Interdiffusion and diffusion structure development in selected refractory metal silicides

Materials Science and Engineering A261 (1999) 64 – 77

Interdiffusion and diffusion structure development in selected refractory metal silicides P.C. Tortorici a, M.A. Dayananda b,* b

a Hewlett-Packard Company, Rohnert Park, CA 94928, USA School of Materials Engineering, Purdue Uni6ersity, West Lafayette, IN 47907, USA

Abstract Solid–solid diffusion couples set up with disks of Mo, W, Re, Nb, or Ta in contact with disks of a single crystal of MoSi2 were annealed at selected temperatures between 1300° and 1700°C for diffusion structure, determination of interdiffusion coefficients and energies of activation for interdiffusion in various silicides developed in the couples. The couples were analyzed and characterized by SEM and optical microscopy, microprobe analysis, X-ray diffraction and orientation imaging microscopy. From the interdiffusion fluxes determined directly from the concentration profiles, integrated and average effective interdiffusion coefficients were calculated for the components in the various binary and ternary silicide layers. The Mo vs. MoSi2 couples developed Mo5Si3 and Mo3Si layers with non-planar interface morphologies; the Mo5Si3 layer exhibited oriented growth and microcracking. The W vs. MoSi2 couples developed W5Si3 and (W,Mo)5Si3 layers with little microcracking. The diffusion structure of Re vs. MoSi2 diffusion couples consisted of layers of Re2Si, (Re,Mo)Si and (Re,Mo)5Si3 phases and cracks were blunted in the (Re,Mo)5Si3 layer. The Nb vs. MoSi2 couples developed the Nb5Si3 and (Nb,Mo)5Si3 phase layers with porosity in the diffusion zone. Layers of Ta2Si, Ta5Si3 and (Ta,Mo)5Si3 phases were observed in the Ta vs. MoSi2 couples. The activation energies (Q) for the interdiffusion of both Si and W are calculated to be about 360 and 450 kJ mol − 1 in the W5Si3 and (W,Mo)5Si3 layers, respectively. Values of Q for the interdiffusion of both Re and Si are about 190 kJ mol − 1 in the Re2Si phase, 325 kJ mol − 1 in the (Re,Mo)Si phase, and 270 kJ mol − 1 in the (Re,Mo)5Si3 phase. For the interdiffusion of Si and Nb in the (Nb,Mo)5Si3 phase, Q values are about 300 and 240 kJ mol − 1, respectively; Q is about 265 kJ mol − 1 in the binary Nb5Si3 phase. Zero-flux planes (ZFP) and uphill diffusion are observed for Mo in the (Re,Mo)Si and (Me,Mo)5Si3 layers where Me =W, Re, Nb or Ta; in these 0 Si layers, the ternary cross coefficient D0 Si MoMe is about as large as the main ternary interdiffusion coefficient D MoMo and the interdiffusion of Mo is enhanced down the Me concentration gradient. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Interdiffusion; Diffusion structures; Refractory metal silicides

1. Introduction As a potential high temperature structural material, MoSi2 has been investigated by many researchers for improving its properties by solid solution alloying and fiber reinforcing [1]. The high temperature strength and the creep resistance of MoSi2 have been shown to improve by alloying with W and Re [2,3]. Attempts at ceramic phase reinforcements have included the addition of SiC and Si3N4 to MoSi2 and the fabrication of composites [4,5]. Several refractory metals, such as W, Ta, and Nb have been processed with MoSi2 as a ductile phase reinforcement [6 – 8]. A key concern re* Corresponding author. Tel.: +1-765-4944113; fax: + 1-7654941204; e-mail: [email protected].

garding the ductile phase additions deals with the compatibility between MoSi2 and the ductile phase at elevated temperatures. MoSi2 reacts extensively with potential refractory metal reinforcements such as Nb and W alloy wires [9,10]; however, no systematic studies of interdiffusion between MoSi2 and refractory elements have been carried out. An understanding of the creep behavior and structural stability of the composites requires interdiffusion studies in MoSi2 and other silicides of Mo. Such studies are limited in literature. Interdiffusion between Mo and Si was investigated [11,12] at temperatures over 900°– 1350°C for the determination of empirical energies of activation for the growth of silicide layers. The growth kinetics of Mo5Si3 and Mo3Si between Mo and MoSi2 was reported over 1200°–1900°C [13,14]. Textures in

0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 9 8 ) 0 1 0 5 0 - 8

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Table 1 Crystal structures, lattice parameters, densities and molar volumes of refractory metal silicides of Mo, W, Re, Nb, and Ta [24–29] Phase

Crystal structure

˚) Lattice parameters (A

a

b

Density (g cm−3)

