Centrifugal spinning of metallic glass filaments

Mat. Res. Bull. Vol. II, pp. 49-54, 1976. P e r g a m o n Press, Inc. Printed in the United States.

CENTRIFUGAL

SPINNING OF METALLIC GLASS FILAMENTS

H. S. Chen and C. E. Miller B e l l Laboratories M u r r a y Hill, New Jersey 07974

(Received N o v e m b e r 14, 1975; C o m m u n i c a t e d by N. B. Hannay)

ABSTRACT A new technique for continuous melt spinning of metal filaments in a glassy state has been developed. The technique consists of casting a stream of the melt from an orifice onto a convex inner surface of a rapidly rotating wheel. A number of glass forming alloys, such as t r a n s i t i o n - s e m i m e t a l s and early t r a n s i t i o n - t r a n s i t i o n metals, have been formed successfully into glassy ribbons. The advantages and limitations of this technique will be discussed.

Introduction Metallic glasses have various p o t e n t i a l l y useful properties, such as high strength with ductility (1-3) and very soft magnetic b e h a v i o r (4,5). Effort in recent years, therefore, has c o n c e n t r a t e d on d e v e l o p i n g methods for f a b r i c a t i n g metal filaments directly from melts so that the inherent costs and difficulties of die drawing and rolling could be avoided. Strange and Pim (6) developed a m e t h o d in w h i c h the m o l t e n stream is cast onto the outside of a rotating roll. Centrifugal force is exerted on the impinging stream i m m e d i a t e l y upon contact w i t h the roll, causing the melt to separate from the roll. In spite of the very short period of contact which results in poor heat transfer b e t w e e n the melt and the quenching roll, this process may be readily employed to form filaments of m a n y p o l y c r y s t a l l i n e metals and p o s s i b l y of some metallic glasses which can be readily quenched into a g l a s s y state. Pond and Maddin (7) developed a method in w h i c h a melt is ejected from a m o v a b l e mold onto the inside surface of a spinning drum. The radial centrifugal force exerted on the melt insures a good thermal contact b e t w e e n the melt and the drum. However the quenched filaments could not readily be pulled out from the drum. This m e t h o d produces only discontinuous lengths. 49

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Chen and Miller (8), in their original experiment, ejected a drop of melt between a pair of rapidly rotating rollers. The melt solidifies while passing through the rollers. This yields short filaments of uniform thickness. This method was later employed to produce continuous filaments of u n i f o r m thickness by injecting a stream of melt between the rollers (9). One drawback of this method is possible plastic deformation of the filaments during the process. In this paper we describe a new technique for cast spinning of metal filaments in a glassy as well as a crystalline state. The technique possesses a combination of the merits of above-mentioned techniques: (1) continuous fabrication of filaments at high speed (2) good thermal contact between the melt and the cooling b l o c k (3) no plastic deformation during the process. Procedures The technique consists of ejecting a stream of the melt from an orifice onto the sloped inner surface of a rapidly rotating wheel (driven by a variable speed motor). The orifice which is about 0.25 mm in diameter is formed by drawing a fused quartz tube. The wheel is about 33 cm in diameter and made of Cu-Be alloy which provides a surface which can be readily polished and yet retains the high thermal conductivity of Cu. The rotational speed of the wheel ranges from 300 to 1800 rpm, corresponding to a tangential velocity of 5 to 30 m/sec.

N

a_

Schematic

FIG. 1 drawing of the centrifugal

j spinning technique.

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The wheel has a convex inner surface as shown in Fig. i. The normal components of the centrifugal force exerted on the impinging melt, FN (=F cos e), insures good thermal contact between the melt and wheel surface. At the same time, the force components along slope, F t = (F sin e), forces the solidified strip to slip down the slope. As the solid strip slides along the slope, F t increases as e increases, and consequently accelerates further-the departure of the strip from the wheel. The convex surface is therefore designed to yield a quick departure of the solid strip while retaining sufficient thermal heat transfer between the impinging melt and the wheel during solidification. It also provides the flexibility for achieving proper quenching conditions for various alloy melts by simply repositioning the impinging melt along the slope. Results and Discussion A number of alloy compositions, Ml_yXy with 0.15 ~ Y 0.30 have been formed successfully into a glassy state. Here M = Pt, Pd, Fe, Co and/or Ni, and X = Si, P, B, C and/or AI. Early transition-transition alloys, such as Zr-Cu, Zr-Ni, Nb-Ni etc., also have been quenched into a glassy phase. These specimens have been employed for thermal, mechanical, magnetic and superconducting properties measurements. The quenched ribbons are typically 0.5 mm wide and 20 ~m thick and i00 m long. Wider ribbons could be obtained by modifying the nozzle shape, for example, using a slit or paralleled multi-orifices instead of a single circular orifice. Typical cross-sections of ribbons prepared at various spinning velocity are shown in Fig. 2. The ribbons are surprisingly uniform in thickness in view of the fact that the melt has never attained hydrostatic equilibrium during the process, as will be discussed in the next section. The thickness of ribbons varies roughly as the reciprocal of the sPinning velocity V.

