. For the liquidus projection in the investigated Si-rich portion, there exist four primary solidification regions, Diamond–Si, b(Nb,W)5Si3, Nb(W)Si2 and W(Nb)Si2, and two eutectic invariant reactions, L-b(Nb,W)5Si3 þW(Nb)Si2 þ Nb(W)Si2 and L-W(Nb)Si2 þNb(W)Si2 þ Diamond–Si. The type of the latter invariant reaction was determined with the help of the thermodynamic assessment of the system. Together with literature reported investigations for the Nb–W-rich portion, the whole liquidus projection of the Nb–Si–W ternary system was constructed. For the isothermal section at 1873 K in the investigated Si-rich portion, there are three three-phase regions, liquid þ Nb(W)Si2 þW(Nb)Si2, Nb(W)Si2 þ aNb(W)5Si3 þ b(Nb,W)5Si3 and Nb(W)Si2 þ W(Nb)Si2 þ b(Nb,W)5Si3, and seven two-phase regions, liquid þNb(W)Si2, Nb(W)Si2 þ aNb(W)5Si3, aNb(W)5Si3 þ b(Nb,W)5Si3, Nb(W)Si2 þ b(Nb,W)5Si3, liquid þ W(Nb)Si2, Nb(W)Si2 þ W(Nb)Si2 and W(Nb)Si2 þ b(Nb,W)5Si3. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Nb–Si–W ternary system Liquidus projection Isothermal section Microstructure Solidification path
1. Introduction Nbss/Nb5Si3 in situ composites exhibit a good balance in mechanical properties between the room temperature toughening provided by the Nb-rich solid solution phase Nbss and the high temperature strength up to 1500 1C, provided by the intermetallic phase Nb5Si3 [1]. Nbss/Nb5Si3 in situ composites, generally based on the phase equilibrium relationship of the Nb-rich region of the Nb–Si phase diagram, have been studied by many researchers [2–5]. Mendiratta et al. [2] investigated the as-castþhot-extrudedþheat-treated NbssþNb5Si3 two-phase alloys with compositions of 10 and 16 at% Si, showing the ductile-phase toughening at low temperatures and the reasonable strength retention at high temperatures (1400–1600 1C). The pffiffiffiffiffi fracture toughness was about 21 MPa m for the Nb–10Si alloy with NbssþNb5Si3 two-phases, while that of Nb5Si3 intermetallic pffiffiffiffiffi phase has been reported to be about 3 MPa m at room temperature [6]. The fracture toughnesses for the as-cast þhot-extrudedþheat-treated Nb–16.5Si alloy were also found to be
n
Corresponding author. Tel.: þ86 10 82377789; fax: þ86 10 62333772. E-mail address: [email protected] (C. Li).
0364-5916/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.calphad.2013.01.002
pffiffiffiffiffi pffiffiffiffiffi 12.8 MPa m at room temperature and 17 MPa m at 1400 1C by Mendiratta et al. [3]. Bewlay et al. [4] found the fracture toughness of the directionally solidified Nb–Si in situ composites (10–25 at% Si) was about 50% more than that of the as-cast alloys and 20% less than that of the extruded alloys of the equivalent compositions. Swkido et al. [5] investigated the directionally solidified and arc-melted Nb–17.5Si alloys with or without 10 at% Ti. The directionally solidified alloys had the higher fracture toughness compared with the arc-melted ones, in accordance with Ref. [4]; the fracture toughness should be improved by low-rate solidification, Ti alloying or heat-treatment; for the pffiffiffiffiffi Nb–17.5Si alloy, the fracture toughness of 14.5 MPa m at room temperature and the compressive strength of 580 MPa at 1673 K were achieved by directional solidification at the rate of 10 mm/h. However, one major impediment of the Nbss/Nb5Si3 alloys for high-temperature applications is their catastrophic oxidation behaviour. To obtain material systems with improved environmental resistance while retaining their structural properties, studies with addition of alloying elements, such as Ti, Al, Hf, Cr, Mo and W, were initiated on the Nbss/Nb5Si3 in situ composites. Sha et al. [7,8] studied the effect of W addition on the yield strength of Nb–10Mo–10Ti–18Si-based in situ composites, and
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found that the yield strength (s0.2) and the specific yield strength (s0.2S) (s0.2 divided by the density) at 1670 K increased markedly with increasing W content; the directionally solidified samples showed higher s0.2 and s0.2S than the as-cast samples; even at 1770 K, the directionally solidified sample with 15 at% W showed a high s0.2 of about 650 MPa. Sha et al. [9] also found that the Nb–10W alloy showed the high fracture toughness of about pffiffiffiffiffi 15.3 MPa m and the low s0.2 of about 90 MPa at 1670 K while the Nb–10Si-10W showed the high s0.2 of about 330 MPa at pffiffiffiffiffi 1670 K and the low fracture toughness of about 8.2 MPa m, and concluded that the toughness was supplied by the metallic Nbss phase and the high-temperature strength was mainly provided by the brittle silicide phase. Ma et al. [10] found that both the lowtemperature deformability and the high-temperature strength of Nbss/Nb–silicide in situ composites were improved by the addition of appropriate amounts of Mo and/or W. By the addition of W, Xiong et al. [11] also improved significantly the oxidation resistance of Nbss/Nb5Si3 in situ composite (Nb–20Si–10W alloy). Consequently, by alloying with W, the Nbss/Nb5Si3 in situ composites may get a good balance among high-temperature strength, room temperature toughness and high-temperature oxidation resistance. The Nb–Si–W ternary system is becoming an important system to be investigated for the development of the Nb–Si-based high temperature materials. The phase relationship is an important guidance for materials design and also an essential basis for the CALPHAD (CALculation of PHAse Diagrams) approach. In order to get a comprehensive and consistent thermodynamic description for one of the phases of a ternary system, it is necessary to understand the phase equilibrium behaviour of the other related phases in the system. For the Nb–Si–W ternary system, literature reports are mainly concentrated on the Nb–W-rich portion with the application importance. From the view point of the thermodynamic assessment of the system, the purpose of the present paper is to study the constituent phases and the solidification processes of the as-cast alloys in the Si-rich portion of the Nb–Si–W ternary system, so that the whole liquidus projection of the Nb–Si–W ternary system can be constructed together with the literature reported investigations for the Nb–W-rich portion. Meanwhile, some selected as-cast alloys were heat-treated at 1873 K for 72 h and by studying the phase equilibria of these alloys, the isothermal section for the Si-rich portion was obtained.
Table 2 Invariant reactions involving the liquid phase in the W–Si system [13].
2. Literature study on the phase diagrams of the binary and the ternary systems
Ma et al. [14] constructed a partial liquidus projection of the ternary system in the Nb–W rich region up to 37.5 at% Si, as shown in Fig. 1. There exist three primary phase regions, Bcc, b(Nb,W)5Si3 and Nb(W)3Si, and one invariant point for the transitional invariant reaction Lþ b(Nb,W)5Si3-Nb(W)3SiþBcc. The liquidus projection for the Si-rich portion has not been reported.
2.1. The Nb–Si binary phase diagram The Nb–Si binary phase diagram is based on the assessment by Geng et al. [12]. The stable phases in this system are the terminal phases Bcc-Nbss and Diamond–Si, the intermetallic compounds Nb3Si, aNb5Si3, bNb5Si3 and NbSi2 and the liquid phase referred to
Table 1 Invariant reactions involving the liquid phase in the Nb–Si system [12]. Reaction type
Reaction
Composition of L (at% Si )
T (1C)
Congruent Congruent Eutectic Peritectic Eutectic Eutectic
L-bNb5Si3 L-NbSi2 L-Bcc þ Nb3Si Lþ bNb5Si3-Nb3Si L-bNb5Si3 þNbSi2 L-NbSi2 þDiamond–Si
39.0 66.7 17.3 20.3 57.7 98.3
2526 1962 1916 1977 1873 1407
Reaction type
Reaction
Composition of L (at% Si )
T (1C)
Congruent Congruent Eutectic Eutectic Eutectic
L-W5Si3 L-WSi2 L-Bcc þ W5Si3 L-W5Si3 þ WSi2 L-WSi2 þ Diamond–Si
0.375 0.667 0.329 0.525 0.960
2108 2002 2089 1940 1388
Fig. 1. Liquidus projection for the Nb–W-rich portion of the Nb–Si–W ternary system [14].
as ‘‘L’’ hereafter. The invariant reactions involving the liquid phase are listed in Table 1. 2.2. The W–Si binary phase diagram The W–Si binary phase diagram is based on the assessment by Vahlas at al. [13]. The stable phases in this system are the terminal phases Bcc-Wss and Diamond–Si, the intermetallic compounds W5Si3 and WSi2 and the liquid phase. The invariant reactions involving the liquid phase are listed in Table 2. 2.3. The Nb–W binary phase diagram The Nb–W binary system was relatively simple, involving an infinite soluble solid phase and a liquid phase. No invariant reaction exists. 2.4. Previous experiments on the liquidus projection of the Nb–Si–W ternary system
2.5. Previous experiments on the isothermal section of the Nb–Si–W ternary system There is not much information about the phase equilibria of the Nb–Si–W ternary system. Ma et al. [14] speculated a partial isothermal section for the Nb–W rich portion of Nb–Si–W ternary system at 1973 K. The samples were arc-melted followed by annealing at 1973 K for 48 h and furnace cooled. 3. Experimental procedures Each sample with a gross weight about 5 g was arc-melted by a non-consumable tungsten electrode in a water-cooled copper
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crucible under an argon atmosphere, and was remelted five times to get a homogeneous specimen. The purities of the raw materials are 99.999 wt% for Si, 99.95 wt% for W and 99.9 wt% for Nb. Since the weight loss of each sample measured after preparation was less than 1 wt%, the compositions of the alloys were considered to be equal to their nominal compositions. The isothermal heat treatment was conducted at 1873 K for 72 h in an argon furnace. The as-cast alloys were used to determine the primarily crystallised phases, to construct the liquidus projection diagram, and to analyse the solidification processes. And the as-cast þheattreated samples were prepared to measure the phase equilibria in the isothermal section. The microstructure observation, the phase identification and the composition measurement were performed using scanning
electron microscopy (SEM) working in a back-scattered electron image (BEI) mode, X-ray diffraction (XRD) and electron probe microanalysis (EPMA), respectively. The Materials Data Inc. software Jade 5.0 [15] and Powder Diffraction File (PDF released 2002) were used for XRD analysis.
