Melting during solidification
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Melting During Solidification D. J. ALLEN AND J. D. HUNT The Scheil equation' has often been used to describe microsegregation in castings. Flemings et aZ 2 - 1 have shown that this considerably overestimates the severity of segregation, even when modified to allow for solidstate diffusion. In this note, we discuss a new factor which affects microsegregation-temperature gradient zone melting in interdendritic pools. An organic 'metal analogue'S alloy, succinonitrile-6 wt pet camphor, was frozen unidirectionally by means of a temperature gradient stage" on an optical microscope. The alloy was sandwiched between two glass cover slides about 125 J.l apart. The cell thus formed lay across the gap between two plates maintained at different temperatures. In this apparatus (unlike that described in Ref. 6) the cell was fixed relative to the stage: solidification was accomplished by driving the hot and cold plates, linked rigidly together, across the D. J. ALLEN andJ. D.HUNT areResearch Student andLecturer, respectively, Department ofMetallurgy and Science of Materials, Oxford University, Oxford, England. Manuscript submitted July 22, 1975. METALLURGICAL TRANSACTIONS A
Fig. 1-A fixed area of the cell photographed at different times during steady-state solidification of succinonitrile-6 wt pet camphor. R = 3/.Lm s-t, G ~ 50°C crn' ". The vertical line marks the position of a rather indistinct stationary dust particle. Magnification 114 times. (a) 0 s , (b) 180 s, (c) 480 s. stage. Hence a selected area of the cell, such as that shown in Fig. l(a) to (c), was kept in view throughout solidification. This was confirmed by using stationary dust particles as reference marks. A sequence of intermediate photographs was used to follow structural changes and identify individual secondary dendrite arms. Some stdearms have melted off or coalesced in surface energy-driven coarsening processes.I>" More im portantly, from the point of view of this pape r, the sidearms have moved two or three secondary armspacings up the temperature gradient, while (as shown by observations on dust particles trapped inside the solid) the primary stalk remained stationary. So, while solidification has occurred on the cold sides of the pools between secondary arms, the hot sides have melted back at a nearly equal rate. Almost all the original solid outside the primary arm cores has remelted. It is suggested that the movement
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