FIG. 2 Cross-sections of spin ribbons of (Fe.4Ni.6).75P.I~B.o6AI_o 3 glass at various spinning rate. From top down: i~00, 1500 aria 700 rpm. If the impinging melt is in a hydrostatic

equilibrium

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C H E N a n d G. E . M I L L E R

at the moment of solidification, thickness of the ribbons as: t ~

Vol.

a simple analysis

yields

11, N o .

the

(fTR/pV2) I/2

where p and 7 are respectively the density and surface tension of the melt, R the radius of the spinning wheel and f = 2(1+ cos ~) with a the contact angle between the melt and the surface of the wheel. Taking p ~ 8 g/cm3 (i0), Y = 1500 dynes/cm (ii) and a = 90 ° (see Fig. 2) for (Fe 4Ni 6) 7~P 16B 06AI Oq glasses, we obtain t = 200 ~m at V = 30 mTsec~ ~ The calculated value is about one order of magnitude larger than the observed value tex p = 20 ~m. A l t h o u g h both calculated and experimental t's varles as V -I, this seems to indicate that the melt has not attained hydrostatic equilibrium during solidification.

The flow pattern

FIG. 3 of impinging melt during

solidification.

Figure 3 illustrates flow patterns of the impinging melt during the process. The melt stream collapses and a melt pool is formed at the point of impingement. The spinning wheel drags the melt out of the pool tangentially in the direction of spinning, at the same time surface tension tends to pull the melt towards the center of the ribbon. The thickness t and w i d t h w of the ribbons therefore are determined by a transient flow pattern in a very complicated way. The cross-sectional area A(=t.w) may be estimated simply according to conservation of mass as: A --

2

(v/V)

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The thickness t (=A/w) varies as V -I when w is constant, as observed, d and v are respectively~the diameter and velocity of the melt stream, v is estimated to be i00 to 200 cm/sec. The angle ~% (in Fig. i), corresponding to the angle of the impinging melt in contact with the wheel for solidification, is given by: ~i ~ V'At/2~R where At[~(Tm-Tg)/T] is the time required.to quench the melt from T m to the ~lass temperature Tg, and T the quenching rate. As t ~ V -I and T ~ t -n with n : i to 2,.~ I decreases with increasing spin velocity V. Typically T = 100°K/sec, so ~i : i0 ° and the departure angle ~2 = 30°" It is apparent therefore that a m u l t i - s p i n n i n g of ribbons is possible by inserting nozzles at a separating angle ~ ~ ~2" It appears that the maximum spin velocity Vcr is determined only by the strength of the wheel material (~). As Vcr = ~7~ with ~ : ixl09 dyngs/cm 2 and ~ = i0 g/cm ~ for the Cu-Be alloy, we have Vcr ~ 10~cm/sec. Because of its operational flexibility, a desirable flow pattern can be obtained by adjusting the relative velocities of the stream and the wheel, the orifice size, the angle of the slope etc., which may improve the structure sensitive properties, such as magnetic and mechanical properties. It is obvious that this technique can be employed readily for filament spinning of very ductile as well as brittle crystalline materials. In conclusion, the present "centrifugal spinning" technique has several advantages over previous techniques. It permits the continuous fabrication of ribbons of various materials at a high spinning and quenching rate and provides considerable operational flexibility. References !. 2.

J. J. Gilman, J. Appl. Phys. 4_~6, 1625 (1975). H. J. Leamy, H. S. Chen and T. T. Wang, Met. Trans. ~, 69

3. 4.

6. 7.

T. Masumoto and R. Maddin, Mater. Sci. Eng. in press. R. C. Sherwood, E. M. Gyorgy, H. S. Chert, S. D. Ferris, G. Norman and H. J. Leamy, AIP Conference Proceedings, No. 24, 745 (1975). T. Egami, P. J. Flanders and C. D. Graham, Jr., Appl. Phys. Letters 26, 128 (1975). Strange and Pim, U.S. Patent No. 905,758. R. Pond Jr. and R. Maddin, Trans. Met. Soc. AiME 245, 2475

8. 9.

H. S. Chen and C. E. Miller, Rev. Sci~ Inst. 41, 1237 I1970). H. S. Chert and D. E Polk, J. Non-Crypt. Solid-s 15, 174

(1972).

5.

(197 ).

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H. S. Chert, J. T. Krause and E. Coleman,

Vol. II, No. 1

Scripta Met. 9,

411 (1975). ii.

V. F. Ukhov, E. L. Dubinin, 0. A. Esin and N. A. Vatolin, Russian J. of Phys. Chemistry 42, 1391 (1968).