4. Results and discussion 4.1. Liquidus projection in the Si-rich portion A set of 10Nb–Si–W ternary alloys were studied to investigate the microstructure and phase relationship for the Si-rich portion of the system. The compositions (at%) of the experimental ingots
Fig. 2. Liquidus projections of the Nb–Si–W ternary system for (a) the Si-rich portion and (b) the whole system.
Table 3 Phases present in the as-cast microstructures together with the bulk alloy compositions. Primary phase
Nominal composition
As-cast phases
Solidification path
b(Nb,W)5Si3
Nb–50Si–10W (]1)
b(Nb,W)5Si3
L-b(Nb,W)5Si3 L-b(Nb,W)5Si3 þNb(W)Si2
Nb–50Si–20W (]2) Nb–50Si–30W (]3) Nb–50Si–40W (]4)
W(Nb)Si2
Nb–60Si–30W (]7)
Nb–70Si–20W (]9)
Nb–80Si–10W (]10)
Nb(W)Si2
Nb–60Si–10W (]5) Nb–60Si–20W (]6) Nb–70Si–10W (]8)
a
Nb(W)Si2 gNb(W)5Si3a b(Nb,W)5Si3 Nb(W)Si2 b(Nb,W)5Si3 Nb(W)Si2 b(Nb,W)5Si3 Nb(W)Si2 W(Nb)Si2
L-b(Nb,W)5Si3 L-b(Nb,W)5Si3 þNb(W)Si2 L-b(Nb,W)5Si3 L-b(Nb,W)5Si3 þNb(W)Si2 L-b(Nb,W)5Si3 L-b(Nb,W)5Si3 þW(Nb)Si2 L-b(Nb,W)5Si3 þW(Nb)Si2 þ Nb(W)Si2
W(Nb)Si2 Nb(W)Si2 b(Nb,W)5Si3 W(Nb)Si2 Nb(W)Si2 Diamond–Si W(Nb)Si2 Nb(W)Si2 Diamond–Si
L-W(Nb)Si2 L-W(Nb)Si2 þ b(Nb,W)5Si3 L-W(Nb)Si2 þ b(Nb,W)5Si3 þ Nb(W)Si2 L-W(Nb)Si2 L-W(Nb)Si2 þNb(W)Si2 L-W(Nb)Si2 þNb(W)Si2 þ Diamond–Si L-W(Nb)Si2 L-W(Nb)Si2 þNb(W)Si2 L-W(Nb)Si2 þNb(W)Si2 þ Diamond–Si
Nb(W)Si2 b(Nb,W)5Si3 Nb(W)Si2 b(Nb,W)5Si3 Nb(W)Si2 Diamond–Si
L-Nb(W)Si2 L-Nb(W)Si2 þ b(Nb,W)5Si3 L-Nb(W)Si2 L-Nb(W)Si2 þ b(Nb,W)5Si3 L-Nb(W)Si2 L-W(Nb)Si2 þNb(W)Si2 L-W(Nb)Si2 þNb(W)Si2 þ Diamond–Si
A metastable phase which often occurs in the multiple alloys of the Nb–Si-based systems.
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Table 4 Crystallographic results of different phases in different Nb–Si–W ternary alloys. Phases
Structure type
Compositions (at%) Nb
bNb5Si3
tI32–Si3W5
W5Si3
tI32–Si3W5
NbSi2
hP9–CrSi2
0.625
Si
W
a
0.375 0.375
0.333
Lattice parameters (nm)
0.625
0.667 0.667
c
1.0026
0.50717
Reference [16]
0.9605(5)
0.4964(5)
Reference [16]
0.4819(2)
0.6592(2)
Reference [16]
WSi2
tI6–MoSi2
0.3210
0.7829
Reference [16]
bNb(W)5Si3
tI32–Si3W5
49.6 13.32 22 42.71 46.67 16.16
37.70 34.15 34.84 39.15 38.66 37.59
12.7 52.53 43.16 18.14 14.67 46.25
0.9994(1) 0.9824(5) 0.9894(7) 0.9970(7) 0.9982(6) 0.9865(7)
0.5007(2) 0.5009(7) 0.5018(5) 0.5156(7) 0.5051(7) 0.4960(6)
In In In In In In
as-cast ]1 alloy as-cast ]4 alloy as-cast ]7 alloy as-cast ]5 alloy heat-treated ]1 alloy heat-treated ]7 alloy
Nb(W)Si2
hP9–CrSi2
30.7 18.91 19.12 19.7 29.08 32.39 20.23
66.38 64.75 65.55 65.53 65.42 66.79 66.78
2.92 16.34 15.33 14.77 5.5 0.82 12.99
0.4800(4) 0.4722(9) 0.4744(2) 0.4749(4) 0.4798(8) 0.4804(5) 0.4757(7)
0.6690(2) 0.6561(1) 0.6544(7) 0.6525(3) 0.6656(2) 0.6695(4) 0.6588(9)
In In In In In In In
as-cast ]1 alloy as-cast ]4 alloy as-cast ]7 alloy as-cast ]9 alloy as-cast ]5 alloy heat-treated ]1 alloy heat-treated ]7 alloy
W(Nb)Si2
tI6–MoSi2
3.46 4.12 5.13 6.04
66.14 66.2 66.15 66.75
30.4 29.68 28.72 27.21
0.3250(2) 0.3253(1) 0.3258(4) 0.3259(6)
0.7859(4) 0.7782(6) 0.7850(0) 0.7905(7)
In In In In
as-cast ]4 alloy as-cast ]7 alloy as-cast ]9 alloy heat-treated ]7 alloy
are marked in Fig. 2(a) and their constituent phases as well as solidification processes are summarized in Table 3. The lattice parameters of the phases with different compositions in different ternary alloys are shown in Table 4, and those of the binary compounds listed in the table are from Daams et al. [16]. The compounds bNb5Si3 and W5Si3 are of the same crystal structure with D8m-Strukturbericht designation/Si3W5-prototype and can form an infinite solid solution which is referred to as b(Nb,W)5Si3 in the present work. The compounds NbSi2 and WSi2 are of different crystal structures, with C40-Strukturbericht designation/ CrSi2-prototype and C11b-Strukturbericht designation/ MoSi2-prototype respectively and cannot form an infinite solid solution. The intermetallic compound NbSi2 with the dissolved W is referred to as Nb(W)Si2 and the WSi2 with the dissolved Nb as W(Nb)Si2. The constructed liquidus projection for the Si-rich portion of the Nb–Si–W ternary system is shown in Fig. 2(a). The univariant curves and the invariant points in the liquidus projection were determined according to both the experimental data and the optimization results, and the latter will be described in detail in the thermodynamic assessment part of the subsequent article. There exist four primary solidification regions, Diamond–Si, b(Nb,W)5Si3, Nb(W)Si2 and W(Nb)Si2, one transitional invariant reaction Lþ W(Nb)Si2-b(Nb,W)5Si3 þNb(W)Si2 and another eutectic one L-Nb(W)Si2 þDiamond–SiþW(Nb)Si2. Together with the literature reported investigations for the Nb–W-rich portion, the liquidus projection of the whole Nb–Si–W ternary system is shown in Fig. 2(b). More detailed descriptions of Table 3 and Fig. 2 are given in the following sections. 4.1.1. Primary solidification region of b(Nb,W)5Si3 4.1.1.1. Nb–50Si–(10,20,30)W (]1–3). X-ray diffractogram of Nb– 50Si–10W (]1) and BEI micrographs of Nb–50Si–(10,20,30)W (]1–3) as-cast alloys are shown in Fig. 3. For ]1 alloy, the microstructure consists of a large and white primary phase of tetragonal morphology, a whiteþdark two-phase eutectic, in which a small quantity of grey flakes of a third phase are dispersed. The crystal
0.333
Remark
structure and the composition of the primary phase were determined, using XRD and EPMA respectively, to be b(Nb,W)5Si3 with an average composition of Nb–37.70Si–12.70W, those of the dark phase in the eutectic to be Nb(W)Si2 of Nb–66.38Si–2.92W and, those of the dispersed grey phase to be gNb(W)5Si3 of Nb–37.39Si– 7.48W. The symbols of X-ray results are labelled according to the experimental measurements. For example, the solid square for Nb(W)Si2 in Fig. 3(a), the element W can substitute Nb in the NbSi2 with CrSi2-prototype crystal structure. Since the atomic radius of the element W (rW ¼0.139 nm) is smaller than that of Nb (rNb ¼0.146 nm), the lattice parameters as well as the spaces between the atomic planes d will decrease with the increase of W content. According to Bragg’s diffraction condition, nl ¼ 2dsin y, when the integer n and the incident wavelength l are kept constant, the angle y between the incident ray and the scattering planes will increase with the decrease of d. Therefore, the experimental value 2y of the ternary Nb(W)Si2 in Fig. 3(a) moves to the right with the increase of W content, resulting the smaller lattice parameters than those of the binary compound NbSi2 as listed in Table 4. Similar to ]1 alloy, the microstructures of both ]2 and ]3 alloys consist of a large and white primary phase b(Nb,W)5Si3 of tetragonal morphology and a whiteþdark two-phase eutectic b(Nb,W)5Si3 þNb(W)Si2. For ]2 and ]3 alloys, the average compositions of the primary phase b(Nb,W)5Si3 are Nb–35.34Si– 25.37W and Nb–34.78Si–37.53W, those of the dark phase Nb(W)Si2 in the eutectic are Nb–65.38Si–8.12W and Nb–66.02Si– 12.23W, respectively. The only difference from ]1 alloy is no gNb(W)5Si3 dispersed in the eutectic. The three alloys ]1, ]2 and ]3 are in the area with the same primarily crystallized phase b(Nb,W)5Si3, which is of a large and white tetragonal morphology. Since there is no gNb(W)5Si3 dispersed in the eutectic structure of ]2 and ]3 alloys, the small quantity of grey flakes gNb(W)5Si3 in ]1 alloy is suspected to be a metastable phase. gNb5Si3 is a metastable phase in the Nb–Si binary system, which can be obtained by the addition of C [17,18] or Ti [19] or Hf [20] in the ternary or multi-component alloy systems because of the stabilization by the interstitial C atoms or the isostructure
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Fig. 3. Experimental results of Nb–50Si–(10,20,30)W (]1–3) as-cast alloys. (a) X-ray diffractograms of ]1 and ]4 alloys; (b) BEI micrograph of ]1 alloy; (c) BEI micrograph of ]2 alloy and (d) BEI micrograph of ]3 alloy.
