Solid State Phenomena Vols. 101-102 (2005) pp 111-116 online at http://www.scientific.net © (2005) Trans Tech Publications, Switzerland Online available since 2005/Jan/15
Formation of Amorphous and Nanostructural Powder Particles from Amorphous Metallic Glass Ribbons using Ball Milling and Electrical Discharge Milling A. Calka,1,a) D. Wexler,1 D. Oleszak2 and J Bystrzycki3 1-Faculty of Engineering, University of Wollongong. Wollongong, NSW 2522, Australia 2- Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska, Warsaw. Poland 3-Department of Materials Technology, Military University of Technology, Kaliskiego 2, 00908 Warsaw, Poland a)
andrzej
[email protected]
Keywords: metallic glass powders, amorphous particles, ball milling, electric discharge milling
Abstract. In this paper both electric discharge assisted milling [1, 2] and conventional mechanosynthesis techniques were applied to investigate the effects of milling conditions on the fracture and agglomeration of amorphous CoSiB ribbons produced by planar flow casting. The effect of spark energy on particle shape and size produced by discharge milling was studied. Conventional milling in inert atmosphere for extended periods generally leads to the formation of porous powder particle aggregates, each particle comprised of small amorphous or, after extended milling times, nanocrystalline elements. The mechanism of agglomeration was believed to originate from repeated fracture, deformation and cold welding of individual ribbon elements. In contrast to conventional milling, spark discharge milling was found to induce the formation of predominantly sub-micron single particles of amorphous powder. The morphology of individual particles varied from sub-micron irregular shaped particles to remelted particles, depending on selection of vibrational amplitude during discharge. For high vibrational amplitudes and high energy input a wider range of particles as produced. These included sub-micron particles, remelted particles and welded agglomerates, and nano-sized particles produced as a fume and collected during discharge milling under flowing argon. These results combined with observations that most re-melted particles produced by discharge milling were also amorphous confirmed that extremely high heating and cooling rates are associated with discharge milling of metals. They also confirm the potential of electrical discharge milling as a new route for the synthesis of ultrafine and nanosized powder particles from amorphous ribbon, for possible processing into 3-D shapes. Introduction Powder particles with useful soft magnetic properties have been used for many years as precursors for production of monolithic magnetic materials. One attractive form of such materials is 3-D net-shaped compacts with isotropic or tailored anisotropic magnetic properties; used as the active magnetic material in electrical components including high frequency power transformers cores, pick-up heads, electro-magnetic sensors and other devices for conversion of motion into electrical signals or vice versa [3]. Compacts can be made by mixing of soft-magnetic particles (iron, iron-oxides, nickel based alloys, grain-oriented silicon etc.) with organic or inorganic binders. A low conductivity and relatively high electrical resistivity is essential for applications where eddycurrent losses must be minimised in high frequency alternating fields. Conventional high temperature sintering is not generally a suitable compaction technique since it increases contacts between particles, increasing electrical conductivity and, consequentially, reducing electromagnetic properties in alternating fields. Metallic glasses and thin strip nanocrystalline materials produced by appropriate crystallisation of metallic glasses are attractive for such applications. These materials have low anisotropy combined with high resistivity and low coercive force. As a result they exhibit low power losses at intermediate frequencies. In the preferred planar-flow casting method of production,
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the metal is rapidly quenched from the melt onto cooled rotating drums to form long strip, typically up to around 70 µm in thickness. Typical Fe- and Co- based metallic glasses and nanocrystalline strip materials have limited applications, mainly as a transformer cores and pick-up heads, as it is difficult to fabricate them into the complicated 3-D shapes required for many modern electromagnetic components. Despite this, it has been recently demonstrated that 3-D shapes with excellent soft magnetic properties can be produced via by appropriate milling of FeSiBCuNb nanocrystalline strip followed by consolidation with polymer or mineral binder [4, 5]. In this paper we study the milling of amorphous CoSiB strip using conventional mechanotechniques and the new technique of electric discharge assisted mechanical milling [1, 2]. Of particular interest is the break up and agglomeration of powder particles to produce particle size ranges and size distributions which may be of use for the fabrication of 3-D magnetic materials. Attempts to fabricate metallic glasses and nanocrystalline soft magnetic materials in the form of powders include the well-known methods of gas atomization and spray deposition, both of which produce well defined particle shapes with well defined size distributions. Limited studies of ball milled Co-based metallic glasses have included demonstration that CoFe- based metallic glass ribbon can be reamorphised by milling [6] and that milling of CoSiB amorphous powder can result in cyclic transformations involving the formation of nanocrystalline and amorphous powder [7, 8]. Experimental As quenched Co75Si10B15 metallic glass ribbons, 16mm wide and 40µm thick, were produced by planar flow casting in air. Ribbon pieces were milled using three different devices, as shown in Fig 1: (i) a Fritch planetary ball mill, Type P5 (Fig. 1(a)), (ii) a magneto-ball mill [9] with milling under shearing mode (Fig. 1(b)) or impact mode (Fig. 1(c) and (iii) an electric discharge mill [1, 2] (Fig. 1(d)).
