8/11/2019 Pages From Casting Aluminum Alloys

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Structure and Microstructure of Aluminum Alloys in As-cast State 171

mixed cell/subgrain microstructure. Typically, several dislocation cells will belocated inside each dendritic cell. At higher solidification rates, in particular,in welded joints where cooling rate may attain 1000 K/s, the sizes of dendritic

and dislocation cells coincide. Similarly to rapidly solidified thin films [163165],dislocation forests will be observed only on the periphery of dendritic cells,along their boundaries.

If cooling rate for binary alloys is reduced to 0.65 K/s, the distributionsof dislocations become more uniform, and cell dislocation microstructure lesspronounced. In slowly cooled (Vc = 0.2 K/s) industrial castings made of copper-bearing aluminum alloy 203.0 with 0.2%Cd dislocations do not form cells at all,while dislocation density attains a value of0.3 109 cm2. This is half orderof magnitude less than in binary alloys at cooling rates >0.7 K/s. In industrial

castings made of alloy 203.0 with 0.2%Cd the total dislocation density turned outto be significantly lower even at relatively high solidification rates: (0.5 0.1)109

at Vc = 0.7 K/s and (0.7 0.2)109 cm2 at Vc = 7.5 K/s. Additionally, systematic

growth of was observed as solidification rate increased.Although in AlCu casting alloys dislocations mostly form bundles (forest),

in certain conditions one may expect the formation of planar dislocation bound-aries. This depends upon the concentration of copper in alloys and also on theapplied cooling rate. In AlCu alloy containing 0.5%Cu subgrain boundaries are

not formed at any solidification rates. On the other hand, in an alloy containing4%Cu, planar dislocation boundaries are observed at rates