Inverse segregation in directionally solidified Al-Cu-Ti alloys with equiaxed grains
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I.
INTRODUCTION
PREVIOUS investigations j-5
have shown that inverse segregation occurs in many unidirectionally solidified alloys. In these investigations, solute distributions in ingots containing a columnar structure were determined, and these agreed well with the theory of inverse segregation. ~ The theory of inverse segregation ''6 should be applicable to any kind of grain structures grown under a temperature gradient. However, some recent reports 7's'9 on macrosegregation in ingots containing an equiaxed structure present conflicting results on the nature of segregation in ingots with an equiaxed structure. Nakano et al. 7 analyzed the solute distribution in equiaxed copper-tin alloys, and obtained inverse segregation similar to that in the columnar structure. However, Motegi and Ohno s did not find inverse segregation in horizontally solidified A1-Cu alloy ingots containing equiaxed structures or in ingots containing a columnar-equiaxed transition structure. In recent work, 9 the present authors found inverse segregation in equiaxed A1-Cu-Ti alloy ingots similar to that in columnar A1-Cu alloy ingots. From these reports, the nature of segregation occurring in an equiaxed structure is not clear. Therefore, the present work is concerned with the determination of solute distribution in directionally solidified ingots containing an equiaxed structure and the comparison of the experimental solute distribution with the theoretical one calculated from the theory of Kirkaldy-Youdelis.' Directionally solidified, A14 wt pct Cu-0.2 wt pct Ti alloys were used with the Ti added to induce the formation of equiaxed grains.
II.
EXPERIMENTAL
A. Preparation of lngots Two A1-4 wt pct Cu-0.2 wt pct Ti alloy ingots were obtained using different casting techniques to obtain different H. KATO, on leave from Saitama University, Urawa, Saitama, Japan, is Visiting Professor, and J. R. CAHOON is Professor and Head, both with the Department of Mechanical Engineering, Uaiversity of Manitoba, Winnipeg, MB R3T 2N2, Canada. Manuscript submitted July 23, 1984. METALLURGICALTRANSACTIONS A
solidification rates. The first melt (A) at 1020 K was poured into a rectangular sand mold (35 x 100 mm 2 in inner area, 160 mm in height, 30 mm in wall thickness) preheated to 970 K. The apparatus is shown in Figure l(a). The mold was placed on the water cooled copper plate immediately prior to pouring. The second melt (B) at 970 K was poured into a cylindrical graphite mold (55 mm in inner diameter, 110 mm in height, 10 mm in wall thickness) which was preheated to 970 K in a furnace. The mold and furnace were situated on a steel base plate as shown in Figure l (b). Immediately after pouring, the ingot was solidified by impinging water on the steel base plate. A third ingot, C, with a nominal composition of 4 pct Cu but containing no Ti was cast under identical conditions to ingot A. This was done to obtain an ingot with a columnar structure exhibiting typical inverse segregation so that a comparison with the equiaxed ingots could be made. All melts
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