Fig. 4. BEI micrographs of Nb–50Si–40W (]4) as-cast alloy at (a) low and (b) high magnification.
compounds of the hexagonal Ti5Si3 and Hf5Si3. Geng et al. [21] studied the as-cast solidification of the Si-rich alloys of the Nb–Si–Mo ternary system, and found that Nb(Mo)Si2–C40 can
also stabilize the g(Nb,Mo)5Si3 because both the phases Nb(Mo)Si2 and g(Nb,Mo)5Si3 are of the hexagonal structure, a low energy interface can be formed between Nb(Mo)Si2
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and g(Nb,Mo)5Si3 when the crystallographic relationship of (0001)C40//(0001)g and [1100]C40//[2110]g is satisfied and a high coincidence site lattice is obtained. In the present work, the existence of the metastable gNb(W)5Si3 is deduced also due to the divorced Nb(W)Si2–C40 in the eutectic structure. After the isothermal heat-treatment at 1873 K as shown in Section 4.2, gNb(W)5Si3 in ]1 alloy cannot be found any more, which is another proof that gNb(W)5Si3 is a metastable phase. The solidification path is revealed by the microstructural morphology. The large primary phase b(Nb,W)5Si3 first solidified from the liquid, L-b(Nb,W)5Si3; then the liquid composition reached the univariant line, L-b(Nb,W)5Si3 þNb(W)Si2, along which the liquid composition moved forward until the solidification process terminated. 4.1.1.2. Nb–50Si–40W (]4). X-ray diffractogram and BEI micrographs of Nb–50Si–40W (]4) as-cast alloy are shown in Figs. 3(a) and 4, respectively. The microstructure consists of a large and white primary phase b(Nb,W)5Si3 of tetragonal morphology, a grey phase W(Nb)Si2 of tetragonal morphology and a dark phase Nb(W)Si2 in the eutectic. The average compositions of b(Nb,W)5Si3, W(Nb)Si2 and Nb(W)Si2 are Nb–34.15Si–52.53W, Nb–66.14Si–30.4W and Nb–64.75Si–16.34W, respectively. The solidification path of ]4 alloy is that the primary phase b(Nb,W)5Si3 first solidified from liquid, L-b(Nb,W)5Si3; after that the liquid composition reached the univariant line, L-b(Nb,W)5 Si3 þW(Nb)Si2, where the divorced eutectic solidification took place in some places; finally, the liquid composition reached the eutectic invariant reaction, L-b(Nb,W)5Si3 þW(Nb)Si2 þNb(W)Si2. 4.1.2. Primary solidification region of W(Nb)Si2 4.1.2.1. Nb–60Si–30W (]7). X-ray diffractogram and BEI micrographs of Nb–60Si–30W (]7) as-cast alloy are shown in Fig. 5. The microstructure consists of a large and grey primary phase W(Nb)Si2 of tetragonal morphology, a coarse eutectic of greyþ white two phases W(Nb)Si2 þ b(Nb,W)5Si3 and an eutectic of whiteþdarkþgrey three phases b(Nb,W)5Si3 þNb(W)Si2 þW(Nb) Si2. The average compositions of W(Nb)Si2, b(Nb,W)5Si3 and Nb(W)Si2 in Fig. 5(c) are Nb–66.2Si–29.68 W, Nb–34.84Si–43.16 W, and Nb–65.55Si–15.33W, respectively. The solidification path of ]7 alloy is that the primary phase W(Nb)Si2 first solidified from liquid, L-W(Nb)Si2; after that the liquid composition reached the univariant line, L-b(Nb,W)5Si3 þ W(Nb)Si2, where the coarse eutectic of W(Nb)Si2 þ b(Nb,W)5Si3 formed; finally, the liquid composition reached the eutectic invariant reaction, L-b(Nb,W)5Si3 þW(Nb)Si2 þNb(W)Si2. 4.1.2.2. Nb–70Si–20W (]9). X-ray diffractogram and BEI micrographs of Nb–70Si–20W (]9) as-cast alloy are shown in Figs. 5(a) and 6, respectively. The microstructure consists of a white primary phase W(Nb)Si2 of tetragonal morphology with an average composition of Nb–66.15Si–28.72W, a grey Nb(W)Si2 phase of Nb–65.53Si–14.77W, a dark Diamond–Si phase of Nb–99.99Si, and a small quantity of the eutectic Nb(W)Si2 þW(Nb)Si2 þDiamond–Si. In this three-phase eutectic structure near the Si corner, Diamond–Si is the dominant phase with a large volume fraction and the distinct eutectic feature is hardly observed. The solidification path is that the primary phase W(Nb)Si2 first solidified from liquid, L-W(Nb)Si2; then the liquid composition reached the univariant line, L-W(Nb)Si2 þ Nb(W)Si2, where the divorced eutectic solidification took place; finally, the liquid composition reached the eutectic invariant reaction, L-W(Nb)Si2 þNb(W)Si2 þ Diamond–Si.
Fig. 5. Experimental results of Nb–60Si–30W (]7) as-cast alloy. (a) X-ray diffractograms of ]7 and ]10 alloys; (b) and (c) BEI micrographs of ]7 alloy at low and high magnification.
4.1.2.3. Nb–80Si–10W (]10). BEI micrographs of Nb–80Si–10W (]10) as-cast alloy are shown in Fig. 6. Similar to ]9 alloy, the microstructure of ]10 alloy consists of a white primary phase W(Nb)Si2 of tetragonal morphology, a grey phase Nb(W)Si2 of hexagonal morphology, a dark Diamond–Si phase, and a small quantity of the eutectic Nb(W)Si2 þW(Nb)Si2 þDiamond–Si.
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Fig. 6. BEI micrographs of Nb–70Si–20W (]9) and Nb–80Si–10W (]10) as-cast alloys. (a) and (b) ]9 alloy at low and high magnification; (c) and (d) ]10 alloy at low and high magnification.
The difference between the two alloys is the relative quantities of the three phases, especially the volume fraction of the W(Nb)Si2 phase of ]10 alloy is much less than that of ]9 alloy. This means that the initial composition of ]10 alloy is very near the univariant line. The solidification path is the same as ]9 alloy.
4.1.3. Primary solidification region of Nb(W)Si2 4.1.3.1. Nb–60Si–(10,20)W (]5–6). X-ray diffractogram of Nb–60Si– 10W (]5) as-cast alloy and BEI micrographs of Nb–60Si–(10,20)W (]5,6) as-cast alloys are shown in Fig. 7. The microstructures of both the alloys are very similar and consist of a large and dark primary phase Nb(W)Si2 of hexagonal morphology and a darkþwhite twophase eutectic Nb(W)Si2 þ b(Nb,W)5Si3. The average compositions of the primary phase Nb(W)Si2 are Nb–65.42Si–5.5W and Nb– 66.49Si–8.23W in ]5 and ]6 alloys respectively, and those of the white phase b(Nb,W)5Si3 in the eutectic Nb–39.5Si–18.14W and Nb–37.8Si–25.13W. The solidification paths of both ]5 and ]6 alloys are that the large primary phase Nb(W)Si2 first solidified from the liquid, LNb(W)Si2; then the liquid composition reached the univariant
line, L-Nb(W)Si2 þ b(Nb,W)5Si3, along which the liquid composition moved forward until the solidification process terminated. 4.1.3.2. Nb–70Si–10W (]8). BEI micrograph of Nb–70Si–10W (]8) as-cast alloy is shown in Fig. 8. The microstructure consists of a large and grey Nb(W)Si2 phase with an average composition of Nb–65.72Si–10.21W and a small amount of dark Diamond–Si phase of Nb–99.99Si. The solidification path is that the Nb(W)Si2 phase first solidified and followed by the divorced eutectic-type solidification of Nb(W)Si2 þW(Nb)Si2. Finally, the liquid composition reached the eutectic invariant reaction, L-W(Nb)Si2 þNb(W)Si2 þDiamond–Si. 4.2. Isothermal section at 1873 K in the Si-rich portion The constituent phases and their equilibrium compositions of the selected alloys heat-treated at 1873 K are summarized in Table 5. Based on both the experimental data and the optimization results, the partial isothermal section of the Nb–Si–W ternary system at 1873 K was shown in Fig. 9. In the investigated Si-rich portion, there are seven two-phase regions, liquidþNb(W)Si2, Nb(W)Si2 þ aNb(W)5Si2, aNb(W)5Si3 þ b(Nb,W)5Si3, Nb(W)Si2 þ b(Nb,W)5Si3, liquidþ W(Nb)
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Fig. 8. BEI micrograph of Nb–70Si–10W (]8) as-cast alloy.
