Grinding peculiarities of rapidly quenched powders

Materia& Science and Engineering, 98 (1988) 453 456

453

Grinding Peculiarities of Rapidly Quenched Powders* V. A, VASILJEV, B. S. MITIN, V. M. ARCHANGELSKY AND A. A. SKURIDIN

MATI named alter K. E. l~iolkocsky, Petrouka 27. Moscow (U.S.S.R.)

Abstract

2. Experimental procedure

There are two trends in the development of amorphous metallic alloys ( A M A ) technology at present. According to one, the alloys are produced in the Jbrm ~ff'foils which are subsequently used to manufacture products. The second trend implies production o[ A MA powders which are subsequently compacted into products ~ff" t,arious shapes in compliance with powder metallurgy techniques. The advanced highly-productire technique o[ super-rapid metallic melt-quenching makes it possihle to produce A M A powders with an arerage particle size q[ 500-1000 mcm. These powders are now wideO' used, e.g. in solders, filters and magnetic circuits. However, producing electrotechnical articles which/imction at high Jrequencies calls .[or the use ~['powders with particle sizes o.1"less than 50 mcm. An ~lficient technique jor producing dispersice A M A powders ther~ffbre needs to be developed.

MATI after K. E. Tsiolkovsky researchers investigated the grinding technique of the AMA produced in the form of fibres by super-rapid melt solidification (SMS), which is also known as melt extraction. Interaction of the metallic melt with a heat removal disc in the free state is a distinctive feature of this method, which is why the shape of the future material is determined by the configuration of the disc working surface which can be assigned in advance. The method makes it possible to produce AMA in the form of fibres (short or continuous) and powders. In addition to the width and thickness of the material produced, the cross-sectional shape can be controlled. The crosssectional shape can be planar, circular or semicircular, but is best as a crescent shape, which enables the compression diagram and the bend diagram to be drawn in grinding units. For grinding purposes, AMA fibre of CoToFesSi~sB~o was used. It was produced by the SMS method at a pilot plant. The fibre was 5 mm long, 200-400 mcm wide and 30-50 mcm thick. The annealed fibre was ground for 1 h at 300 C and the unannealed fibre was ground in a laboratory centrifugal mill. The mill drums were manufactured from natural jasper. Highest quality hard alloy balls 5-7 mm in diameter were used as grinding bodies; according to the chemical analysis of the powder, the grinding body material constituted no more than 3% of the powder. The granule matrix composition was determined by the sieve method. The average geometrical dimensions were determined by a mathematical statistical technique from measurements conducted under a light microscope. The presence of an amorphous structure of the investigated powder was confirmed by X-ray diffractometry radiation K-Co data and also by the exothermicity effect while conducting differential thermal analysis (DTA) at a heating rate of 50 K min 1. Magnetic spectra in the area of the natural magnetic resonance frequencies were determined according to a coaxial measuring line method using a computer calculation of the determined parameters from data under no-load and short-circuit conditions.

1. Introduction

The technique of producing dispersive amorphous metallic alloy (AMA) powders [1] by grinding foils (mechanical comminution) is well known. In this case the foils undergo preliminary embrittlement annealing at temperatures lower than the solidification temperature. But the grinding efficiency is low, despite the brittleness of the material because of an unfavourable foil shape, whose cross-section leads to the formation of the stressed state in the material according to the bend diagram. The powder particles obtained with this technology fail easily in subsequent processing stages, i.e. they diminish in size. This, in turn, leads to instability of the product properties. The latest work in the field of local magnetic anisotropy [2] indicates growth of the AMA magnetic anisotropy constant after such processing which deteriorates magnetic characteristics.

*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montr6al, August 3 7, 1987. 0025-5416/88,$3.50

~' Elsevier Sequoia:Printed in The Netherlands

454 3. Results

A comparative study of the comminution processes of short AMA fibre, either unannealed or annealed, revealed that the nature and the mechanism of the destruction of these materials were opposed. The results of the work on unannealed AMA SMS fibre comminution are now considered. Figure 1 illustrates the changes in the granule matrix composition of the powder in the grinding process. Even at the first grinding stage, there are particles less than 50 mcm in size, but most of the powder consists of particles 100200 mcm in size. At the first grinding stage, the granulemetric composition change as the particles increase in size and the weight of the larger particles diminishes. When the proportion of particles 100-200 mcm in size reaches about 60%, the output of the fine fraction increases. Thus the process of grinding is uneven and particles 100-200 mcm in size constitute the bulk of the material. This behaviour of the AMA powders can be explained by two comminution mechanisms: brittle and ductile failure. The first mechanism is inherent at the