Molar volume (m3 mol−1)×105

Formula units per cell

c

Si

Diamond cubic

5.43

2.33

1.21

8

Mo Mo3Si Mo5Si3 MoSi2

bcc Cubic Tetragonal Tetragonal

3.15 4.89 9.62 3.20

4.90 7.87

10.22 8.97 8.26 6.24

0.94 3.52 6.83 2.42

8 2 4 2

W W5Si3 WSi2

bcc Tetragonal Tetragonal

5.05 9.65 3.12

4.97 7.88

19.35 14.41 9.75

0.95 6.96 2.44

8 4 2

Re Re2Si

4.61 6.44

6.41 5.38

20.5 –

0.91 5.0

4 4

ReSi Re5Si3

Hexagonal Monoclinic (b =94.16°) Cubic Tetragonal

4.77 9.53

4.81

13.1 15.4

1.64 6.58

4 4

Nb Nb3Si Nb5Si3

bcc Tetragonal Tetragonal

3.31 10.2 6.56

5.18 11.9

8.53 – 7.14

1.09 4.06 7.68

2 8 4

Ta Ta3Si Ta2Si Ta5Si3

bcc Tetragonal Tetragonal Tetragonal

3.31 10.2 6.16 9.88

5 18 5.04 5.06

16.26 14.13 13.54 13.44

1.09 4.06 2.88 7.44

2 8 4 4

9.60

disilicide layers of Mo and W diffusion-grown in vapor–solid diffusion couples at temperatures between 1000° and 1200°C have also been reported [15]. Diffusion studies with Mo vs. Si diffusion couples have been recently carried out over 900° – 1350°C for the determination of interdiffusion coefficients for the MoSi2 and Mo5Si3 phases; the energies of activation for interdiffusion in these phases are reported to be 130 9 20 and 2109 10 kJ mol − 1, respectively [16,17]. In addition, the MoSi2 diffusion layer developed a columnar structure and the Si to Mo ratio across the layer varied over the approximate range of 1.9 to 2 [16]. The objective of this study was to investigate interdiffusion and formation of silicide layers between MoSi2 and selected refractory metals. In this paper are reported the interdiffusion studies that were carried out at selected temperatures over 1300° – 1700°C with solid–solid diffusion couples assembled with disks of a single crystal MoSi2 and disks of polycrystalline W, Re, Nb and Ta. New results are presented on the diffusion structures, diffusion paths, interdiffusion coefficients and energies of activation for interdiffusion calculated for the silicide layers developed in the couples. The diffusion analysis employs multicomponent diffusion phenomenology and the concepts of integrated and average effective interdiffusion coefficients. Diffusional interactions among the components are explored in the light of zero-flux planes [18] and uphill diffusion ob-

served for Mo in the ternary silicides developed in the various couples.

2. Experimental procedure Plates (25× 25×0.5 mm) of 99.95% Mo, W, Re, Nb, Ta were obtained from Johnson-Matthey Alfa/Aesar, Ward Hill, MA. Samples of polycrystalline MoSi2 (20× 5× 5 mm) prepared from powder by hot pressing at 1800°C to 95% density [19] and a single crystal

Fig. 1. Mo vs. MoSi2 diffusion couple annealed at 1500°C for 6 h.

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P.C. Tortorici, M.A. Dayananda / Materials Science and Engineering A261 (1999) 64–77

Fig. 2. Contoured inverse pole figures based on 001, 010, 100 orientations measured in electron back-scattered patterns of the Mo5Si3 layer in Mo vs. MoSi2 couple annealed at 1600°C for 8 h.

sample of MoSi2 (diameter 7.5 ×25 mm) grown by a float zone technique were furnished by J. Petrovic [20] of Los Alamos National Laboratory. The rod-axis of the crystal was oriented at about 18° from [110] and the crystal cross-section corresponded approximately to (210). Disks approximately 1×1 ×0.5 cm were cut from the starting materials. The disks were mechanically

Fig. 3. (a) Back-scattered electron micrograph and (b) experimental concentration profiles for the W vs. MoSi2 diffusion couple annealed at 1500°C for 6 h.

Fig. 4. Experimental diffusion path for the W vs. MoSi2 diffusion couple annealed at 1500°C for 6 h.

polished on 600 grit SiC paper through 1-mm diamond paste and used in assembling solid–solid diffusion couples. A Centorr Testorr™ vacuum furnace specially equipped with tungsten heating elements and Mo– TZM rams was employed for diffusion annealing at temperatures between 1300° and 1700°C in an atmosphere of Ar–5%H2 gas. Three disks were assembled in each diffusion stack; two Al2O3 or MgO plates were placed on each side of the diffusion stack to prevent the terminal alloys from reacting with the TZM rams. A compressive stress between 2 and 5 MPa was maintained on the diffusion stack during annealing. The annealed diffusion couples were mounted in an epoxy resin and were sectioned parallel to the diffusion direction with a low speed diamond saw. The cross sections were prepared for metallographic examination by standard metallographic techniques through 1-mm diamond paste. Final polishing was conducted on Chemomet® disks employing colloidal silica as a lubricant. The slightly basic nature of the colloidal silica aided in slightly etching the diffusion structures that developed in the diffusion zones. A solution of 25 ml HNO3 and 5 ml HF in 50 ml of H2O was used for additional etching of the diffusion couples. The diffusion couples were examined with a Phillips Electroscan environmental scanning electron microscope (ESEM) equipped with secondary and back-scattered electron detectors. Concentration profiles were determined by point-to-point counting techniques with a Cameca SX-50 microprobe equipped with four wavelength dispersive spectrometers at an accelerating voltage of 15 kV and a probe current of 20 nA. Pure elemental standards were used for the conversion of X-ray intensities into compositions. Additional chemi-

P.C. Tortorici, M.A. Dayananda / Materials Science and Engineering A261 (1999) 64–77

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Table 2 Integrated interdiffusion coefficientsa in the W5Si3 and (W, Mo)5Si3 layers in the W vs. MoSi2 diffusion couples annealed between 1400° and 1700°C Temperature (°C)

Annealing time (h)