Table 5 Constituent phases and their equilibrium compositions of the heat-treated alloys. Nominal composition Constituent phases Composition (at%)
Remark
Si
Nb
66.79 38.66
32.39 0.82 46.67 14.67
Tie-line
b(Nb,W)5Si3 Nb–50Si–20W (]2)
Nb(W)Si2 b(Nb,W)5Si3
65.69 38.22
29.28 5.03 32.66 29.12
Tie-line
Nb–60Si–10W (]5)
Nb(W)Si2 b(Nb,W)5Si3
66.82 38.64
29.31 3.87 34.97 26.39
Tie-line
Nb–70Si–10W (]8)
Nb(W)Si2 Liquid
66.78 Si rich
23.19 10.03
Tie-line
Nb–60Si–30W (]7)
Nb(W)Si2 W(Nb)Si2 b(Nb,W)5Si3
66.78 66.75 37.59
20.23 12.99 6.03 27.21 16.16 46.25
Tie-triangle
Nb–70Si–20W (]9)
Nb(W)Si2 W(Nb)Si2 Liquid
66.62 66.60 Si rich
19.95 13.43 6.22 27.18
Tie-triangle
Nb–80Si–10W (]10)
Nb(W)Si2 W(Nb)Si2 Liquid
66.52 66.66 Si rich
20.18 13.30 6.14 27.2
Tie-triangle
Nb–50Si–10W (]1)
Nb(W)Si2
W
Fig. 7. Experimental results of Nb–60Si–(10,20)W (]5,6) as-cast alloys. (a) X-ray diffractogram of ]5 alloy; (b) BEI micrograph of ]5 alloy and (c) BEI micrograph of ]6 alloy.
Si2, Nb(W)Si2 þW(Nb)Si2 and W(Nb)Si2 þ b(Nb,W)5Si3, and three three-phase regions, Nb(W)Si2 þW(Nb)Si2 þ b(Nb,W)5Si3, Nb(W) Si2 þ aNb(W)5Si3 þ b(Nb,W)5Si3 and liquidþNb(W)Si2 þW(Nb)Si2. BEI micrographs of the three as-castþ heat-treated alloys, Nb–50Si–10W (]1), Nb–50Si–20W (]2) and Nb–60Si–10W (]5) and the X-ray diffractogram of ]1 alloy are shown in Fig. 10. All the microstructures consist of Nb(W)Si2 and b(Nb,W)5Si3 two
Fig. 9. Partial isothermal section of the Nb–Si–W ternary system at 1873 K.
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Fig. 10. Experimental results of Nb–50Si–10W (]1), Nb–50Si–20W (]2) and Nb–60Si–10W (]5) heat-treated alloys (a) X-ray diffractograms of ]1 alloy; (b) BEI micrograph of ]1 alloy; (c) BEI micrograph of ]2 alloy and (d) BEI micrograph of ]3 alloy.
Fig. 11. BEI micrograph of Nb–70Si–10W (]8) heat-treated alloys.
Fig. 12. BEI micrograph of Nb–60Si–30W (]7) heat-treated alloys.
equilibrium phases. The stable 5:3 silicide is b(Nb,W)5Si3 at 1873 K instead of aNb(W)5Si3 like aNb5Si3 in the Nb–Si binary system [12], in which aNb5Si3 remains stable up to 1921 K. That is to say, the
element W can stabilize b(Nb,W)5Si3 to a lower temperature. One other thing to note is that after heat-treatment gNb(W)5Si3 no longer exists in the microstructure of ]1 alloy.
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Fig. 13. BEI micrographs of Nb–70Si–20W (]9) and Nb–80Si–10W (]10) heat-treated alloys. (a) BEI micrograph of ]9 alloy and (b) BEI micrograph of ]10 alloy.
BEI micrograph of the heat-treated Nb–70Si–10W (]8) alloy is shown in Fig. 11. Different from the as-cast microstructure in Fig. 8, the heat-treated one consists of the grey Nb(W)Si2 phase accompanied by some holes where used to be the Si-rich liquid. This is because the heat-treated temperature 1873 K is higher than the melting point 1685 K of Diamond–Si, resulting in the evaporation of Diamond–Si and the appearance of holes. That is to say, the microstructure of ]8 alloy consists of two equilibrium phases, Nb(W)Si2 and liquid, which contains mainly Si. BEI micrograph and X-ray diffractogram of the heat-treated Nb–60Si–30W (]7) alloy are shown in Figs. 12 and 10(a), respectively. The microstructure consists of three equilibrium phases, the large and grey phase W(Nb)Si2, the white phase b(Nb,W)5Si3 and the dark phase Nb(W)Si2. BEI micrographs of the heat-treated Nb–70Si–20W (]9) and Nb–80Si–10W (]10) alloys are shown in Fig. 13. The microstructure of both alloys consists of three equilibrium phases, the white phase W(Nb)Si2, the grey phase Nb(W)Si2 and the liquid which evaporates and forms holes similar to ]8 alloy. The equilibrium phase compositions of W(Nb)Si2 and Nb(W)Si2 in ]9 and ]10 alloys are nearly the same as shown in Table 5, which confirms that ]9 and ]10 are in the same Tie-triangle at 1873 K.
5. Conclusions Both the liquidus projection and the isothermal section at 1873 K of the Nb–Si–W ternary system in the Si-rich portion were studied experimentally. To investigate the liquidus projection, the microstructures and the solidification paths of the as-cast alloys were analysed. To determine the isothermal section, the constituent phases and their equilibrium compositions of the as-castþheat-treated alloys were researched. The conclusions are obtained as follows: (1) For the liquidus projection in the investigated Si-rich portion, there exist four primary solidification regions, Diamond–Si, b(Nb, W)5Si3, Nb(W)Si2 and W(Nb)Si2, and two eutectic invariant reactions, L-b(Nb,W)5Si3 þW(Nb)Si2 þNb(W)Si2 and L-W(Nb)Si2 þNb(W) Si2 þDiamond–Si. Together with literature reported investigations for the Nb–W-rich portion, the whole liquidus projection of the Nb–Si–W ternary system was constructed. (2) For the partial isothermal section at 1873 K, there are three three-phase regions, liquid þNb(W)Si2 þW(Nb)Si2, Nb-
(W)Si2 þ aNb(W)5Si3 þ b(Nb,W)5Si3 and Nb(W)Si2 þW(Nb)Si2 þ b (Nb,W)5Si3, and seven two-phase regions, liquidþNb(W)Si2, Nb(W)Si2 þ aNb(W)5Si3, aNb(W)5Si3 þ b(Nb,W)5Si3, Nb(W)Si2 þ b(Nb,W)5Si3, liquidþW(Nb)Si2, Nb(W)Si2 þW(Nb)Si2 and W(Nb) Si2 þ b(Nb,W)5Si3.
Acknowledgements The authors would like to acknowledge National Natural Science Foundation of China (Nos. 51271027 and 50731002) and the Ministry of Science and Technology of China (No. 2005DKA32800) for the financial supports. Thanks to Royal Institute of Technology and CompuTherm LLC for supplying Thermo-Calc and Pandat Software packages respectively.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.calphad.2013. 01.002.
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[12] T. Geng, C.R. Li, X.Q. Zhao, H.B. Xu, Z.M. Du, C.P. Guo, CALPHAD 34 (2010) 363–376. [13] C. Vahlas, P.Y. Chevalier, E. Blanquet, CALPHAD 13 (3) (1989) 273–292. [14] C.L. Ma, Y. Tan, A. Kasama, S. Hanada, Mater. Trans. 43 (4) (2002) 688–693. [15] Jade 5.0, XRD Pattern Processing Materials Data Inc. (1999). [16] J.L.C. Daams, P. Villars, J.H.N. van Vucht, Pearson’s Handbook of Crystallographic Data for Intermetallic Phase, ASM International, Metals Park, OH (1991).
[17] H. Schachner, E. Cerwenka, H. Nowotny, Monatsh. Chem. 85 (1954) 245–254(in German). [18] J.H. Kim, T. Tabaru, H. Hirai, A. Kitahara, S. Hanada, Scripta Mater. 48 (10) (2003) 1439–1444. [19] Y.X. Tian, J.T. Guo, L.Y. Sheng, G.M. Cheng, L.Z. Zhou, L.L. He, H.Q. Ye, Intermetallics 16 (2008) 807–812. [20] J. Geng, P. Tsakiropoulos, Intermetallics 15 (3) (2007) 382–395. [21] T. Geng, C.R. Li, X.Q. Zhao, H.B. Xu, C.P. Guo, Z.M. Du, Intermetallics 18 (2010) 1007–1015.
