Influence of Solidification Parameters on the Microstructure of Rapidly Quenched Fe-Ni-B Alloys
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INFLUENCE OF SOLIDIFICATION PARAMETERS ON THE MICROSTRUCTURE OF RAPIDLY QUENCHED Fe-Ni-B ALLOYS
UWE KOSTER Abt. Chemietechnik, Universitdt Dortmund D - 4600 Dortmund, F.R.Germany URSULA HEROLD, DIETER KRAUSE Institut ffr Werkstoffe, Ruhr-Universit~t Bochum D -4630 Bochum, F.R.Germany
ABSTRACT The microstructure of Fe-Ni-B alloys rapidly quenched by melt spinning has been investigated by means of light and electron microscopy. Depending on the surface velocity of the quenching wheel and the boron concentration a large variety of microstructures has been observed: increased solubility of boron, microstructures with different metastable crystalline compounds or metallic glasses. Plotting the as-cast microstructures versus the quenching rate has been found to be very useful for arranging the large amount of results and/or for predicting particular microstructures. Due to an extremely large gradient in the microstructure across the ribbon thickness this method of forming particular microstructures seems to be only of limited advantage. Controlled crystallization reactions of metallic glasses result in different, but more homogeneous microstructures.
INTRODUCTION to:
very high cooling rates achieved in rapid solidification processes are known - increase the solubility limits of solute elements in the parent metal matrix; - form metastable crystalline phases; - form in certain alloy systems amorphous phases (metallic glasses).
In Fe-Ni-B alloys a large number of papers has been published on metallic glass formation and crystallization /1/, however, very little is known on the increase of boron solubility or the formation of metastable crystalline phases at lower cooling rates. Whereas Ray and Hasegawa /2/ were able to obtain Fe-B solid solutions up to 12 at%boron, Herold et al. /3,4/ observed in splatquenched alloys with 5 at%boron a microstructure consisting ofOd-Fe and metastable Fe 2 3 B6 (T-phase). At higher boron concentrations eutectic solidification ofc.-Fe and metastable orthorhombic Fe 3 B occurred. A similar microstructure was reported by Choo and Kaplow /5/ to be formed by splat-cooling alloys with 7 at% boron. In addition to the formation of the metastable Ni 2 3 B6 (T-phase)/6/ only a small increase of boron solubility has been reported for Ni-B alloys with low boron contents after splat-quenching. To the authors knowledge nothing has been published so far on the behaviour of rapidly quenched Fe-Ni-B alloys with low boron contents. The aim of this paper is to investigate the formation of metastable phases and microstructures in Fe-Ni-B alloys after rapid solidification more systematically and to compare these microstructures with those formed by crystallization reactions of metallic glasses.
180 EXPERIMENTAL METHODS Fe-B and (Fe 0 5 Ni 0 5 )-B ribbons with boron contents ranging from 6 to 24 at% were prepared by'melt'spinning under an ambient pressure of about 200 mbar He. For the melt spinning process a copper wheel 20 cm in diameter was used with surface velocities ranging from 10 to 50 m/s. Mic
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