W5Si3 layer 1400 1500 1500 1500b 1600 1700

16 6 12 12 8 6

(W,Mo)5Si3 layer 1400 1500 1500 1500b 1600 1700

16 6 12 12 8 6

−1 −1 D0 int s ) Mo (mol m

– – – – – – −1.5×10−13 −8.0×10−13 −4.0×10−12 −4.2×10−12 −8.8×10−12 −4.2×10−11

−1 −1 D0 int s ) W (mol m

−1 −1 D0 int s ) Si (mol m

8.1×10−13 1.8×10−11 8.1×10−12 8.3×10−12 6.5×10−12 9.6×10−11

8.1×10−13 1.8×10−11 8.1×10−12 8.3×10−12 6.5×10−12 9.6×10−11

2.6×10−13 1.6×10−12 6.2×10−12 6.7×10−12 1.3×10−11 3.1×10−11

4.9×10−13 2.4×10−12 8.9×10−12 7.7×10−12 2.2×10−11 7.7×10−11

a The D0 int values can be expressed in units of m2 s−1 on multiplying with the appropriate molar volumes of the binary silicides or an average i of molar volumes of the binary silicides that make up a ternary silicide (see Table 1). b This corresponds to a couple assembled with a W (100) single crystal vs. single crystal MoSi2.

cal analyses were conducted on a JEOL-35CF scanning electron microscope (SEM) equipped with a Tracor Northern Series II energy dispersive spectrometer.

where #Cj /#x is the concentration gradient of component j and D0 nij are (n− 1)2 composition-dependent interdiffusion coefficients. On the basis of Eq. (3), Eq. (2) becomes [22]:

3. Integrated and average effective interdiffusion coefficients

D0 int i,Dx = %

n−1 j=1

&

Cj (x1)

D0 nij dCj

Cj(x 2)

n−1

For an isothermal, solid – solid diffusion couple the interdiffusion fluxes of all components can be determined at any section x directly from the concentration profiles. The interdiffusion flux J0 i(x)of component i at any time t is given by [18,21]: J0 i(x)=

1 2t

&

Ci (x)

(x − x0) dCi

(i =1, 2, …n)

Ci+ or Ci−

+ i

(1)

D int i,Dx =

x2

J0 i dx

(i =1, 2, …n)

(2)

where D0( nij correspond to average values of the interdiffusion coefficients over the concentration range in the diffusion zone from x1 to x2. An average effective interdiffusion coefficient D0 eff for the component i over i the interval is also defined by [22]: D0 eff i =

D0 int i,Dx Ci (x1)− Ci (x2)

Temperature (°C)

Annealing time (h)

j=1

(Cj (x

(W,Mo)5Si3 2 −1 2 −1 D0 eff ) D0 eff ) Mo (m s W (m s

where x2 is greater than x1 for positive fluxes and x2 is smaller than x1 for negative fluxes. On the basis of Onsager’s formalism of Fick’s law for multicomponent diffusion n−1

(5)

Table 3 Average effective interdiffusion coefficients for Mo and W in the (W,Mo)5Si3 phase layer in the W vs. MoSi2 diffusion couples annealed between 1400° and 1700°C

x1

J0 i = − % D0 nij

(4)

j=1

− i

where C and C are the concentrations in the terminal alloys, and x0 refers to the location of the Matano plane. Over a concentration range from Ci (x1) to Ci (x2) in the diffusion zone, an integrated interdiffusion coefficient, D0 int i,9C [22,23], is defined by:

&

= % D(0 nij[Cj (x1)− Cj (x2)]

(i =1, 2, …n −1)

(3)

1400 1500 1500 1500a 1600 1700

16 6 12 12 8 6

−2.4×10−17 −1.0×10−15 −2.6×10−16 −3.0×10−16 −1.0×10−15 −4.9×10−15

3.2×10−17 1.9×10−15 7.2×10−16 6.5×10−16 1.5×10−15 3.6×10−15

a This corresponds to a couple assembled with a W (100) single crystal vs. single crystal MoSi2.

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binary and ternary silicides are also determined from integrated interdiffusion coefficients. Binary phase diagrams for the Mo vs. Si, W vs. Si, Re vs. Si, Nb vs. Si and Ta vs. Si systems are available [24–28]. The crystal structures, lattice parameters and molar volumes of the various binary silicides in each system are presented in Table 1 [24–29]. There exist limited experimental isotherms for the ternary Me– Mo–Si systems; isotherms for the Nb–Si–Mo and W–Si–Mo systems are available at 800° and 1900°C, respectively [30,31].

4.1. Non-planar morphologies and texture de6elopment in Mo 6s. MoSi2 couples

Fig. 5. Ln (D0 int i ) vs. 1/T for the (a) W5Si3 and (b) (W, Mo)5Si3 phase layers in the W vs. MoSi2 diffusion couples annealed between 1300° and 1700°C.

From Eq. (4), Eq. (5) becomes: DCj 0( nii + % D0( nij D0 eff i =D DCi j"i n−2

(6)

where DCj and DCi are concentration differences of the components over x1 to x2.