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CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry journal homepage: www.elsevier.com/locate/calphad
Experimental study on phase relationships in the Si-rich portion of the Nb–Si–W ternary system Yan Li a, Changrong Li a,n, Zhenmin Du a, Cuiping Guo a, Jingbo Li b a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2012 Received in revised form 9 January 2013 Accepted 12 January 2013 Available online 4 February 2013
This paper describes the experimental study on liquidus projection and the isothermal section at 1873 K of the Nb–Si–W ternary system in the Si-rich portion. The microstructures and solidification paths of the as-cast alloys were analysed. The constituent phases and their equilibrium compositions of the as-cast þ heat-treated alloys were determined. The microstructure observation, the phase identification and the composition measurement were performed using scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron probe microanalysis (EPMA), respectively. No ternary compound was found. For the liquidus projection in the investigated Si-rich portion, there exist four primary solidification regions, Diamond–Si, b(Nb,W)5Si3, Nb(W)Si2 and W(Nb)Si2, and two eutectic invariant reactions, L-b(Nb,W)5Si3 þW(Nb)Si2 þ Nb(W)Si2 and L-W(Nb)Si2 þNb(W)Si2 þ Diamond–Si. The type of the latter invariant reaction was determined with the help of the thermodynamic assessment of the system. Together with literature reported investigations for the Nb–W-rich portion, the whole liquidus projection of the Nb–Si–W ternary system was constructed. For the isothermal section at 1873 K in the investigated Si-rich portion, there are three three-phase regions, liquid þ Nb(W)Si2 þW(Nb)Si2, Nb(W)Si2 þ aNb(W)5Si3 þ b(Nb,W)5Si3 and Nb(W)Si2 þ W(Nb)Si2 þ b(Nb,W)5Si3, and seven two-phase regions, liquid þNb(W)Si2, Nb(W)Si2 þ aNb(W)5Si3, aNb(W)5Si3 þ b(Nb,W)5Si3, Nb(W)Si2 þ b(Nb,W)5Si3, liquid þ W(Nb)Si2, Nb(W)Si2 þ W(Nb)Si2 and W(Nb)Si2 þ b(Nb,W)5Si3. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Nb–Si–W ternary system Liquidus projection Isothermal section Microstructure Solidification path
1. Introduction Nbss/Nb5Si3 in situ composites exhibit a good balance in mechanical properties between the room temperature toughening provided by the Nb-rich solid solution phase Nbss and the high temperature strength up to 1500 1C, provided by the intermetallic phase Nb5Si3 [1]. Nbss/Nb5Si3 in situ composites, generally based on the phase equilibrium relationship of the Nb-rich region of the Nb–Si phase diagram, have been studied by many researchers [2–5]. Mendiratta et al. [2] investigated the as-castþhot-extrudedþheat-treated NbssþNb5Si3 two-phase alloys with compositions of 10 and 16 at% Si, showing the ductile-phase toughening at low temperatures and the reasonable strength retention at high temperatures (1400–1600 1C). The pffiffiffiffiffi fracture toughness was about 21 MPa m for the Nb–10Si alloy with NbssþNb5Si3 two-phases, while that of Nb5Si3 intermetallic pffiffiffiffiffi phase has been reported to be about 3 MPa m at room temperature [6]. The fracture toughnesses for the as-cast þhot-extrudedþheat-treated Nb–16.5Si alloy were also found to be
n
Corresponding author. Tel.: þ86 10 82377789; fax: þ86 10 62333772. E-mail address: [email protected] (C. Li).
0364-5916/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.calphad.2013.01.002
pffiffiffiffiffi pffiffiffiffiffi 12.8 MPa m at room temperature and 17 MPa m at 1400 1C by Mendiratta et al. [3]. Bewlay et al. [4] found the fracture toughness of the directionally solidified Nb–Si in situ composites (10–25 at% Si) was about 50% more than that of the as-cast alloys and 20% less than that of the extruded alloys of the equivalent compositions. Swkido et al. [5] investigated the directionally solidified and arc-melted Nb–17.5Si alloys with or without 10 at% Ti. The directionally solidified alloys had the higher fracture toughness compared with the arc-melted ones, in accordance with Ref. [4]; the fracture toughness should be improved by low-rate solidification, Ti alloying or heat-treatment; for the pffiffiffiffiffi Nb–17.5Si alloy, the fracture toughness of 14.5 MPa m at room temperature and the compressive strength of 580 MPa at 1673 K were achieved by directional solidification at the rate of 10 mm/h. However, one major impediment of the Nbss/Nb5Si3 alloys for high-temperature applications is their catastrophic oxidation behaviour. To obtain material systems with improved environmental resistance while retaining their structural properties, studies with addition of alloying elements, such as Ti, Al, Hf, Cr, Mo and W, were initiated on the Nbss/Nb5Si3 in situ composites. Sha et al. [7,8] studied the effect of W addition on the yield strength of Nb–10Mo–10Ti–18Si-based in situ composites, and
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found that the yield strength (s0.2) and the specific yield strength (s0.2S) (s0.2 divided by the density) at 1670 K increased markedly with increasing W content; the directionally solidified samples showed higher s0.2 and s0.2S than the as-cast samples; even at 1770 K, the directionally solidified sample with 15 at% W showed a high s0.2 of about 650 MPa. Sha et al. [9] also found that the Nb–10W alloy showed the high fracture toughness of about pffiffiffiffiffi 15.3 MPa m and the low s0.2 of about 90 MPa at 1670 K while the Nb–10Si-10W showed the high s0.2 of about 330 MPa at pffiffiffiffiffi 1670 K and the low fracture toughness of about 8.2 MPa m, and concluded that the toughness was supplied by the metallic Nbss phase and the high-temperature strength was mainly provided by the brittle silicide phase. Ma et al. [10] found that both the lowtemperature deformability and the high-temperature strength of Nbss/Nb–silicide in situ composites were improved by the addition of appropriate amounts of Mo and/or W. By the addition of W, Xiong et al. [11] also improved significantly the oxidation resistance of Nbss/Nb5Si3 in situ composite (Nb–20Si–10W alloy). Consequently, by alloying with W, the Nbss/Nb5Si3 in situ composites may get a good balance among high-temperature strength, room temperature toughness and high-temperature oxidation resistance. The Nb–Si–W ternary system is becoming an important system to be investigated for the development of the Nb–Si-based high temperature materials. The phase relationship is an important guidance for materials design and also an essential basis for the CALPHAD (CALculation of PHAse Diagrams) approach. In order to get a comprehensive and consistent thermodynamic description for one of the phases of a ternary system, it is necessary to understand the phase equilibrium behaviour of the other related phases in the system. For the Nb–Si–W ternary system, literature reports are mainly concentrated on the Nb–W-rich portion with the application importance. From the view point of the thermodynamic assessment of the system, the purpose of the present paper is to study the constituent phases and the solidification processes of the as-cast alloys in the Si-rich portion of the Nb–Si–W ternary system, so that the whole liquidus projection of the Nb–Si–W ternary system can be constructed together with the literature reported investigations for the Nb–W-rich portion. Meanwhile, some selected as-cast alloys were heat-treated at 1873 K for 72 h and by studying the phase equilibria of these alloys, the isothermal section for the Si-rich portion was obtained.
Table 2 Invariant reactions involving the liquid phase in the W–Si system [13].
2. Literature study on the phase diagrams of the binary and the ternary systems
Ma et al. [14] constructed a partial liquidus projection of the ternary system in the Nb–W rich region up to 37.5 at% Si, as shown in Fig. 1. There exist three primary phase regions, Bcc, b(Nb,W)5Si3 and Nb(W)3Si, and one invariant point for the transitional invariant reaction Lþ b(Nb,W)5Si3-Nb(W)3SiþBcc. The liquidus projection for the Si-rich portion has not been reported.
