The effect of quenching on mushy-zone microstructures
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A technique used to study the evolution of dendrites during solidification is to quench a partially solidified material in hopes of freezing the high temperature structure against a background of quenched liquid.~'2'3 The purpose of this work is to outline the conditions when this procedure is valid and to identify cases when the changes which occur on cooling can be estimated and corrected. Figure 1 illustrates the problem which is being considered. In Figure l(a) three primary dendrites are pictured as they grow by directional solidification into an undercooled liquid. Secondary arms grow from these as the local temperature decreases. Finally, in the case considered here an invariant temperature is reached and the remaining liquid solidifies. During solidification, a component of the alloy is rejected into the liquid and produces the composition profile shown in Figure l(b). Also, a dendrite volume profile develops across the two-phase, mushy zone as shown in Figure l(c). When the environment of the sample is changed suddenly, for example by plunging the crucible containing the semi-solid material into water, the cooling rate increases and the dendrites experience an increased driving force to grow from the increased supersaturation which develops. SUBHASH R. PATI is with the Department of Materials and Chemistry Development, Combustion Engineering, Inc., CEPCODE 9454-501, 1000 Prospect Hill Road, Windsor, CT 06095. J.E. MORRAL is with the Department of Metallurgy and Institute of Materials Science, University of Connecticut, U-136, 97 North Eagleville Road, Storrs, CT 06268. Manuscript submitted March 4, 1985. 360--VOLUME 17A, FEBRUARY 1986
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REFERENCE
The Effect of Quenching on Mushy-Zone Microstructures
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The author is thankful to Professor A. K. Chakraborty, Professor S. C. Panigrahi, and Professor M. R. Das for their timely help.
I. Asoka K. Misra: M e t a l l . Trans. A , 1985, vol. 16A, pp. 1354-55.
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Fig. l - - T h e mushy zone before and after quenching. (a) The microstructure before quenching. Primary and secondary arm spacings are labeled A~ and A2. The dendrite periphery outlines regions with an interdendritic spacing less than A_,. (b) Concentration of the rejected component in the liquid vs distance from the invariant reaction. After quenching, the concentration profile is higher. (c) Dendrite volume vs distance. Growth during quenching has increased the local dendrite volume at each position. (d) Correction factor for dendrite volume vs distance. For infinite quenching rate (T) the correction factor is one (no correction needed). At slower rates, the correction needed increased as the solidification growth front is approached.
Figures l(b) and (c) illustrate the changes that result from the growth. Figure 1(b) shows that growth adds solute to the interdendrit
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