4. Results and discussion Several series of solid – solid diffusion couples, Me vs. MoSi2, where Me=Mo, W, Re, Nb, or Ta were assembled with disks of Mo, W, Re, Nb and Ta and disks of a MoSi2 single crystal; they were annealed at selected temperatures in the range 1300° – 1700°C. Experimental diffusion structures and concentration profiles are presented for selected couples and discussed in the context of silicide layer development, morphologies of interfaces and diffusion paths. Integrated and average effective interdiffusion coefficients are calculated for the individual silicide layers observed in the diffusion zone. Energies of activation for interdiffusion in the various

Several couples assembled with disks of Mo and MoSi2 single crystal were annealed at selected temperatures between 1300° and 1700°C. Mo3Si and Mo5Si3 phase layers developed in the diffusion zone and exhibited appreciable non-planar morphologies at the Mo/ Mo3Si and Mo3Si/Mo5Si3 interfaces. A secondary electron micrograph of the diffusion zone of a couple annealed at 1500°C for 6 h is presented in Fig. 1. In addition to the non-planar interface morphologies, microcracks that run parallel in each growth front are observed in the tetragonal Mo5Si3 phase; their propagation seems to get arrested in the adjacent cubic Mo3Si phase. The microcracking can be a result of anisotropy in the coefficient of thermal expansion in the tetragonal Mo5Si3 phase. The non-planar growth fronts of the Mo5Si3 layer may reflect anisotropy in diffusion. A prelimanary texture analysis of the diffusion layer developed in a Mo vs. MoSi2 couple annealed at 1600°C for 8 h was carried out by orientation imaging microscopy at the TexSem Laboratories, Utah and is presented in the form of contoured inverse pole figures in Fig. 2. The Mo5Si3 layer appears to develop a strong 001 texture, but details on the orientation relationships between the Mo5Si3 and Mo3Si phases require further study.

4.2. Interdiffusion and diffusion structures in W 6s. MoSi2 couples Several W vs. single-crystal MoSi2 couples were annealed at selected temperatures between 1400° and 1700°C. A micrograph, as well as the concentration profiles, for a couple annealed for 6 h at 1500°C are presented in Fig. 3. Layers of W5Si3 and (Mo,W)5Si3 phases develop in the diffusion zone and the W/W5Si3 interface is non-planar A marker plane (xm) is visible near the section between the W5Si3 and (Mo,W)5Si3 regions. Diffusion along grain boundaries in the (Mo,W)5Si3 layer is also apparent by the dark contrast in Fig. 3(a).

P.C. Tortorici, M.A. Dayananda / Materials Science and Engineering A261 (1999) 64–77

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Fig. 6. (a) Back-scattered electron micrograph and (b) experimental concentration profiles for the Re vs. MoSi2 diffusion couple annealed at 1700°C for 6 h.

Fig. 4 shows the experimental diffusion path for the couple presented in Fig. 3. The dashed line segments of the path indicate the two-phase equilibria between W and W5Si3 and between the (W,Mo)5Si3 and MoSi2 phases. The solid line segment is the experimental composition path within the (W,Mo)5Si3 layer. In Table 2, values of the integrated interdiffusion coefficients D0 int calculated on the basis of Eq. (2) for i Mo, W and Si in the W5Si3 and (W,Mo)5Si3 layers for the various couples are presented. These D0 int values can i be expressed in units of m2 s − 1 by multiplying with the appropriate molar volumes of the individual silicide phases reported in Table 1; for the ternary silicides an average of the corresponding binary silicides is employed. The average effective interdiffusion coefficients calculated from Eq. (5) for W and Mo in the (W,Mo)5Si3 layer are presented in Table 3. In the binary W5Si3 layer, the D0 int values are common to both W and Si. In the ternary (W,Mo)5Si3 layer, D0 int Si is int 0 larger than D0 int and D in magnitude by a factor of W Mo

Fig. 7. Experimental diffusion path for the Re vs. MoSi2 diffusion couple annealed at 1700°C for 6 h.

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Table 4 Integrated interdiffusion coefficientsa for the Re2Si, (Re,Mo)Si and (Re,Mo)5Si3 phase layers in the Re vs. MoSi2 diffusion couples annealed between 1425° and 1700°C Temperature (°C)

Annealing time (h)

Re2Si layer 1425 1500 1600 1700

20 12 16 6

(Re,Mo)Si layer 1425 1500 1600 1700

20 12 16 6

(Re,Mo)5Si3 layer 1425 1500 1600 1700

20 12 16 6

−1 −1 D0 int s ) Mo (mol m

−1 −1 D0 int s ) W (mol m

−1 −1 D0 int s ) Si (mol m

1.41×10−12 2.9×10−12 6.7×10−12 8.3×10−12

1.41×10−12 2.9×10−12 6.7×10−12 8.3×10−12

−3.54×10−13 −3.1×10−13 −2.0×10−13 −1.5×10−13

4.27×10−12 9.3×10−12 4.0×10−11 8.5×10−13

3.71×10−12 1.0×10−11 4.0×10−11 8.7×10−11

−1.2×10−12 −1.8×10−12 −5.2×10−12 −1.3×10−11

4.6×10−13 1.2×10−12 2.5×10−12 6.9×10−12

1.4×10−12 3.0×10−12 8.2×10−12 2.1×10−11

– – – –

a The D0 int values can be expressed in units of m2 s−1 on multiplying with the appropriate molar volumes of the binary silicides or an average i of molar volumes of the binary silicides that make up a ternary silicide (see Table 1).