2.1. The Nb–Si binary phase diagram The Nb–Si binary phase diagram is based on the assessment by Geng et al. [12]. The stable phases in this system are the terminal phases Bcc-Nbss and Diamond–Si, the intermetallic compounds Nb3Si, aNb5Si3, bNb5Si3 and NbSi2 and the liquid phase referred to
Table 1 Invariant reactions involving the liquid phase in the Nb–Si system [12]. Reaction type
Reaction
Composition of L (at% Si )
T (1C)
Congruent Congruent Eutectic Peritectic Eutectic Eutectic
L-bNb5Si3 L-NbSi2 L-Bcc þ Nb3Si Lþ bNb5Si3-Nb3Si L-bNb5Si3 þNbSi2 L-NbSi2 þDiamond–Si
39.0 66.7 17.3 20.3 57.7 98.3
2526 1962 1916 1977 1873 1407
Reaction type
Reaction
Composition of L (at% Si )
T (1C)
Congruent Congruent Eutectic Eutectic Eutectic
L-W5Si3 L-WSi2 L-Bcc þ W5Si3 L-W5Si3 þ WSi2 L-WSi2 þ Diamond–Si
0.375 0.667 0.329 0.525 0.960
2108 2002 2089 1940 1388
Fig. 1. Liquidus projection for the Nb–W-rich portion of the Nb–Si–W ternary system [14].
as ‘‘L’’ hereafter. The invariant reactions involving the liquid phase are listed in Table 1. 2.2. The W–Si binary phase diagram The W–Si binary phase diagram is based on the assessment by Vahlas at al. [13]. The stable phases in this system are the terminal phases Bcc-Wss and Diamond–Si, the intermetallic compounds W5Si3 and WSi2 and the liquid phase. The invariant reactions involving the liquid phase are listed in Table 2. 2.3. The Nb–W binary phase diagram The Nb–W binary system was relatively simple, involving an infinite soluble solid phase and a liquid phase. No invariant reaction exists. 2.4. Previous experiments on the liquidus projection of the Nb–Si–W ternary system
2.5. Previous experiments on the isothermal section of the Nb–Si–W ternary system There is not much information about the phase equilibria of the Nb–Si–W ternary system. Ma et al. [14] speculated a partial isothermal section for the Nb–W rich portion of Nb–Si–W ternary system at 1973 K. The samples were arc-melted followed by annealing at 1973 K for 48 h and furnace cooled. 3. Experimental procedures Each sample with a gross weight about 5 g was arc-melted by a non-consumable tungsten electrode in a water-cooled copper
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crucible under an argon atmosphere, and was remelted five times to get a homogeneous specimen. The purities of the raw materials are 99.999 wt% for Si, 99.95 wt% for W and 99.9 wt% for Nb. Since the weight loss of each sample measured after preparation was less than 1 wt%, the compositions of the alloys were considered to be equal to their nominal compositions. The isothermal heat treatment was conducted at 1873 K for 72 h in an argon furnace. The as-cast alloys were used to determine the primarily crystallised phases, to construct the liquidus projection diagram, and to analyse the solidification processes. And the as-cast þheattreated samples were prepared to measure the phase equilibria in the isothermal section. The microstructure observation, the phase identification and the composition measurement were performed using scanning
electron microscopy (SEM) working in a back-scattered electron image (BEI) mode, X-ray diffraction (XRD) and electron probe microanalysis (EPMA), respectively. The Materials Data Inc. software Jade 5.0 [15] and Powder Diffraction File (PDF released 2002) were used for XRD analysis.
4. Results and discussion 4.1. Liquidus projection in the Si-rich portion A set of 10Nb–Si–W ternary alloys were studied to investigate the microstructure and phase relationship for the Si-rich portion of the system. The compositions (at%) of the experimental ingots
Fig. 2. Liquidus projections of the Nb–Si–W ternary system for (a) the Si-rich portion and (b) the whole system.
Table 3 Phases present in the as-cast microstructures together with the bulk alloy compositions. Primary phase
Nominal composition
As-cast phases
Solidification path
b(Nb,W)5Si3
Nb–50Si–10W (]1)
b(Nb,W)5Si3
L-b(Nb,W)5Si3 L-b(Nb,W)5Si3 þNb(W)Si2
Nb–50Si–20W (]2) Nb–50Si–30W (]3) Nb–50Si–40W (]4)
W(Nb)Si2
Nb–60Si–30W (]7)
Nb–70Si–20W (]9)
Nb–80Si–10W (]10)
Nb(W)Si2
Nb–60Si–10W (]5) Nb–60Si–20W (]6) Nb–70Si–10W (]8)
a
Nb(W)Si2 gNb(W)5Si3a b(Nb,W)5Si3 Nb(W)Si2 b(Nb,W)5Si3 Nb(W)Si2 b(Nb,W)5Si3 Nb(W)Si2 W(Nb)Si2
L-b(Nb,W)5Si3 L-b(Nb,W)5Si3 þNb(W)Si2 L-b(Nb,W)5Si3 L-b(Nb,W)5Si3 þNb(W)Si2 L-b(Nb,W)5Si3 L-b(Nb,W)5Si3 þW(Nb)Si2 L-b(Nb,W)5Si3 þW(Nb)Si2 þ Nb(W)Si2
W(Nb)Si2 Nb(W)Si2 b(Nb,W)5Si3 W(Nb)Si2 Nb(W)Si2 Diamond–Si W(Nb)Si2 Nb(W)Si2 Diamond–Si
L-W(Nb)Si2 L-W(Nb)Si2 þ b(Nb,W)5Si3 L-W(Nb)Si2 þ b(Nb,W)5Si3 þ Nb(W)Si2 L-W(Nb)Si2 L-W(Nb)Si2 þNb(W)Si2 L-W(Nb)Si2 þNb(W)Si2 þ Diamond–Si L-W(Nb)Si2 L-W(Nb)Si2 þNb(W)Si2 L-W(Nb)Si2 þNb(W)Si2 þ Diamond–Si
Nb(W)Si2 b(Nb,W)5Si3 Nb(W)Si2 b(Nb,W)5Si3 Nb(W)Si2 Diamond–Si
L-Nb(W)Si2 L-Nb(W)Si2 þ b(Nb,W)5Si3 L-Nb(W)Si2 L-Nb(W)Si2 þ b(Nb,W)5Si3 L-Nb(W)Si2 L-W(Nb)Si2 þNb(W)Si2 L-W(Nb)Si2 þNb(W)Si2 þ Diamond–Si
A metastable phase which often occurs in the multiple alloys of the Nb–Si-based systems.
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Table 4 Crystallographic results of different phases in different Nb–Si–W ternary alloys. Phases
Structure type
Compositions (at%) Nb
bNb5Si3
tI32–Si3W5
W5Si3
tI32–Si3W5
NbSi2
hP9–CrSi2
0.625
Si
W
a
0.375 0.375
0.333
Lattice parameters (nm)
0.625
0.667 0.667
c
1.0026
0.50717
Reference [16]
0.9605(5)
0.4964(5)
Reference [16]
0.4819(2)
0.6592(2)
Reference [16]
WSi2
tI6–MoSi2
0.3210
0.7829
Reference [16]
bNb(W)5Si3
tI32–Si3W5
49.6 13.32 22 42.71 46.67 16.16
37.70 34.15 34.84 39.15 38.66 37.59
12.7 52.53 43.16 18.14 14.67 46.25
0.9994(1) 0.9824(5) 0.9894(7) 0.9970(7) 0.9982(6) 0.9865(7)
0.5007(2) 0.5009(7) 0.5018(5) 0.5156(7) 0.5051(7) 0.4960(6)
In In In In In In
as-cast ]1 alloy as-cast ]4 alloy as-cast ]7 alloy as-cast ]5 alloy heat-treated ]1 alloy heat-treated ]7 alloy
Nb(W)Si2
hP9–CrSi2
30.7 18.91 19.12 19.7 29.08 32.39 20.23
66.38 64.75 65.55 65.53 65.42 66.79 66.78
2.92 16.34 15.33 14.77 5.5 0.82 12.99
0.4800(4) 0.4722(9) 0.4744(2) 0.4749(4) 0.4798(8) 0.4804(5) 0.4757(7)
0.6690(2) 0.6561(1) 0.6544(7) 0.6525(3) 0.6656(2) 0.6695(4) 0.6588(9)
In In In In In In In
as-cast ]1 alloy as-cast ]4 alloy as-cast ]7 alloy as-cast ]9 alloy as-cast ]5 alloy heat-treated ]1 alloy heat-treated ]7 alloy
W(Nb)Si2
tI6–MoSi2
3.46 4.12 5.13 6.04
66.14 66.2 66.15 66.75
30.4 29.68 28.72 27.21
0.3250(2) 0.3253(1) 0.3258(4) 0.3259(6)
0.7859(4) 0.7782(6) 0.7850(0) 0.7905(7)
In In In In
as-cast ]4 alloy as-cast ]7 alloy as-cast ]9 alloy heat-treated ]7 alloy
are marked in Fig. 2(a) and their constituent phases as well as solidification processes are summarized in Table 3. The lattice parameters of the phases with different compositions in different ternary alloys are shown in Table 4, and those of the binary compounds listed in the table are from Daams et al. [16]. The compounds bNb5Si3 and W5Si3 are of the same crystal structure with D8m-Strukturbericht designation/Si3W5-prototype and can form an infinite solid solution which is referred to as b(Nb,W)5Si3 in the present work. The compounds NbSi2 and WSi2 are of different crystal structures, with C40-Strukturbericht designation/ CrSi2-prototype and C11b-Strukturbericht designation/ MoSi2-prototype respectively and cannot form an infinite solid solution. The intermetallic compound NbSi2 with the dissolved W is referred to as Nb(W)Si2 and the WSi2 with the dissolved Nb as W(Nb)Si2. The constructed liquidus projection for the Si-rich portion of the Nb–Si–W ternary system is shown in Fig. 2(a). The univariant curves and the invariant points in the liquidus projection were determined according to both the experimental data and the optimization results, and the latter will be described in detail in the thermodynamic assessment part of the subsequent article. There exist four primary solidification regions, Diamond–Si, b(Nb,W)5Si3, Nb(W)Si2 and W(Nb)Si2, one transitional invariant reaction Lþ W(Nb)Si2-b(Nb,W)5Si3 þNb(W)Si2 and another eutectic one L-Nb(W)Si2 þDiamond–SiþW(Nb)Si2. Together with the literature reported investigations for the Nb–W-rich portion, the liquidus projection of the whole Nb–Si–W ternary system is shown in Fig. 2(b). More detailed descriptions of Table 3 and Fig. 2 are given in the following sections. 4.1.1. Primary solidification region of b(Nb,W)5Si3 4.1.1.1. Nb–50Si–(10,20,30)W (]1–3). X-ray diffractogram of Nb– 50Si–10W (]1) and BEI micrographs of Nb–50Si–(10,20,30)W (]1–3) as-cast alloys are shown in Fig. 3. For ]1 alloy, the microstructure consists of a large and white primary phase of tetragonal morphology, a whiteþdark two-phase eutectic, in which a small quantity of grey flakes of a third phase are dispersed. The crystal
