silicon systems

Scripta

METALLURGICA

V o l . 22, p p . 1 0 1 1 - 1 0 1 4 , 1988 Printed in the U.S.A.

Pergamon Press plc All rights reserved

MECHANICAL ALLOYING BEHAVIOR IN GROUP V TRANSITION METAL/SILICON SYSTEMS R.K. Viswanadham, S.K. Mannan and S. Kumar Martin Marietta Laboratories 1450 S. Rolling Road Baltimore, MD 21227-3898

(Received February 12, 1988) (Revised March 16, 1988)

INTRODUCTION Mechanical alloying (MA) is the intimate mixing, on an atomic scale, of constituents that results from intense mechanical working in a high-intensity mill of a powder mixture. This method was first used successfully to produce oxide-dispersion-strengthened Ni and Al-base alloys (1). The process has been scaled up to produce these materials in commercial quantities. Though highly deformed, the crystalline nature of the constituente usually, but not always, is retained in the final product. In recent years it has been demonstrated by several workers that MA of certain elemental powders can result in the formation of an amorphous structure whose chemical composition corresponds to that of the powder mixture (2-6). In some cases, mechanical alloying of elemental powders has been shown to result in the formation of a compound (7). Mechanical alloying is by no means the only method for amorphization. Other methods such as ion-beam mixing (8), solid-state reaction (9), and hydrogenation (10), can also result in amorphization. However, amorphization by mechanical alloying (AMA) appears to be very attractive for processing materials that will eventually be used in bulk quantities. Group IV, V, and VI transition metals form a variety of compounds with silicon (11). These silicides posess many interesting properties, such as high melting point, low density, and good oxidation resistance (12). However, their compositional range often is very narrow and their high melting points make processing by conventional methods very difficult. Because AMA followed by a low-temperature crystallization anneal might provide an alternate, low-energy processing route, the MA behavior of V-Si, Nb-Si, and Ta-Si systems were examined at metal to silicon atomic ratios of 5:3 and 1:2. These particular ratios were chosen because the crystalline compounds at these ratios are stable up to their melting points and have similar crystal structures. In a study on the mechanical alloying behavior of Nb-Ge alloys, Politis examined the MA behavior of Nb to Si in the ratio 3:1 (13). Amorphization was observed after prolonged alloying time. However, the crystalline counterpart at this composition is only stable over a narrow temperature range (11). EXPERIMENTAL PROCEDURE Mechanical alloying was carried out at room temperature in a Spex Model 8000 high-intensity mixer/mill with a carbide-lined stainless steel vial (76- x 57-ram diameter) and hardened 52100 steel balls (12-mm diameter). Approximately 20 g powder and 100 g steel balls were used. For low density systems such as V-Si, the amount of powder was reduced to I0 g to enable ~ to proceed at a reasonable rate. The premixed powder and the balls were loaded into the vial in an argon-filled bag to avoid oxidation during alloying. The end caps of the vial were sealed with gaskets to prevent entry of oxygen. Periodically the mill was stopped, the vial was allowed to cool for a few minutes, and a small quantity of the powder was withdrawn for x-ray diffraction analysis. The maximum temperature reached during ~ did not exceed 80 °C. The starting powders were -325 mesh with a nominal purity exceeding 99.5%. 1011 0036-9748/88 $3.00 + .00