Table 5 Average effective interdiffusion coefficients for Mo and Re in the (Re,Mo)Si and (Re,Mo)5Si3 phase layers in the Re vs. MoSi2 diffusion couples annealed between 1425° and 1700°C Temperature (°C)

1425 1500 1600 1700

Annealing time (h)

20 12 16 6

(Re,Mo)Si

(Re,Mo)5Si3

2 −1 D0 eff ) Mo (m s

2 −1 D0 eff ) Re (m s

2 −1 D0 eff ) Mo (m s

2 −1 D0 eff ) Re (m s

−2.3×10−17 −1.9×10−17 −1.7×10−17 −1.1×10−17

2.7×10−16 7.4×10−16 3.4×10−15 6.0×10−15

−1.1×10−15 −2.1×10−15 −5.2×10−15 −7.7×10−15

4.8×10−16 1.6×10−15 2.6×10−15 4.0×10−15

2 –3. D0 int Mo is negative and indicates that the cumulative interdiffusion of Mo in the (W,Mo)5Si3 layer is against its own concentration gradient. The average effective interdiffusion coefficients of Mo and W in the (W,Mo)5Si3 layer are also of opposite signs, but of similar magnitude. The negative values of D0 eff Mo imply that Mo interdiffuses uphill against its own concentration gradient. The diffusion layers in these ternary couples were found to be smaller in thickness than those for the Mo vs. MoSi2 binary couples. This indicates that the W addition to MoSi2 can decrease the width of the diffusion zone. Another effect of the W addition was in the reduction of the microcracking in the ternary (Mo,W)5Si3 silicide relative to that in the Mo5Si3 layer. An examination of the W5Si3 layer by orientation imaging microscopy indicated a texture development similar to that observed for the MoSi2 vs. Mo couples presented in Fig. 2. Plots of Ln (D0 int i ) vs. 1/T for the W5Si3 and (W,Mo)5Si3 layers are presented in Fig. 5. The activation energies (Q) for the interdiffusion of both Si and

W were calculated to be 3609 60 and 450955 kJ mol − 1 in the W5Si3 and (W,Mo)5Si3 layers, respectively.

4.3. Interdiffusion and diffusion structures for couples between Re and MoSi2 Interdiffusion and diffusion structures were investigated in several Re and MoSi2 diffusion couples at selected temperatures over 1425°–1700°C. A back-scattered electron micrograph and the experimental concentration profiles for a Re vs. MoSi2 diffusion couple annealed at 1700°C for 6 h are presented in Fig. 6. The Re vs. MoSi2 couples developed three layers in the diffusion zone; one of the layers was Re2Si with negligible Mo content and the other two layers were ternary phases (Re,Mo)Si and (Re, Mo)5Si3. Fig. 7 presents the experimental diffusion path for the Re vs. MoSi2 diffusion couple presented in Fig. 6. The dashed line segments correspond to two-phase equilibria between phase layers of Re and Re2Si, Re2Si and (Re,Mo)Si, (Re,Mo)Si and (Re,Mo)5Si3, and (Re,Mo)5Si3 and

P.C. Tortorici, M.A. Dayananda / Materials Science and Engineering A261 (1999) 64–77

Fig. 8. Ln (D0 int i ) vs. 1/T for the Re2Si phase layer in the Re vs. MoSi2 diffusion couples annealed between 1425° and 1700°C.

Fig. 9. Ln (D0 int i ) vs. 1/T for the (Re,Mo)Si phase layer developed in the Re vs. MoSi2 diffusion couples annealed between 1425° and 1700°C.

MoSi2. Re solubility in MoSi2 is about 4 atom%. The diffusion paths of all the Re vs. MoSi2 diffusion couples were similar to the one shown in Fig. 7. The phases Re2Si and (Re,Mo)Si are consistent with the binary Re2Si and ReSi phases in the Re vs. Si binary phase diagram [26]. There exist no published ternary Re–Si– Mo phase diagrams [32]. The phase layers in Fig. 6(a) are essentially free of microcracks; cracks propagating from the MoSi2 phase appear to get blunted in the (Re,Mo)5Si3 phase. This beneficial effect is similar to that observed in the MoSi2 vs. W couples. In Table 4, the integrated interdiffusion coefficients calculated from Eq. (2) for the Re2Si, (Re,Mo)Si and (Re,Mo)5Si3 phase layers in the various couples are

71

Fig. 10. Ln (D0 int i ) vs. 1/T for the (Re,Mo)5Si3 phase layer developed in the Re vs. MoSi2 diffusion couples annealed between 1425° and 1700°C.

presented. On the basis of Table 1, the D0 int values reported in Table 4 may be expressed in units of m2 s − 1 by multiplying them with the molar volumes of 5.0× 10 − 5, 1.64×10 − 5 and 6.70×10 − 5 m3 . mol − 1, for the Re2Si, (Re,Mo)Si and (Re,Mo)5Si3 phases, respectively. Average effective interdiffusion coefficients calculated for Mo and Re in the (Re,Mo)Si and (Re,Mo)5Si3 phases from Eq. (5) are reported in Table 5. For the binary Re2Si layer, the D int values are the i same for both Re and Si. For the ternary (Re,Mo)Si 0 int values are quite similar, phase, the D0 int Re and D Si int whereas the D0 Mo values are negative and smaller in 0 int values. In the magnitude than the D0 int Re and D Si int (Re,Mo)5Si3 phase layer, the D0 Mo values are also negative, but larger in magnitude than those of D0 int Re and . comparable to those of D0 int Si Ln (D0 int i ) vs. 1/T plots for the Re2Si, (Re,Mo)Si and (Re,Mo)5Si3 phase layers are presented in Figs. 8–10, respectively. For the interdiffusion of Re and Si in the Re2Si and (Re,Mo)Si layers, the activation energies are approximately 1909 15 and 325 910 kJ mol − 1, respectively. The activation energies for interdiffusion calculated for Si and Re in the (Re,Mo)5Si3 layer are approximately 275 kJ mol − 1. This value is considerably less than the Q value of 450 kJ mol − 1 for the interdiffusion of W and Si in the (Mo,W)5Si3 layer calculated from Fig. 5. The effective interdiffusion coefficients in Table 4 provide additional insight on the interdiffusion behavior of Mo and Re in the (Re,Mo)Si and (Re,Mo)5Si3 phases. The negative values for D0 eff Mo imply that Mo interdiffuses up its own concentration gradient but down a Re concentration gradient in these ternary silicides of Mo and Re. The D0 eff Mo values are smaller in