0.333
Remark
structure and the composition of the primary phase were determined, using XRD and EPMA respectively, to be b(Nb,W)5Si3 with an average composition of Nb–37.70Si–12.70W, those of the dark phase in the eutectic to be Nb(W)Si2 of Nb–66.38Si–2.92W and, those of the dispersed grey phase to be gNb(W)5Si3 of Nb–37.39Si– 7.48W. The symbols of X-ray results are labelled according to the experimental measurements. For example, the solid square for Nb(W)Si2 in Fig. 3(a), the element W can substitute Nb in the NbSi2 with CrSi2-prototype crystal structure. Since the atomic radius of the element W (rW ¼0.139 nm) is smaller than that of Nb (rNb ¼0.146 nm), the lattice parameters as well as the spaces between the atomic planes d will decrease with the increase of W content. According to Bragg’s diffraction condition, nl ¼ 2dsin y, when the integer n and the incident wavelength l are kept constant, the angle y between the incident ray and the scattering planes will increase with the decrease of d. Therefore, the experimental value 2y of the ternary Nb(W)Si2 in Fig. 3(a) moves to the right with the increase of W content, resulting the smaller lattice parameters than those of the binary compound NbSi2 as listed in Table 4. Similar to ]1 alloy, the microstructures of both ]2 and ]3 alloys consist of a large and white primary phase b(Nb,W)5Si3 of tetragonal morphology and a whiteþdark two-phase eutectic b(Nb,W)5Si3 þNb(W)Si2. For ]2 and ]3 alloys, the average compositions of the primary phase b(Nb,W)5Si3 are Nb–35.34Si– 25.37W and Nb–34.78Si–37.53W, those of the dark phase Nb(W)Si2 in the eutectic are Nb–65.38Si–8.12W and Nb–66.02Si– 12.23W, respectively. The only difference from ]1 alloy is no gNb(W)5Si3 dispersed in the eutectic. The three alloys ]1, ]2 and ]3 are in the area with the same primarily crystallized phase b(Nb,W)5Si3, which is of a large and white tetragonal morphology. Since there is no gNb(W)5Si3 dispersed in the eutectic structure of ]2 and ]3 alloys, the small quantity of grey flakes gNb(W)5Si3 in ]1 alloy is suspected to be a metastable phase. gNb5Si3 is a metastable phase in the Nb–Si binary system, which can be obtained by the addition of C [17,18] or Ti [19] or Hf [20] in the ternary or multi-component alloy systems because of the stabilization by the interstitial C atoms or the isostructure
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Fig. 3. Experimental results of Nb–50Si–(10,20,30)W (]1–3) as-cast alloys. (a) X-ray diffractograms of ]1 and ]4 alloys; (b) BEI micrograph of ]1 alloy; (c) BEI micrograph of ]2 alloy and (d) BEI micrograph of ]3 alloy.
Fig. 4. BEI micrographs of Nb–50Si–40W (]4) as-cast alloy at (a) low and (b) high magnification.
compounds of the hexagonal Ti5Si3 and Hf5Si3. Geng et al. [21] studied the as-cast solidification of the Si-rich alloys of the Nb–Si–Mo ternary system, and found that Nb(Mo)Si2–C40 can
also stabilize the g(Nb,Mo)5Si3 because both the phases Nb(Mo)Si2 and g(Nb,Mo)5Si3 are of the hexagonal structure, a low energy interface can be formed between Nb(Mo)Si2
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and g(Nb,Mo)5Si3 when the crystallographic relationship of (0001)C40//(0001)g and [1100]C40//[2110]g is satisfied and a high coincidence site lattice is obtained. In the present work, the existence of the metastable gNb(W)5Si3 is deduced also due to the divorced Nb(W)Si2–C40 in the eutectic structure. After the isothermal heat-treatment at 1873 K as shown in Section 4.2, gNb(W)5Si3 in ]1 alloy cannot be found any more, which is another proof that gNb(W)5Si3 is a metastable phase. The solidification path is revealed by the microstructural morphology. The large primary phase b(Nb,W)5Si3 first solidified from the liquid, L-b(Nb,W)5Si3; then the liquid composition reached the univariant line, L-b(Nb,W)5Si3 þNb(W)Si2, along which the liquid composition moved forward until the solidification process terminated. 4.1.1.2. Nb–50Si–40W (]4). X-ray diffractogram and BEI micrographs of Nb–50Si–40W (]4) as-cast alloy are shown in Figs. 3(a) and 4, respectively. The microstructure consists of a large and white primary phase b(Nb,W)5Si3 of tetragonal morphology, a grey phase W(Nb)Si2 of tetragonal morphology and a dark phase Nb(W)Si2 in the eutectic. The average compositions of b(Nb,W)5Si3, W(Nb)Si2 and Nb(W)Si2 are Nb–34.15Si–52.53W, Nb–66.14Si–30.4W and Nb–64.75Si–16.34W, respectively. The solidification path of ]4 alloy is that the primary phase b(Nb,W)5Si3 first solidified from liquid, L-b(Nb,W)5Si3; after that the liquid composition reached the univariant line, L-b(Nb,W)5 Si3 þW(Nb)Si2, where the divorced eutectic solidification took place in some places; finally, the liquid composition reached the eutectic invariant reaction, L-b(Nb,W)5Si3 þW(Nb)Si2 þNb(W)Si2. 4.1.2. Primary solidification region of W(Nb)Si2 4.1.2.1. Nb–60Si–30W (]7). X-ray diffractogram and BEI micrographs of Nb–60Si–30W (]7) as-cast alloy are shown in Fig. 5. The microstructure consists of a large and grey primary phase W(Nb)Si2 of tetragonal morphology, a coarse eutectic of greyþ white two phases W(Nb)Si2 þ b(Nb,W)5Si3 and an eutectic of whiteþdarkþgrey three phases b(Nb,W)5Si3 þNb(W)Si2 þW(Nb) Si2. The average compositions of W(Nb)Si2, b(Nb,W)5Si3 and Nb(W)Si2 in Fig. 5(c) are Nb–66.2Si–29.68 W, Nb–34.84Si–43.16 W, and Nb–65.55Si–15.33W, respectively. The solidification path of ]7 alloy is that the primary phase W(Nb)Si2 first solidified from liquid, L-W(Nb)Si2; after that the liquid composition reached the univariant line, L-b(Nb,W)5Si3 þ W(Nb)Si2, where the coarse eutectic of W(Nb)Si2 þ b(Nb,W)5Si3 formed; finally, the liquid composition reached the eutectic invariant reaction, L-b(Nb,W)5Si3 þW(Nb)Si2 þNb(W)Si2. 4.1.2.2. Nb–70Si–20W (]9). X-ray diffractogram and BEI micrographs of Nb–70Si–20W (]9) as-cast alloy are shown in Figs. 5(a) and 6, respectively. The microstructure consists of a white primary phase W(Nb)Si2 of tetragonal morphology with an average composition of Nb–66.15Si–28.72W, a grey Nb(W)Si2 phase of Nb–65.53Si–14.77W, a dark Diamond–Si phase of Nb–99.99Si, and a small quantity of the eutectic Nb(W)Si2 þW(Nb)Si2 þDiamond–Si. In this three-phase eutectic structure near the Si corner, Diamond–Si is the dominant phase with a large volume fraction and the distinct eutectic feature is hardly observed. The solidification path is that the primary phase W(Nb)Si2 first solidified from liquid, L-W(Nb)Si2; then the liquid composition reached the univariant line, L-W(Nb)Si2 þ Nb(W)Si2, where the divorced eutectic solidification took place; finally, the liquid composition reached the eutectic invariant reaction, L-W(Nb)Si2 þNb(W)Si2 þ Diamond–Si.
Fig. 5. Experimental results of Nb–60Si–30W (]7) as-cast alloy. (a) X-ray diffractograms of ]7 and ]10 alloys; (b) and (c) BEI micrographs of ]7 alloy at low and high magnification.
4.1.2.3. Nb–80Si–10W (]10). BEI micrographs of Nb–80Si–10W (]10) as-cast alloy are shown in Fig. 6. Similar to ]9 alloy, the microstructure of ]10 alloy consists of a white primary phase W(Nb)Si2 of tetragonal morphology, a grey phase Nb(W)Si2 of hexagonal morphology, a dark Diamond–Si phase, and a small quantity of the eutectic Nb(W)Si2 þW(Nb)Si2 þDiamond–Si.
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Fig. 6. BEI micrographs of Nb–70Si–20W (]9) and Nb–80Si–10W (]10) as-cast alloys. (a) and (b) ]9 alloy at low and high magnification; (c) and (d) ]10 alloy at low and high magnification.
The difference between the two alloys is the relative quantities of the three phases, especially the volume fraction of the W(Nb)Si2 phase of ]10 alloy is much less than that of ]9 alloy. This means that the initial composition of ]10 alloy is very near the univariant line. The solidification path is the same as ]9 alloy.