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RESULTS AND DISCUSSION Powder mixtures corresponding to the compositions V-Si (1:2), Nb-Si (1:2), and Ta-Si (1:2) were mechanically alloyed using the procedure described above. X-ray diffraction patterns taken at different times for the three systems are shown in Figs. 1-3. In the initial stages, the principal changes observed are broadening of the metal peaks and decreasing intensity of the Si peaks. The low solubility of Si in group V metals (11), and the lack of any significant shift in the peak positions of the transition metal lead us to conclude that the decrease in the intensity of the Si peaks is due to fracture of Si particles and coverage of these fractured particles by highly deformed metal layers. The rate of change of the intensity of the Si peaks is highest for Ta-Si and lowest for V-Si. Since the atomic number and hence the x-ray absorption are highest for Ta, this observation is in agreement with coverage of the Si particles by deformed metal layers. In Nb-Si, once the intensity of the Si peaks became negligible, formation of crystalline NbSi~ was observed (Fig. 1). Agreement of the peak positions and their relative intensities with published data is very good (14). Formation of the disilicide in the Nb-Si system appears to be fairly rapid since no intermediate stage where the elemental mixture and the crystalline disUicide coexist was found. With continued MA, the disilicide peaks began to broaden. The MA behavior of Ta-Si and V-Si was different. As the x-ray patterns in Fig. 3 demonstrate, formation of VSi s was more gradual. Crystalline VSi 2 and the elemental blend coexist and, over time, the elemental blend is gradually converted to VSi 2. The MA behavior of Ta-Si was intermediate between that of Nb-Si and V-Si (Fig. 2). Powder mixtures corresponding to the compositions V-Si (5:3), Nb-Si (5:3), and Ta-Si (5:3) were mechanically alloyed and the x-ray diffraction patterns taken at different times are shown in Figs. 4-6. In the initial stages a decrease in the intensity of the Si peaks similar to that observed for the disilicides was noted. With futher alloying, Ta-Si and V-Si became amorphous. In V-Si, crystalline VsSi a peaks began to appear if MA were continued for prolonged times. In the Nb-Si system, on the other hand, formation of crystalline NbsSi a was observed.

SUMMARY The mechanical alloying behavior of group V transition metal/silicon systems is not uniform and amorphization is not the rule. A variety of pathways are followed depending on the metal to silicon ratio and the specific transition metal involved. Reasons for why a particular pathway is chosen are not clear. The heat of formation, crystal structure, and the melting points of NbsSi 3 and TasSi 3 are very similar (12) yet the pathways chosen are very different. Amorphization occurs in the Ta-Si (5:3) system but not in the Nb-Si (5:3) system.

ILF~.K!~?~LC~~ 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

J.S. Benzamin, Metall. Trans., 1, 2943 (1970). C.C. Koch, O.B. Cavin, C.G. M c I ~ m e y and J.O. Scarborough, Appl. Phys. Left., 43, 1017 (1983). R.B. Sch~varz, R.R. Petrich and C.K. Shaw, J. Non-cryst. Solids, 76, 281 (1985). E. Hellstern and L. Schultz, Appl. Phys. Left., 48(2), 124 (1986). C. Politis and W.L. Johnson, J. Appl. Phys., 60(3), 1147 (1986). E. Hellstern and L. Schultz, Marl. Sci. Eng., 93, 213 (1987). B.T. McDermott and C.C. Koch, Scr. Metall., 20(5), 669 (1986). B.X. Liu, Phys. Status Solidi A., 94, 11 (1986). H. Schroder, K. Samwer and U. Koster, Phys. Rev. Left., 54(3), 197 (1985). K. Aoki, T. Yamamoto and T. Masumoto, Scr. Metall., 21, 27 (1987). Binary Phase Diagrams, Ed. T.B. Massalski, et al., ASM, Metals Park, OH 44073, 1,2 (1986). G.V. Samsonov, High Temperature Materials, Properties Index, Plenum Press, New York, 2 (1964). C. Politis, Physica, 135B, 286 (1985). ASTM X-ray Diffraction Card Index/Powder Diffraction File- JCPDS, Swarthmore, PA.,~985~.

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• METAL

• METAL

eSILICON

,COMPOUND

• SILICON eCOMPOUNO

t:0h

=~

t=2h

l:5h

!

~

t=3h

t=16h

J 70

r ~

I

t

i 50

i

i

i 30

i

i

i

,

~o -'

50

30

28 DEGREES

20 DEGREES

Fig. 1 MA response of Nb-Si (1:2) • METAL

Fig. 2 MA response of V-SI (1:2) eSlLICON

aCOMPOUND

t=0h

1=2.5h

=

1:3h

t=10h

70

50

30

28 DEGREES

Fig. 3 MA response of Ta-Si (1:2)

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I:0h

•METAL

eSlLICON

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• METAL t=2h

•SILICON

• COMPOUND

t=Oh

m

.< 70 °

Z

50

t=12h

70

30

28 D E G R E E S

S0

30

28 D E G R E E S

Fig. 4 MA response of Nb-Si (5:3)

• METAL

Fig. 5 ~

•SILICON

• COMPOUND

t=0

I=lh

1=511

t=12h

70

50 ~

30

28 D E G R E E S

Fig. 6 MA response of Ta-Si (5:3)

response of V-Si (5:3)

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