Supercooling and Structure of Levitation Melted Fe-Ni Alloys
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I.
INTRODUCTION
FINE grained
structures generally have improved low temperature strength, reduced microsegregation, reduced anisotropy, better dispersion of intermetallics, better heat treatment response, improved castability, and reduced hot tearing tendency. ~-4 Fine grain structures can be attained by increasing the number of active nucleation sites or by grain multiplication via controlling thermal, mechanical and/or chemical conditions during solidification. Thermal controls include rapid cooling and supercooling of the liquid. Mechanical controls include magnetic convection, ultrasonic vibration, and mechanical stirring. The last method of grain refining involves the addition of alloying elements or inoculating powders to the alloy. Increasing the number of effective nucleation sites is based on the homogeneous and heterogeneous nucleation theories) -u According to the former theory, the homogeneous nucleation rate is an exponential function of the supercooling. Therefore, when an alloy is supercooled to a low enough temperature where many nuclei form spontaneously, the resulting microstructure will contain extremely fine grains. The critical supercooling necessary for homogeneous nucleation was originally believed to be approximately equal to about 0.2 T,,, with T,, being the melting point of the metal in absolute scale. ~3However, more recent experiments of finely dispersed droplets of low melting temperature alloys have shown that undercoolings in excess of 0.3 Tm can be reached prior to nucleation. ~4 A number of studies have shown that an abrupt reduction of grain size occurs at a critical supercooling less than that required for homogeneous nucleation. ~5-19The value of the supercooling is shown to be about 175 K for a variety of iron and nickel based alloys. It has been proposed that the grain size reduction is due to the pressure generated by the collapse of shrinkage cavities. Theoretical calculations have been carried out to support this idea by showing that the pressure increase could result in an increased nucleation probability of the supercooled melt. 2~A similar explanation G.J. ABBASCHIAN is Associate Professor, Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611. M.C. FLEMINGS is Professor and Chairman, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted June 25, 1982. METALLURGICALTRANSACTIONS A
has also been given to explain the grain refining effect of ultrasonic vibrationfl ~'22 Another, probably most likely, mechanism to explain the grain size reduction at the critical supercooling postulates that the dendrites, which form at the supercooled state, partially remelt and disintegrate during recalescence.23 Each segment of the partially remelted dendrites then can resolidify and form a grain. With this mechanism, the grain refinement is due to grain multiplication, rather than increased number of nucleation sites. Fluid flow also affects the final casting structure by
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