Competitive growth of stable and metastable Fe- C- X eutectics: Part I. experiments
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I.
INTRODUCTION
I T is well known that one of the most important types of casting alloys, cast iron, solidifies in two different forms, grey and white: 1. Grey iron is predominantly obtained at low cooling rates. Its eutectic phases, y-iron and graphite, are the equilibrium ones. 2. White iron appears at high cooling rates and consists of y-iron and the metastable Fe3C. The properties of the two eutectics are very different. The controlled production of one or the other microstructure is therefore of considerable practical interest. The solidification rate at which the transition between these structures occurs depends strongly upon the melt chemistry, trace elements playing as important a role as alloying elements. A large number of papers on this subject exist. Hillert [1'2] was the first to present a firm theoretical analysis. The influence of alloying elements upon the microstructure has been studied by various authors. [3-1q Unfortunately, the resuits presented are usually qualitative or incomplete. Further, the details of the mechanism of the transition between grey and white cast iron are still unknown. The aim of this work is to provide consistent experimental results and theoretical considerations concerning the competition between stable and metastable phases. The influence of some important trace or alloying elements is determined quantitatively under directional solidification conditions. There are two distinct critical velocities: with increasing growth rate, grey eutectic will remain stable until the critical velocity for the transition from grey to white, Vg__,w,is reached (Figure 1). The transition from white to grey occurs at a lower solidification rate, Vw_~g.In order to quantify the effects of alloying elements upon the competition between the grey and white eutectics, these two critical velocities have been measured using directional solidification experiments. An understanding of these effects requires a knowledge of the influence of these elements on: 1. relative thermodynamic stability of the two eutectics, as reflected by their equilibrium temperatures; tl2] P. MAGNIN and W. KURZ are with the Department of Materials, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland. Manuscript submitted December 11, 1987. METALLURGICAL TRANSACTIONS A
2. nucleation rate of the phases; and 3. growth undercooling of the two eutectics. The growth of lamellar eutectic structures is characterized by the well-known relationships: [13] AT = /~X/-f
[1]
h = K'/V'-V
[2]
where V is the solidification rate, AT is the growth undercooling, and h is the mean lamellar spacing. K and K ' are constants which are related to the material properties. It has been shown [14A5]that these relationships, which were established for regular eutectics (such as white cast iron), are also valid for irregular eutectics (such as grey cast iron) if one uses different constants. Thus there are two different values, Kg and Kw, for the proportionality constant of Eq. [1] in grey and white eutectic, respectively. Becaus
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