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D0 3i2 (C1 = D0 3i1 (C2

(8)

ZFP

From the concentration gradients of Mo and Re at the ZFP compositions for the various couples, the ratios of 0 Si D0 Si MoRe/D MoMo were calculated on the basis of Eq. (8). These ratios of the cross to main ternary interdiffusion coefficients for Mo ranged from 0.9 to 1.4. This indicates that the cross interdiffusion coefficient D0 Si MoRe is about as large as the main interdiffusion coefficient D0 Si MoMo and contributes to the enhancement of the interdiffusion of Mo down a Re concentration gradient.

4.5. Interdiffusion and diffusion structures for couples between Nb and MoSi2

Fig. 11. Interdiffusion flux vs. distance for (a) all components and (b) Mo in the Re vs. MoSi2 diffusion couple annealed at 1700°C for 6 h.

magnitude than those of D0 eff Re for the (Re,Mo)Si phase. On the other hand, in the (Re,Mo)5Si3 phase, the D0 eff Mo values are larger in magnitude than those of D0 eff Re by about a factor of 2 and indicate a significant diffusional interaction between Re and Mo.

4.4. ZFP de6elopment in the (Re, Mo)Si phase All of the Re vs. MoSi2 diffusion couples listed in Table 3 exhibited ZFP development in the (Re,Mo)Si diffusion layer. Interdiffusion flux profiles calculated from the concentration profiles shown in Fig. 6 are presented in Fig. 11. Fig. 11 shows the development of a ZFP in the interdiffusion flux profile of Mo. A ZFP [18,21] is a plane in the diffusion zone where the interdiffusion flux of a component goes to zero and has opposite directions on the two sides of the plane. ZFPs were observed for Mo in all the Re vs. MoSi2 couples. For a component i developing a ZFP in a ternary system, J0 i is zero at the ZFP and Eq. (3) yields: J0 i = − D0 3i1 and hence,

(C1 (C − D0 3i2 2 = 0 (x (x

(7)

Several Nb vs. MoSi2 diffusion couples were diffusion annealed at selected temperatures between 1425° and 1700°C. A back-scattered electron micrograph, as well as experimental concentration profiles for a Nb vs. MoSi2 diffusion couple annealed at 1600°C for 16 h, are presented in Fig. 12. Two phase layers, corresponding to Nb5Si3 and (Nb,Mo)5Si3 are observed in the diffusion zone with appreciable porosity in the ternary silicide layer. These layers are comparable to the W5Si3 and (W,Mo)5Si3 layers observed in the W vs. MoSi2 diffusion couple in Fig. 3. The interface morphology between Nb and Nb5Si3 is relatively planar. The diffusion zone is similar in thickness to that of Re vs. MoSi2 couple shown in Fig. 6. The experimental diffusion path for the Nb vs. MoSi2 diffusion couple is presented in Fig. 13. The dashed line segments indicate two-phase equilibria between Nb and Nb5Si3 phases and (Nb,Mo)5Si3 and MoSi2 phases. The solid line segment is the composition path within the (Nb,Mo)5Si3 phase layer. Integrated interdiffusion coefficients calculated from Eq. (2) for the components in the Nb5Si3 and (Nb,Mo)5Si3 layers developed in the various couples are reported in Table 6. The average effective interdiffusion coefficients calculated for Nb and Mo in the (Nb,Mo)5Si3 phase from Eq. (5) are reported in Table 7. For the (Nb,Mo)5Si3 layer the D0 int Nb values are larger in magnitude than the negative D0 int Mo values, but smaller than the D0 int values. Similarly, values of D0 eff Si Nb are larger eff than those of D0 Mo in magnitude by a factor of 2–3. This observation is different from that for the Re vs. MoSi2 couples, where values of D0 int Mo were greater in magnitude than those of D0 int in the (Re,Mo)5Si3 phase. Re The interdiffusion of Mo against its own gradient but down a Nb gradient is reflected by the negative D0 eff Mo. Activation energies for interdiffusion were calculated from the slopes of the Ln(D0 int i ) vs. 1/T plots and are presented in Fig. 14. The activation energies (Q) for the interdiffusion of Si and Nb in the (Nb,Mo)5Si3 phase are 3009 10 and 2409 40 kJ mol − 1, respectively.

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Fig. 12. (a) Back-scattered electron micrograph and (b) experimental concentration profiles for the Nb vs. MoSi2 diffusion couple annealed at 1600°C for 16 h.