4.1.3. Primary solidification region of Nb(W)Si2 4.1.3.1. Nb–60Si–(10,20)W (]5–6). X-ray diffractogram of Nb–60Si– 10W (]5) as-cast alloy and BEI micrographs of Nb–60Si–(10,20)W (]5,6) as-cast alloys are shown in Fig. 7. The microstructures of both the alloys are very similar and consist of a large and dark primary phase Nb(W)Si2 of hexagonal morphology and a darkþwhite twophase eutectic Nb(W)Si2 þ b(Nb,W)5Si3. The average compositions of the primary phase Nb(W)Si2 are Nb–65.42Si–5.5W and Nb– 66.49Si–8.23W in ]5 and ]6 alloys respectively, and those of the white phase b(Nb,W)5Si3 in the eutectic Nb–39.5Si–18.14W and Nb–37.8Si–25.13W. The solidification paths of both ]5 and ]6 alloys are that the large primary phase Nb(W)Si2 first solidified from the liquid, LNb(W)Si2; then the liquid composition reached the univariant
line, L-Nb(W)Si2 þ b(Nb,W)5Si3, along which the liquid composition moved forward until the solidification process terminated. 4.1.3.2. Nb–70Si–10W (]8). BEI micrograph of Nb–70Si–10W (]8) as-cast alloy is shown in Fig. 8. The microstructure consists of a large and grey Nb(W)Si2 phase with an average composition of Nb–65.72Si–10.21W and a small amount of dark Diamond–Si phase of Nb–99.99Si. The solidification path is that the Nb(W)Si2 phase first solidified and followed by the divorced eutectic-type solidification of Nb(W)Si2 þW(Nb)Si2. Finally, the liquid composition reached the eutectic invariant reaction, L-W(Nb)Si2 þNb(W)Si2 þDiamond–Si. 4.2. Isothermal section at 1873 K in the Si-rich portion The constituent phases and their equilibrium compositions of the selected alloys heat-treated at 1873 K are summarized in Table 5. Based on both the experimental data and the optimization results, the partial isothermal section of the Nb–Si–W ternary system at 1873 K was shown in Fig. 9. In the investigated Si-rich portion, there are seven two-phase regions, liquidþNb(W)Si2, Nb(W)Si2 þ aNb(W)5Si2, aNb(W)5Si3 þ b(Nb,W)5Si3, Nb(W)Si2 þ b(Nb,W)5Si3, liquidþ W(Nb)
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Fig. 8. BEI micrograph of Nb–70Si–10W (]8) as-cast alloy.
Table 5 Constituent phases and their equilibrium compositions of the heat-treated alloys. Nominal composition Constituent phases Composition (at%)
Remark
Si
Nb
66.79 38.66
32.39 0.82 46.67 14.67
Tie-line
b(Nb,W)5Si3 Nb–50Si–20W (]2)
Nb(W)Si2 b(Nb,W)5Si3
65.69 38.22
29.28 5.03 32.66 29.12
Tie-line
Nb–60Si–10W (]5)
Nb(W)Si2 b(Nb,W)5Si3
66.82 38.64
29.31 3.87 34.97 26.39
Tie-line
Nb–70Si–10W (]8)
Nb(W)Si2 Liquid
66.78 Si rich
23.19 10.03
Tie-line
Nb–60Si–30W (]7)
Nb(W)Si2 W(Nb)Si2 b(Nb,W)5Si3
66.78 66.75 37.59
20.23 12.99 6.03 27.21 16.16 46.25
Tie-triangle
Nb–70Si–20W (]9)
Nb(W)Si2 W(Nb)Si2 Liquid
66.62 66.60 Si rich
19.95 13.43 6.22 27.18
Tie-triangle
Nb–80Si–10W (]10)
Nb(W)Si2 W(Nb)Si2 Liquid
66.52 66.66 Si rich
20.18 13.30 6.14 27.2
Tie-triangle
Nb–50Si–10W (]1)
Nb(W)Si2
W
Fig. 7. Experimental results of Nb–60Si–(10,20)W (]5,6) as-cast alloys. (a) X-ray diffractogram of ]5 alloy; (b) BEI micrograph of ]5 alloy and (c) BEI micrograph of ]6 alloy.
Si2, Nb(W)Si2 þW(Nb)Si2 and W(Nb)Si2 þ b(Nb,W)5Si3, and three three-phase regions, Nb(W)Si2 þW(Nb)Si2 þ b(Nb,W)5Si3, Nb(W) Si2 þ aNb(W)5Si3 þ b(Nb,W)5Si3 and liquidþNb(W)Si2 þW(Nb)Si2. BEI micrographs of the three as-castþ heat-treated alloys, Nb–50Si–10W (]1), Nb–50Si–20W (]2) and Nb–60Si–10W (]5) and the X-ray diffractogram of ]1 alloy are shown in Fig. 10. All the microstructures consist of Nb(W)Si2 and b(Nb,W)5Si3 two
Fig. 9. Partial isothermal section of the Nb–Si–W ternary system at 1873 K.
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Fig. 10. Experimental results of Nb–50Si–10W (]1), Nb–50Si–20W (]2) and Nb–60Si–10W (]5) heat-treated alloys (a) X-ray diffractograms of ]1 alloy; (b) BEI micrograph of ]1 alloy; (c) BEI micrograph of ]2 alloy and (d) BEI micrograph of ]3 alloy.
Fig. 11. BEI micrograph of Nb–70Si–10W (]8) heat-treated alloys.
Fig. 12. BEI micrograph of Nb–60Si–30W (]7) heat-treated alloys.
equilibrium phases. The stable 5:3 silicide is b(Nb,W)5Si3 at 1873 K instead of aNb(W)5Si3 like aNb5Si3 in the Nb–Si binary system [12], in which aNb5Si3 remains stable up to 1921 K. That is to say, the
element W can stabilize b(Nb,W)5Si3 to a lower temperature. One other thing to note is that after heat-treatment gNb(W)5Si3 no longer exists in the microstructure of ]1 alloy.
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Fig. 13. BEI micrographs of Nb–70Si–20W (]9) and Nb–80Si–10W (]10) heat-treated alloys. (a) BEI micrograph of ]9 alloy and (b) BEI micrograph of ]10 alloy.
BEI micrograph of the heat-treated Nb–70Si–10W (]8) alloy is shown in Fig. 11. Different from the as-cast microstructure in Fig. 8, the heat-treated one consists of the grey Nb(W)Si2 phase accompanied by some holes where used to be the Si-rich liquid. This is because the heat-treated temperature 1873 K is higher than the melting point 1685 K of Diamond–Si, resulting in the evaporation of Diamond–Si and the appearance of holes. That is to say, the microstructure of ]8 alloy consists of two equilibrium phases, Nb(W)Si2 and liquid, which contains mainly Si. BEI micrograph and X-ray diffractogram of the heat-treated Nb–60Si–30W (]7) alloy are shown in Figs. 12 and 10(a), respectively. The microstructure consists of three equilibrium phases, the large and grey phase W(Nb)Si2, the white phase b(Nb,W)5Si3 and the dark phase Nb(W)Si2. BEI micrographs of the heat-treated Nb–70Si–20W (]9) and Nb–80Si–10W (]10) alloys are shown in Fig. 13. The microstructure of both alloys consists of three equilibrium phases, the white phase W(Nb)Si2, the grey phase Nb(W)Si2 and the liquid which evaporates and forms holes similar to ]8 alloy. The equilibrium phase compositions of W(Nb)Si2 and Nb(W)Si2 in ]9 and ]10 alloys are nearly the same as shown in Table 5, which confirms that ]9 and ]10 are in the same Tie-triangle at 1873 K.
5. Conclusions Both the liquidus projection and the isothermal section at 1873 K of the Nb–Si–W ternary system in the Si-rich portion were studied experimentally. To investigate the liquidus projection, the microstructures and the solidification paths of the as-cast alloys were analysed. To determine the isothermal section, the constituent phases and their equilibrium compositions of the as-castþheat-treated alloys were researched. The conclusions are obtained as follows: (1) For the liquidus projection in the investigated Si-rich portion, there exist four primary solidification regions, Diamond–Si, b(Nb, W)5Si3, Nb(W)Si2 and W(Nb)Si2, and two eutectic invariant reactions, L-b(Nb,W)5Si3 þW(Nb)Si2 þNb(W)Si2 and L-W(Nb)Si2 þNb(W) Si2 þDiamond–Si. Together with literature reported investigations for the Nb–W-rich portion, the whole liquidus projection of the Nb–Si–W ternary system was constructed. (2) For the partial isothermal section at 1873 K, there are three three-phase regions, liquid þNb(W)Si2 þW(Nb)Si2, Nb-
(W)Si2 þ aNb(W)5Si3 þ b(Nb,W)5Si3 and Nb(W)Si2 þW(Nb)Si2 þ b (Nb,W)5Si3, and seven two-phase regions, liquidþNb(W)Si2, Nb(W)Si2 þ aNb(W)5Si3, aNb(W)5Si3 þ b(Nb,W)5Si3, Nb(W)Si2 þ b(Nb,W)5Si3, liquidþW(Nb)Si2, Nb(W)Si2 þW(Nb)Si2 and W(Nb) Si2 þ b(Nb,W)5Si3.
Acknowledgements The authors would like to acknowledge National Natural Science Foundation of China (Nos. 51271027 and 50731002) and the Ministry of Science and Technology of China (No. 2005DKA32800) for the financial supports. Thanks to Royal Institute of Technology and CompuTherm LLC for supplying Thermo-Calc and Pandat Software packages respectively.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.calphad.2013. 01.002.
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