Effect of te on morphological transitions in Fe-C-Si alloys: Part i. Directional solidification
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I.
INTRODUCTION
D I R E C T I O N A L solidification techniques provide several advantages in studying solidification reactions. Particular advantages of interest here are the ability to control the solidification velocity and examine microstructures of solidified alloys vs velocity; and by use of quenching, evaluation of the morphology of the solid/liquid interface and microstructural changes which occur by solid state reactions during cooling is possible. Unfortunately, it is particularly difficult to study solidification involving white cast iron by directional solidification, because experiments have shown I1,21 that in Fe-C-Si alloys of commercial interest (Si ~ 2 pct), solidification velocities of over 1000 t z m / s are required to obtain white iron growth (at lower growth rates, gray iron forms). Directional solidification techniques become very difficult at such high rates, because latent heat effects cause the solid/liquid interface to shift toward the cold sink, and it becomes difficult to maintain unidirectional heat flow with planar isotherms at the interface. Because of these difficulties, many previous directional solidification studies involving white cast iron have been done on alloys with low or zero Si content. ~3'4,5~ Such alloys provide a model of high Si alloys having only limited usefulness, because the white iron eutectic morphology is lamellar at low Si [41 but becomes a complex lamellar-rod structure at high Si contents, t6j It is well established that the addition of very small amounts of Te causes the gray-to-white transition to shift to lower solidification velocities. Because small additions of Te do not produce any apparent effect upon the white iron morphology, they can be utilized to suppress the growth of gray iron and allow directional solidification studies of white iron at low velocities. As an initial step in investigating the use of Te additions for directional solidification studies of white cast iron, this study was initiated to evaluate the solidification velocity range in which white iron growth could be obtained by means of Te additions. Several s t u d i e s [1,2'5,7] have demonstrated that a large hysteresis exists in the critical velocity required for the gray-to-white transition, J.D. VERHOEVEN, Professor and Senior Metallurgist, J.S. PARK, Graduate Student, and L.L. JONES, Assistant Metallurgist, are with the Department of Materials Science and Engineering and Ames Laboratory, Iowa State University, Ames, IA 50011. Manuscript submitted July 21, 1988. METALLURGICAL TRANSACTIONS A
Vg_w, which occurs with increasing velocity, vs the whiteto-gray transition, Vw_g, which occurs with decreasing velocity. Values of Vg_w can be greater than Vw_~ by factors of 10 or more. This study presents an evaluation of these transition velocities in 2 pct Si alloys, and Part II of the study [8] utilizes Auger spectroscopy to help evaluate the role of the Te in causing the large suppression of the transition velocities. II.
EXPERIMENTAL TECHNIQUE
Alloys were made to a target composi
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