Fig. 1 Milling devices used in the current investigations: (a) a Fritch planetary ball mill, a magnetoball mill with milling under (b) shearing mode and (c) impact mode, and (d) an electric discharge mill. Planetary Fritch Milling and Magneto Milling Metallic glass ribbons, ~20 cm in length, were cut into small pieces (~3-4 mm width) and placed into the steel milling cylinders. Fritch milling was performed with 10 mm diameter steel balls (15 balls, 1:50 ball to powder wt ratio) under an inert atmosphere for times of up to 130 h. Magneto-milling was performed using 4 balls, each 25 mm in diameter in an inert atmosphere (400kPa). Separate samples were magneto-milled milled for 10 hrs using either shearing (Fig. 1(b)) or impact milling mode (Fig. 1(c)). Milling in Electric Discharge Milling A vibrational laboratory rod mill was modified in order to a produce milling mode involving repeated impact of a hardened curved rod end on particles placed on a vibrating hemispherical container under a controlled electrical discharge within a controlled atmosphere. During milling, gaps of up to1.5 mm, between the vibrating mill base the powder
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particles and the loosely suspended conducting plunger occur, as illustrated in Fig.1 (d). An electrical discharge was generated within those gaps. The power supply used in this study was connected through an AC high voltage transformer, generating typically 15 kV 50 Hz impulses within the µA range. A locally hot milling condition, described as spark discharge milling, was created during milling, with the spark discharge reinitiated with vibration cycle of the base. By varying the amplitude of the base vibration the average length of the spark varied which, in turn, affected in a complex way the energy input and energy transfer into powder particles during milling. Experiments were carried out using ~160 mm2 pieces of metallic glass ribbon cut into 5 pieces. In one set of experiments, samples were milled under vibrational amplitudes ranging from 0.3mm to 1.2 mm in a still argon atmosphere for periods of 30, 90, and 120 sec. In this series of experiments powder samples after milling were collected from the mill base. Following observations of vapour fumes produced after milling with vibration amplitudes of 1.2 mm and higher, a second set of experiments was performed. In this case discharge milling was carried out with vibrational amplitude of 1.2 mm under a continuous Ar gas flow through the mill, with a flow rate of 1000 cc/min. The finest powder particles created during discharge milling cycle were caught in the gas flow and collected at the end of a gas exit tube located near the mill base. Powder samples were analysed by x-ray diffraction (XRD) using the diffractometer method with CuKα radiation and graphite monochrometer, and scanning electron microscopy (SEM) using a Leica Stereoscan 440 scanning electron microscope. For SEM examination, powder particles were dispersed on conductive tape and gold coated. Results and discussion Magneto-milling of samples using low energy shearing mode (Fig. 1(b)) was found not to result in powder formation, with individual ribbon pieces remaining unfractured after 10 hours of milling. However, higher energy milling resulted in the formation of fine powders. Figure 2 shows SEM micrographs of powders produced by milling of ribbons in the Fritch mill (Fig. 2(a)) and the Magneto-mill working in impact mode (Fig. 2(b)). Both micrographs in Fig. 2 show characteristic powder particle morphologies. Individual particles were generally irregular in shape with a complicated surface texture, each particle comprising clusters of smaller particles which are apparently formed by repeated fracture, deformation and cold welding during the milling process. High internal porosity is a characteristic of such powder agglomerates. Corresponding XRD patterns show broad x-ray peaks, which are typical of those generally associated with an amorphous structure. The results demonstrate that the amorphous structure of the metallic glass ribbon can still be retained after high-energy milling. However, prolonged milling, for up to 500 h, has been found to induce partial crystallisation and formation of a crystalline or nanocrystalline structure in ribbon of similar composition [7, 8]. Potential drawbacks of using a magnetic powder comprising particle agglomerates with internal porosity include limitations on the final density of compacts and difficulties in achieving maximum possible saturation induction. Figure 3 shows a set of SEM micrographs obtained from powders spark milled under a vibrational amplitude of 0.6 mm for 30sec (Fig. 3(a)), 90sec (Fig. 3(b)) and 120sec (Fig. 3(c)). This milling method produced single particle powders. Three types of particles were observed: Those (i) irregular in shape with the size range 5-60µm, presumably formed as a result of fracturing of amorphous ribbons, (ii) spherical particles, believed to form result of rapid local melting and subsequent rapid quenching of individual particles and (iii) fused particles, believed to result from a combination of melting and/or welding of individual particles. Corresponding XRD patterns for powders milled for 90 s or less do not show visible sharp peaks, suggesting that crystallisation during spark milling had not occurred. It therefore appears that both heating rates and quenching rates during discharge milling are sufficiently rapid that crystallisation can be avoided. Rapid cooling during discharge milling
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possibly occurs via self-quenching of small particles and, for larger particles, quenching assisted by heat conduction into the mill base or plunger, both of which have relatively large thermal masses. For samples milled for longer times there were increases in both the proportion and size of spherical particles (c.f. Fig. 3(b) and Fig. 3(c)). It appears that after longer milling an increased fraction of particles are melted and there is increased coalescence into larger melted particles. Due to their size, quench rates within the larger particles must be lower and XRD evidence of partial crystallisation in these samples containing larger particles (Fig. 3(c)) is consistent with this.
