A new amorphous Mo46Co54 alloy prepared by high-rate sputter deposition

Scripta M E T A L L U R G I C A

Vol. 12, pp. 1037-I041, 1978 Printed in the United States

Pergamon Press,

Inc

A NEW A M O R P H O U S Mo~6Cos~ ALLOY P R E P A R E D BY H I G H - R A T E SPUTTER D E P O S I T I O N

R. Wang, M. D. Merz and J. L. Brimhall Battelle Pacific N o r t h w e s t L a b o r a t o r i e s Richland, W a s h i n g t o n 99352 USA (Received August 14, 1978) (Revised September I, 1978)

INTRODUCTION Recently, we reported the formation of an amorphous Ws0Fe~0 alloy made by high-rate sputter d e p o s i t i o n (i). The formation of the amorphous W-Fe alloy indicated that these two inner transition metals with c l o s e - b y group numbers can be made into the amorphous phase near the c o m p o s i t i o n of the e q u i l i b r i u m phase, W6FeT. Based on the same consideration, an amorphous Mo46Cos4 alloy was prepared. R~sults of x-ray diffraction, TEM, m i c r o h a r d n e s s and SEM fractography from an alloy d e p o s i t e d at l i q u i d - n i t r o g e n temperature were c o n s i s t e n t with an a m o r p h o u s structure and in many cases resembled the amorphous WsoFes0 alloy. This new amorphous Mo4~Cos4, formed at the c o m p o s i t i o n of the e q u i l i b r i u m ~-phase, Mo6Co7, adds to the evidence that the ~-phase structure is responsible for the formation and thermal stability of several r e f r a c t o r y amorphous alloys of W-Fe, Nb-Ni and Ta-Ni systems. Based on those p r e l i m i n a r y o b s e r v a t i o n s of structural and p r o p e r t y aspects, we suggest that the structure of amorphous Mo~6Cos4and Ws0Fes0 may be c o n s i d e r e d as a d e v i a t i o n from the dense random p a c k i n g s (DRP) o b s e r v e d for m e t a l - m e t a l l o i d amorphous alloys, and instead have a dense r a n d o m t e t r a h e d r a l p a c k i n g (DRTP) with short-range order similar to a rangom i n t e r p e n e t r a t i o n of Kasper p o l y h e d r a in t h r e e - d i m e n s i o n a l space. EXPERIMENTAL PROCEDURE The s p u t t e r - d e p o s i t e d Mo46Cos4 alloy was p r e p a r e d by triode d.c. sputtering of a Mo-Co c o m p a c t e d powder target. The sputtering was done w i t h Kr at a rate of 1 zm/min on to a l i q u i d - n i t r o g e n - c o o l e d Cu substrate. The resulting deposit was about 0.l-mm thick with a c o m p o s i t i o n of Mo-54 at% Co o b t a i n e d from x-ray elemental analysis. RESULTS AND D I S C U S S I O N X-ray D i f f r a c t i o n The x-ray d i f f r a c t i o n pattern of the a s - d e p o s i t e d Mo46Co~4 resembles those reported for a m o r p h o u s structures (Fig. i). Similar to the amorphous W50Fes0 alloy made at room temperature, the line b r o a d e n i n g of the first d i f f r a c t i o n peak of the Mo46Co5~ alloy c o r r e s p o n d s to a c o h e r e n t l y d i f f r a c t i n g domain size of 1.47nm, w h i c h may be interpreted either as high degree of short-range order in the amorphous structure or as m i c r o c r y s t a l l i n i t y . Transmission Electron Microscopy T r a n s m i s s i o n e l e c t r o n m i c r o s c o p y samples were p r e p a r e d by e l e c t r o c h e m i c a l thinning w i t h a sulfuric a c i d - m e t h a n o l solution. At low magnifications, the a s - d e p o s i t e d alloys showed a featureless m i c r o s t r u c t u r e w i t h broad d i f f r a c t i o n rings. D a r k - f i e l d m i c r o g r a p h s o b t a i n e d from the first broad d i f f r a c t i o n ring did not reveal any grains. At high magnification, the m i c r o s t r u c t u r e s had the "salt and pepper" feature c h a r a c t e r i s t i c of an amorphous solid at this magni1037

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Fig. i. X-ray d i f f r a c t i o n pattern of s p u t t e r - d e p o s i t e d Mo4~Cos~alloy. The line b r o a d e n i n g of the first peak c o r r e s p o n d s to a d i f f r a c t i n g domain size of ].47nm,

f

20o

45 o

70 o

95o

120 o

20 ÷

Fig.

