The 3-dimensional morphology of dendrite during equiaxed solidification of an Al-5 wt.% Cu alloy

doi:10.1016/j.jmst.2018.09.042

Abstract

Abstract:

In a sample quenched during equiaxed solidification of an Al-5?wt.% Cu alloy, the multi-scales 3-dimensional morphology of equiaxed dendrite was observed. The slim primary stem and secondary branches constitute the frame of dendrite, and rows of dense tertiary branches further divide the 3-dimensional space. In the divided space, the quartic branches grow further. The dendritic branches, which are perpendicular to each other, can change their growth directions and coalesce into a whole. In the tertiary branches and quartic branches, the formation of double branch structures is induced by competitive growth. The branch that wins in the competitive growth will produce a cabbage-like structure by wrapping the failed branches. In addition, the side branch can also wrap the original parent branch to produce cabbage-like structures. Depending on the historical growth direction, the dendritic arms can form vein-like and spicate structures, and the shapes of single dendritic arm may be the cylinder, plate and trapezoid platform. According to the compositions and etching morphology, the single dendritic arm in the final solidification structures should coalesce from a fine porous structure. The porous structures at different length-scales are principally induced by the preferential growth. Based on 3-dimensional morphology of equiaxed dendrite, a new research object for the investigation of microsegregation was suggested.

Key words: Equiaxed dendrite, 3-Dimensional morphology, Competitive growth, Coalescence, Segregation

Xiaoping Ma, Dianzhong Li. The 3-dimensional morphology of dendrite during equiaxed solidification of an Al-5 wt.% Cu alloy[J]. J. Mater. Sci. Technol., 2019, 35(3): 239-247.

Figures/Tables 9

Fig. 1. The cooling curve and selected quenching time in a preliminary experiment (on the upper-right); the cooling curve and actual quenching time in current experiment (on the bottom-left).

Fig. 2. The morphology of the whole equiaxed dendrite. (a) the morphology of equiaxed dendrite on a 2-dimensional observation plane; (b) the morphology of equiaxed dendrite on three neighboring but random 2-dimensional observation planes; (c) the morphology of equiaxed dendrite on three neighboring and oriented 2-dimensional observation planes; (d) the sketched 3-dimensional morphology of equiaxed dendrite; (e) the morphology of equiaxed dendrite on an oriented 2-dimensional observation plane; (f) the sketched 3-dimensional morphology of equiaxed dendrite with the 2-dimensional observation plane corresponding to (e).

Fig. 3. The initiation of tertiary branches from a row of secondary branches. (a) and (b) random observation planes; (c), (d) and (e) the initiation positions of tertiary branches traced on slices with different depth; (f) the slim initiation of tertiary branch revealed by heavy etching.

Fig. 4. The intersection, direction conversion and coalescence of perpendicular branches revealed on slices with different depth.

Fig. 5. The double branch structure, the stem alteration and the cabbage-like structure in the branches revealed on slices with different depth. (a)-(d) the double branch structure; (e)-(i) the stem alteration and the cabbage-like structure; (j)-(n) the cabbage-like structure. 10?μm is removed between two figures in a series.

Fig. 6. The double branch structure (a)-(e) and the cabbage-like structure (f)-(l) in the branches with less side arms revealed on slices with different depth. The depth is labeled in the figures.

Fig. 7. The initiation and growth competition of branches revealed on slices with different depth. The depth is labeled in the figures.

Fig. 8. The 3-dimensional morphology of dendritic arms. (a)-(d) cylinder dendritic arms; (e)-(h) plate dendritic arm in a vein-like structure; (i)-(m) trapezoid platform dendritic arm in a spicate structure; (n) the spicate structure produced by several alternation of growth direction; (o) the sketched trapezoid platform for dendritic arm 1?in. (i)-(m). The depth removed is labeled in the figures.

Fig. 9. The porous medium within dendritic arms concealed by coalescence. (a) the hollow dendritic arm in the quenched sample; (b) the coalescence in the sample solidified by air cooling; (c) the composition differentiation in different coalesced positions; (d)-(f) the dendritic arm with significant fluctuation of solute content and the special etching morphology showing residual trace information of coalescence; (g) and (h) the new porous medium produced in the quenching; (i) The sketch for the porous medium at different scales induced by the preferential growth at different scales.

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