Fig. 2 Amorphous agglomerates produced by conventional milling of cut pieces of amorphous CoSi15B10 strip. SEM images and XRD patterns of milling products were obtained from (a) a planetary Fritch mill and (b) a Magneto-mill under ball-particle impact mode. In samples milled with vibrational amplitudes of 1.2 mm and above, much smaller particles were also produced and these were collected at the exit port of the discharge mill during milling under flowing Ar. Figure 4 shows SEM micrographs of these fine powders. In this case the majority of particles were below 5 µm and only a few particles of sizes >10 µm were observed, the morphologies being fine irregular shaped fragments and small remelted particles containing fine fragments on the surface. The corresponding XRD pattern (Fig. 4(d)) confirms that this powder is predominantly amorphous. The mechanism of formation of fine particles during high energy spark milling in flowing gas flow mode is not very clear at this stage. However, increases in vibrational amplitude of the mill result in longer sparks and higher energy input per vibrational cycle. We may speculate that in this case the formation mechanism is a combination of melting, vaporization, rapid quenching and condensation. Further studies, including TEM and microdiffraction investigations of individual particles are required to explain this phenomenon.
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An important result of this study is that the spark milling method can produce single particle powders, which can potentially be 3-D net-shape consolidated using organic or inorganic binder. Using this method relatively high strength elements can be achieved via appropriate heat treatment although annealing temperatures should be below the crystallization and binder degradation temperatures. The effect of stress induced during ball milling and spark milling (thermal and mechanical) was not investigated in this study. It might be expected that a high degree of stress may affect soft magnetic properties such as permeability, coercive force and hysteresis loss. However additional heat treatment of powder particles up to the crystallisation temperature may induce stress relaxation and improve magnetic properties.
Fig. 3 SEM micrographs and associated XRD pattern obtained from powders spark milled under a vibrational amplitude of 0.6 mm for (a) 30s, (b) 90s and (c) 120s.
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Fig. 4. SEM micrographs and XRD patterns obtained from powders spark milled under flowing Ar with a vibrational amplitude of 1.2 mm. Particles were collected at the exit port of the mill. Conclusions Ball milling of CoSi10B15 planar flow cast strip using both the planetary Fritch and Magneto(impact mode) techniques resulted in formation of amorphous particle agglomerates, each particle with relatively high internal porosity. Spark milling resulted in a range of powder particle products, including single fractured particles, remelted particles and welded composite particles. High-energy spark milling resulted in the production of very fine powder particles, which potentially could be 3-D net-shape consolidated using an appropriate binder and thermomechanical treatment. References [1] A. Calka and D. Wexler: NATURE, Vol. 419 (Sept 12, 2002), p. 147. [2] A. Calka and D. Wexler: Mater. Sci. Forum, Vol. 386-388 (2002) p.125. [3] J. A. Bas, J. A. Calero, M. J. Dougan, J. Magn. Mag. Mater, Vol. 254-255, (2003), P. 391. [4] M. Muller, A. Novy, M. Brunner, R. Hilzinger, J. Magn. Mag. Mater., Vol. 196-197, (1999) p. 357. [5] R. Lebourgeois, S. Berenguer, C. Ramiarinjaona and T. Waeckerle, J. Magn. Mag. Mater., Vol. 254-255, (2003) p. 191. [6] A. Calka, A.P. Pogany, R.A. Shanks and H. Engelman, Mater. Sci. Eng. Vol.A128 (1990) p. 107. [7] M. Jachimowicz, Ph.D. Thesis, Phase Transformations in Fe and Co-based alloys induced by high energy bal milling, Warsaw University of Technology, Warsaw, (1998). [8] M. Pekala, M. Jachimowicz, V.I. Fadeeva et al., J. NonCryst. Sol. Vol. 287, (2001) p. 365. [9]. A. Calka and A. Radlinski, Mater. Sci. Eng. Vol. A134 (1991) p. 1351.