Fig.

2.

3.

TEM m i c r o g r a p h of a s - d e p o s i t e d Mo~6Cos4 with the " salt and pepper " feature c h a r a c t e r i s t i c of an a m o r p h o u s solid.

The amorphous Mo46Co5~ alloy c r y s t a l l i z e d into ~- phase c r y s t a l l i t e s after a n n e a l i n g of 900°C for 2 h.

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fication (Fig. 2). The m i c r o s t r u c t u r e of a s - d e p o s i t e d Mo4~Cos~ was similar to the s p u t t e r - d e p o s i t e d amorphous Ws0Fes0 (i) and s p l a t - c o o l e d amorphous Nb~0Ni60 alloys (2). C r y s t a l l i z a t i o n into fine c r y s t a l l i t e s of about 0.i ~m d i a m e t e r was found after a n n e a l i n g at 800 ° and 900°C for 2 hr as shown in Fig. 3. The c r y s t a l l i t e s were identified as e q u i l i b r i u m ~-phase by both e l e c t r o n and x-ray diffraction. It is interesting that the ~-phase c r y s t a l l i t e s contain a high density of stacking faults or microtwins. Hardness and Density Even though both Mo and Co elements are soft, the a s - d e p o s i t e d amorphous Mo4~Cos4 alloy had a high m i c r o h a r d n e s s of 1050 kg/mm 2 w h i c h was c o m p a r a b l e to the ii00 k g / m m 2 of the l i q u i d - q u e n c h e d amorphous FesoB20, and only slightly less than the 1210 kg/mm 2 of the c r y s t a l l i n e b-phase based on a sample annealed at 900°C for 3 hr. This high m i c r o h a r d n e s s of the amorphous Mo46Co54 is indicative of strong c o v a l e n t bonding c h a r a c t e r i s t i c s r e s e m b l i n g those of the crystalline b-phase. However, the c r y s t a l l i z e d samples were brittle in c o n t r a s t to the amorphous alloy for w h i c h a thin sample could be bent e x t e n s i v e l y w i t h o u t breaking or plastic deforming. The a s - d e p o s i t e d sample had a density of 9.55 g/cm 3, w h i c h was only 3% less dense than the 9.85 g/cm of a 900°C a n n e a l e d sample. The density of the annealed sample was within the range of the x-ray density, 9.76-10.02 g/cm 3, o b t a i n e d from the reported lattice p a r a m e t e r s for the solid solutions of the b-phase and the c o m p o s i t i o n Mo6Co7. Thermal Effects Thermal effects for the amorphous Mo~6Cos4 alloy were m e a s u r e d w i t h differential s c a n n i n g c a l o r i m e t r y (DSC) between 400°-890°C. A small e x o t h e r m i c reaction was o b s e r v e d after 5 min at 810°C. Further increases in the DSC t e m p e r a t u r e and s e n s i t i v i t y revealed a 2500 J / m o l e c r y s t a l l i z a t i o n r e a c t i o n near 867°C with 20°C/min scan rate (Fig. 4). The heat of c r y s t a l l i z a t i o n for amorphous Mo46Cos4 was c o n s i d e r a b l y lower than the 4700 J / m o l e of Nb~0Ni60 and 6700 J /mole of FesoB20. No second thermal peak, as p r e v i o u s l y o b s e r v e d for a m o r p h o u s Nb40Ni60, was d e t e c t e d in this t e m p e r a t u r e range, and though studies at higher temperature was limited by the instrument. However, we did not expect to find additional thermal peaks beyond the 867°C-peak because x-ray d i f f r a c t i o n indicated that the amorphous structure t r a n s f o r m e d into e q u i l i b r i u m b-phase. The crystallization, therefore, o c c u r r e d between 800 ° and 900°C, which was also consistent with the x-ray d i f f r a c t i o n and TEM data for annealed samples.

+

/

H

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820°

830 °

840 o

850 °

860°

\ \

8700

880 °

T E M P E R A T U R E °C Fig.

4.

DSC t h e r m o g r a m for s p u t t e r - d e p o s i t e d Mo4~Cos4 heated at 20°C/min indicating a c r y s t a l l i z a t i o n e x o t h e r m of 2500 J / m o l e .