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early stage of grinding, the second occurs at the appearance of the dispersive particles. The study of the surfaces and distribution of alloying elements in the SMS materials indicates that they contain original stress concentrators. These result in slag and other non-metallic inclusions, micropores and macropores, and stresses from the presence of the thermal coefficient of expansion in the material as a result of segregation. The fibre is also subjected to bend stresses during formation when hardening from the melt [3]. This explains why the macrostrength of the AMA SMS fibre is lower than the theoretical value, which is determined on the basis of microhardness data and the known relationship between these two values [4]. The failure surface morphology indicates its brittle nature. Thus the material undergoes a brittle failure because of the original stress concentrators at the first stage. If the quantity of stress concentrators per unit material volume is known, we can determine the critical size which represents the limit of brittle failure, according to the first mechanism of failure. In our case, the critical size is 100-200 mcm. Further comminution follows the ductile failure mechanism pattern under which the powder particles are subjected to plastic deformation which mainly works across the fibre, causing its expansion (Fig. 2). The first maximum in the relationship dependence (Fig. 2(b)) corresponds to the straightening of the fibre of crescent-shaped cross-section. After the failure of the straightened fibre, the flattening process of the powder particles represents the second maximum in the relationship dependence. After that, the deformation-failure process recurs. Photography of the particles appearing at the later stage of grinding which was performed in a scanning electron microscope revealed the vein pattern on their surface which represents a trace of frictional flow of the amorphous alloy under plastic deformation. The failure surface morphology (Fig. 3) with a trace of frictional flow reveals ductile failure of the material.

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Fig. 1. Changes in granulemetric composition of AMA powder during grinding.

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Fig. 2. Changes in A M A during grinding.

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Fig. 3. AMA powder particle after ductile failure (at 1250 K).

Fig. 4. AMA powder obtained by grinding with preliminary annealing (embrittlement failure).

4. Discussion

It is well known that amorphous metallic alloys possess high strength. In analysing the failure of particles of less than the critical size, we must consider not the macrostrength but the theoretical strength. The energy delivered by the balls to the material in the grinding process is no more than l0 s erg cm 2, which is insufficient for failure of the material. Under repeated attacks, however, the energy appears to be sufficient to cause plastic deformation and to form secondary stress concentrators in the material and cause it to fail. We also studied the annealed A M A SMS fibre grinding, in which the failure is quite different. The grinding process is more active although the comminution process of the particles of all fractions proceeds at the same rate. There is no vein pattern on the particles' surface in this case. The failure surface morphology (Fig, 4) indicates its brittle nature. As yet, there is no unambiguous explanation of the A M A embrittlement as a result of annealing at temperatures lower than the solidification temperature. In our opinion, based on X-ray analysis data, embrittlement occurs because of the ordering of the amorphous structure. In this process the appearance of metastable crystal-phase clusters is possible even at annealing temperatures below the solidification tem-

perature. The above-mentioned phase contributes to the internal stress in the material. The frequency of the natural ferromagnetic resonance is determined by the internal magnetic anisotropy field which, according to the Landau theory, depends on the magnetic structure of the material. We have studied the influence of various processing methods of A M A powders on the natural ferromagnetic reasonance frequency. Figure 5 shows the magnetic spectra of the imaginary and real complex permeability parts of the amorphous alloy powders of the composition under investigation. The annealing after

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Fig. 5. Magnetic spectra of AMA powder: 1', 1", ,u', it" after grinding; 2', 2", ,u', it" after annealing.

456 grinding at a temperature below the solidification temperature diminishes the resonance frequency. This confirms the reduction in the material magnetic anisotropy and the relief of the stress which appeared as a result of comminution. Thus, the comminution of an annealed A M A SMS fibre proceeds less efficiently than it does after an embrittling annealing, but the powder particles do not lose their high plastic properties and they can be subsequently processed without failure. The A M A magnetic properties can be restored by thermal treatment of the powders at temperatures below the solidification temperature.

References 1 R. Ranjan, 75/251, B22F9/04, B22F9/06, 4290808, March 1979, 23411, September 1981, Allied Chemical Corporation, U.S.A. 2 R. S. Ishakov, V. I. Kirko and A. A. Kusovnikov, Dokla. Akad. Nauk, S.S.S.R, 284 (1985) 854-857. 3 V. M. Archangelsky, V. A. Vasiljev, B. S. Mitin and A. A. Skuridin, Poroshk. Metall., 6 (1986) 1(~14. 4 A. M. Gleser, B. V. Molotilov and O. L. Utevskaja, Metallofizika, 6 (1) ( 1983) 29-45. 5 V. M. Archangelsky, V. A. Vasiljev, B. S. Mitin and A. A. Skuridin, Fizikohimija Amorfnih (Stekloobrasnix) Mettalitcheskih Splavov, Moscow, Nauka, 1985, pp. 77-78.