These are comparable to the activation energies for interdiffusion in the (Re,Mo)5Si3 layer and considerably less than those for interdiffusion in the (W,Mo)5Si3 layer. Q for the binary (Nb)5Si3 phase is 2659 30 kJ mol − 1. Zero-flux planes were observed in the (Nb, Mo)5Si3 layer in the Nb vs. MoSi2 couples annealed at 1600° and 1700°C. At the ZFP composition of 38.4 Si–8.3 Mo–52.3 Nb observed for the couple annealed at 1600°C, the ratio of the cross to main interdiffusion 0 Si coefficients, D0 Si MoNb/D MoMo was calculated to be around 0.8. Hence, the cross interdiffusion coefficient D0 Si MoNb is about as large as the main interdiffusion coefficient D0 Si MoMo and contributes to the enhancement of the interdiffusion of Mo down a Nb concentration gradient.

couple annealed at 1600°C for 8 h are presented in Fig. 15. Three phase layers, corresponding to Ta2Si, Ta5Si3, and (Ta,Mo)5Si3, developed in the diffusion zone. The

4.6. Interdiffusion and diffusion structures for couples between Ta and MoSi2 Two diffusion couples assembled with disks of Ta and polycrystalline MoSi2 were annealed at 1500° and 1600°C. The diffusion structure and the experimental concentration profiles for a Ta vs. MoSi2 diffusion

Fig. 13. Experimental diffusion path for the Nb vs. MoSi2 diffusion couple annealed at 1600°C for 16 h.

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Table 6 Integrated interdiffusion coefficientsa for the Nb5Si3, and (Nb,Mo)5Si3 phase layers in the Nb vs. MoSi2 diffusion couples annealed between 1425° and 1700°C Temperature (°C)

Annealing time (h)

Nb5Si3 layer 1425 1600 1600 1700

20 8 16 6

(Nb,Mo)5Si3 layer 1425 1600 1600 1700

20 8 16 6

−1 −1 D0 int s ) Mo (mol m

– – – – −4.2×10−13 −1.3×10−12 −5.2×10−12 −1.6×10−11

−1 −1 D0 int s ) Nb (mol m

−1 −1 D0 int s ) Si (mol m

1.8×10−12 5.6×10−12 7.0×10−12 3.0×10−11

1.8×10−12 5.6×10−12 7.0×10−12 3.0×10−11

9.4×10−13 6.3×10−12 8.3×10−12 1.1×10−11

1.6×10−12 8.2×10−12 1.4×10−11 3.0×10−11

a The D0 int values can be expressed in units of m2 s−1 on multiplying with the appropriate molar volumes of the binary silicides or an average i of molar volumes of the binary silicides that make up a ternary silicide (see Table 1).

Table 7 Average effective interdiffusion coefficients for Nb and Mo in the (Nb,Mo)5Si3 phase layer in the Nb vs. MoSi2 diffusion couples annealed between 1425° and 1700°C Temperature (°C)

Annealing time (h)

(Nb,Mo)5Si3 2 −1 2 −1 D0 eff ) D0 eff ) Mo (m s Re (m s

1425 1600 1600 1700

20 8 16 6

−6.3×10−17 −2.0×10−16 −7.1×10−16 −2.1×10−15

1.2×10−16 7.8×10−16 1.1×10−15 7.8×10−15

Ta5Si3 and (Ta,Mo)5Si3 layers are similar to the silicides formed in the Nb vs. MoSi2 couples. The Ta2Si layer, with negligible Mo content, corresponds to the Ta2Si layer on the binary Ta vs. Si phase diagram [28] and is analogous to the Re2Si layer observed in the Re vs. MoSi2 couples. The experimental diffusion path for the Ta vs. MoSi2 diffusion couple in Fig. 15 is presented in Fig. 16. The two-phase equilibrium along the path are separated by dashed line segments. The calculated integrated interdiffusion coefficients and average effective interdiffusion coefficients for the Ta2Si, Ta5Si3 and (Ta,Mo)5Si3 layers are presented in Tables 8 and 9, respectively. In the Ta2Si layer, the D0 int i values are about 2 – 3 times smaller than those reported values for the Ta5Si3 and for Re2Si in Table 4. The D0 int i (Ta,Mo)5Si3 layers are smaller than those for the (Nb,Mo)5Si3 layer reported in Table 6. The average effective interdiffusion coefficients, D0 eff Mo, are negative and about 3 – 5 times smaller in magnitude 0 eff than D0 eff Ta in the (Ta,Mo)5Si3 layer. The D Ta values are about an order of magnitude smaller than those of D0 eff Nb reported in Table 7. The negative interdiffusion coeffi-

Fig. 14. Ln (D0 int i ) vs. 1/T for the (a) Nb5Si3 and (b) (Nb, Mo)5Si3 phase layers in the Nb vs. MoSi2 diffusion couples annealed between 1425° and 1700°C.

cients for Mo reflect the uphill interdiffusion of Mo against its own concentration gradient, but down a Ta gradient. A zero-flux plane was also identified for Mo in the (Ta,Mo)5Si3 layer.

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Fig. 15. (a) Back-scattered electron micrograph and (b) experimental concentration profiles for a couple assembled with polycrystalline Ta and MoSi2 and annealed at 1600°C for 8 h. Table 8 Integrated interdiffusion coefficientsa for the Ta2Si, Ta5Si3 and (Ta, Mo)5Si3 phase layers in the Ta vs. polycrystalline MoSi2 diffusion couples. Temperature (°C)

Annealing time (h)

Ta2Si layer 1500 1600

12 8

Ta5Si3 layer 1500 1600

12 8

(Ta,Mo)5Si3 layer 1500 1600

12 8

−1 −1 D0 int s ) Mo (mol m

−1 −1 D0 int s ) Ta (mol m

−1 −1 D0 int s ) Si (mol m

– –

9.5×10−13 3.6×10−12

9.5×10−13 3.6×10−12

– –

5.9×10−13 9.6×10−13

5.9×10−13 9.6×10−13

4.6×10−13 2.7×10−12

6.8×10−13 3.4×10−12

−7.0×10−14 −1.1×10−12

a The D0 int values can be expressed in units of m2 s−1 on multiplying with the appropriate molar volumes of the binary silicides or an average i of molar volumes of the binary silicides that make up a ternary silicide (see Table 1).