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Fracture Pattern A common feature observed in amorphous alloys is the unique fracture surface containing vein-like patterns. The SEM fractography of the as-deposited Mo~6Cos4 alloy, (Fig. 5a) shows a fine, equally spaced fracture pattern, such as those observed for amorphous Ws0Fe50 alloy. We have observed this equally spaced vein pattern in a number of high-rate sputter-deposited amorphous alloys containing metalloids, such as FesoB20 and Fe~0PI~C7 (Fig. 5b). These fracture patterns of sputter-deposited amorphous alloys were different from that obtained from liquid-quenched amorphous alloys. The fine spacings are aligned at various directions and are unrelated in orientation to the growth direction.

a.

Fig. 5.

Fracture surface patterns for sputter-deposited a. Mo4~Cos4, and b. Fee0P13CT.

b.

amorphous alloys;

Discussion of the Amorphous Structure There are many reasons to believe that the atomic bonding in elemental and compound amorphous solids resembles the bonding of the crystalline counterparts as suggested by Wang and Merz (3) and Goodman (4). So far, for transition metal systems, several binary amorphous phases, such as W-Fe, Nb-Ni and Ta-Ni were formed near the composition of the ~-phase. The amorphous M046C05~ alloy provides additional evidence that the crystal structure of the crystalline ~-phase may be responsible for the formation and the stability of the amorphous structure. For this consideration, it is possible to comprehend the amorphous structure based on the unique short and long-range atomic orders of the ~-phase. The u-phase has a giant hexagonal unit cell containing 39 atoms and is made of interpenetrated ~-phase polyhedra in a planar arrangement. The ~-phase polyhedron, with a coordination number of 15, is one of the Kasper polyhedra that constitute a unique space-filling geometry different from the cubic or hexagonal close-packed structures. The cubic or hexagonal close-packed structures have both tetrahedra and octahedra voids, but the Kasper structures contain only

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irregular t e t r a h e d r a voids and form a tetrahedral c l o s e - p a c k e d structure. It is then r e a s o n a b l e to expect that the basic tetrahedral c l o s e - p a c k e d structure would be e n e r g e t i c a l l y favorable in an a m o r p h o u s state. The i r r e g u l a r i t y of the tetrahedra r e s u l t i n g from the varying length of their edges and the edge angles allows for a d j u s t m e n t s in the atomic size and f l e x i b i l i t y in approaching special bond angles. This may lower the free energy of the amorphous structure and lead to the ease of formation and high thermal stability of the amorphous alloy. We b e l i e v e that amorphous Mo46Co54 and Ws0Fes0 and, possibly, the liquidq u e n c h e d Nb-Ni and Ta-Ni alloys near this c o m p o s i t i o n consist b a s i c a l l y of random t e t r a h e d r a l c l o s e - p a c k i n g (RTCP) of atoms w i t h short-range order similar to a random i n t e r p e n e t r a t i o n of Kasper polyhedra in t h r e e - d i m e n s i o n a l space. Evidence for the short-range order came from p r e l i m i n a r y x-ray d i f f r a c t i o n data. The close s i m i l a r i t y between the short-range order of amorphous and crystalline structures may explain the small heat of c r y s t a l l i z a t i o n for amorphous Mo~6Co54. Moreover, Nagel et al. (5) r e c e n t l y indicated that the formation and stability of the l i q u i d - q u e n c h e d Nb-Ni glasses can be i n t e r p r e t a t e d by a decrease in density of states at the Fermi level as shown by p h o t o e m i s s i o n experiments. Our atomistic concept for the short-range order of amorphous Mo~6Co54 that contains random tetrahedral c l o s e - p a c k i n g to lower the free energy, is parallel to the e l e c t r o n i c structural i n t e r p r e t a t i o n for anorphous Nb-Ti alloys. In general, a c o m b i n e d picture of both atomistic and electronic structural concepts enhances u n d e r s t a n d i n g of the r e f r a c t o r y amorphous alloys.

ACKNOWLEDGEMENT The authors thank A. G. Graybeal, L. D. Fetrow and H. E. Kjarmo for experimental assistances. The work was supported by Battelle Memorial Institute, C o r p o r a t e Technical D e v e l o p m e n t Project 333-413.

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R. Wang, M. D. Merz, J. L. Brimhall and S. D. Dahlgren, Third International C o n f e r e n c e on Rapidly Quenched Metals, Brighton, England, July (1978).

2.

B. C. Giessen,

3.

R. Wang and M. D. Merz, 35, (1976).

4.

C. H. L. Goodman,

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S. R. Nagel,

M. Madhava and D. E. Polk, Mat. Phy.

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