Effect of Cooling Rate on the Structure of Rapidly Cooled Fe 75 Si 15 B 10
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EFFECT OF COOLING RATE ON THE STRUCTURE OF RAPIDLY COOLED Fe 75Si5 BI0
JOHN L. WALTER,* AND AMI E. BERKOWITZ* *General Electric Company, Corporate Research and Development, Schenectady, NY 12301
INTRODUCTION Solidification of metal alloys by rapid quenching can result in the formation of amorphous or microcrystalline solids, or materials with improved microstructural homogeneity, all with the view of forming new phases or obtaining improved properties. Some alloys may be cooled at high rates to achieve "microcrystallinity" but cannot be cooled rapidly enough to become amorphous. Other alloys may achieve both conditions depending on the cooling rate. We have examined the effects of cooling rate on the structure of one alloy that can, depending on the cooling rate, be made partially or completely amorphous. The alloy is Fe 7 5 SiI 5 BI 0 (atom percent) which was formed as ribbon by melt spinning and as powder by spark erosion in dielectrics of different cooling characteristics and by gas-water atomization. The structural characteristics were determined by x-ray diffraction, measurements of magnetic properties and by optical and scanning electron microscopy.
EXPERIMENTAL PROCEDURE Ribbon Ribbons were prepared by the melt-spinning process [11 and were about 27 pm thick and 2 mm wide. The cooling rate for the ribbons was determined 6 to be in excess of about 10 K/s [1]. Powders Powders of the Fe 7 5 Si 1 5 B1 0 alloy were prepared by two methods; spark erosion [2] and gas-water atomization [3,4]. Spark erosion consists essentially of maintaining a repetitive spark discharge between two electrodes immersed in a dielectric fluid [2,5]. The spark discharge produces highly localized melting and/or vaporization of the electrodes and the particles result from the freezing of the molten droplets as they are ejected into the dielectric where they are rapidly cooled. The electrodes were fabricated from the alloy and the dielectric fluids used were dodecane (CH 3 (CH 2 )IoCH 3 ) and liquid argon. In gas-water atomization a stream of liquid metal is impacted by an atomizing gas at very high velocity. The atomizing gas breaks the stream into droplets which are then rapidly cooled by water jets positioned so that their focal point is on the axis of the metal stream at a small distance below the focal point of the gas jets [3]. The particles then fall to the bottom of the collection chamber. Cooling rates for this process have been 6 estimated to be between 105 and 10 K/s [4].
EXPERIMENTAL RESULTS Determination of Some Cooling Rates Cooling rates of some particles were estimated by spark eroding a nickel-base superalloy in liquid nitrogen and in dodecane and measuring the secondary-dendrite arm spacing or grain size. These dimensions were then Mat. Res.
Soc.
Syrup. Proc.
Vol.
28 (1984)
OElsevier Science Publishing Co.,
Inc%
176
plotted on a curve of arm spacing as a function of cooling rate taken from the literature and extrapolated to higher cooler rates [2]. X-ray diffraction The x-ray diffraction patterns of 27 pm thi
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