4.7. Diffusional interactions of W, Re, Nb and Ta on Mo in ternary silicides In this study, all of the Me vs. MoSi2 ternary diffusion couples, where Me=W, Re, Nb, or Ta, developed

a zero-flux plane in one of the silicide layers and the interdiffusion behavior of Mo in the ternary (Me,Mo)5Si3 silicides could be characterized by a nega0 eff tive D0 eff Mo. On the basis of Eq. (5), D Mo is related to the main and cross ternary interdiffusion coefficients by:

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Table 9 Average effective interdiffusion coefficients for Mo and Ta in the (Ta,Mo)5Si3 phase layer in the Ta vs. MoSi2 diffusion couples Temperature (°C)

Annealing time (h)

(Ta,Mo)5Si3 2 −1 2 −1 D0 eff ) D0 eff ) Mo (m s Ta (m s

1500 1600

12 8

−1.1×10−17 −1.7×10−16

DCMe (0 Si (0 Si D0 eff Mo = D MoMo + D MoMe DCMo

5.9×10−17 3.3×10−16

(9)

where DCMe =[CMe(x1) −CMe(x2)] and DCMo = [CMo(x1)−CMo(x2)] represent the concentration ranges for the layers. For the (Me,Mo)5Si3 silicide layers, DCMe/DCMo : − 1. This implies that the cross coefficient, D0 Si MoMe is positive and can be larger than the main 0 eff coefficient D0 Si MoMo, since D Mo is negative. Hence, the interdiffusion of Mo is enhanced down the Me gradient. Based on the magnitudes of the D0 eff Mo, the Re additions are observed to have the most significant effect on Mo interdiffusion, followed by W, Nb and Ta.

5. Summary and conclusions Interdiffusion was investigated at selected temperatures between 1400° and 1700°C with several series of MoSi2 vs. Me ternary couples, where Me = W, Re, Nb, or Ta. From an analysis of concentration profiles, integrated and average effective interdiffusion coefficients were determined for the components in the various silicide layers developed in the diffusion zone. The main observations and conclusions are as follows. The Mo3Si and Mo5Si3 phase layers developed in the Mo vs. MoSi2 diffusion couples exhibited non-planar interface morphologies; the Mo5Si3 layer exhibited 001 texture. Oriented microcracking was also observed in the Mo5Si3 layer. For the W vs. MoSi2 diffusion couples, W5Si3 and (W,Mo)5Si3 layers developed with little microcracking. Layers of Re2Si, (Re,Mo)Si and (Re,Mo)5Si3 layers were observed in the diffusion zone of the Re vs. MoSi2 diffusion couples. Cracks appear to get blunted in the (Re,Mo)5Si3 layer. The Nb vs. MoSi2 couples developed layers of Nb5Si3 and (Nb,Mo)5Si3 phases with porosity in the diffusion zone. The diffusion structure of Ta vs. MoSi2 diffusion couples consisted of layers of Ta2Si, Ta5Si3 and (Ta,Mo)5Si3 with no microcracking. Values of the average effective interdiffusion coefficients, D0 eff Mo, for Mo in the ternary (Me,Mo)5Si3 silicide layers were negative. The negative D0 eff Mo indicates that Mo interdiffuses up its own concentration gradient.

Fig. 16. Experimental diffusion path for the Ta vs. MoSi2 diffusion couple annealed at 1 600°C for 8 h.

This is also reflected in the ZFPs for Mo identified in the ternary silicide layers. The diffusional interactions determined at the ZFPs indicate that the cross ternary interdiffusion coefficient D0 Si MoMe is about as large as the main ternary interdiffusion coefficient D0 Si MoMo and contributes to the enhancement of interdiffusion of Mo down the Me concentration gradient. In this context, Re additions are more effective than additions of W, Nb and Ta. The activation energies (Q) for the interdiffusion of both Si and W are calculated to be about 360 and 450 kJ mol − 1 in the W5Si3 and (W,Mo)5Si3 layers, respectively. For the interdiffusion of both Re and Si, values of Q are determined to be about 190 kJ mol − 1 in the Re2Si phase, 325 kJ mol − 1 in the (Re,Mo)Si phase and 270 kJ mol − 1 in the (Re,Mo)5Si3 phase. Q-values for the interdiffusion of Si and Nb in the (Nb,Mo)5Si3 phase are about 300 and 240 kJ mol − 1, respectively; in the binary Nb5Si3 phase, Q is about 265 kJ mol − 1.

Acknowledgements This research was supported by the Office of Naval Research under the grant No. N00014-95-0466 and is based on a dissertation submitted by P.C. Tortorici to Purdue University in partial fulfillment of the requirements for the Ph.D. degree. Sincere thanks are expressed to J.J. Petrovic of Los Alamos National Laboratory for providing the polycrystalline and single crystal MoSi2 samples employed in this study. Sincere appreciation is also expressed to M.C. Petri of Argonne National Laboratory for his assistance in the calculation of the interdiffusion coefficients.

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