For the alloys with large-sized grains (>150 µm), their mi- crostructural evolutions operate in a different way during the structure separation stage. Herein, the unrefined AZ91D alloy (grain size = 311µm) is taken as an example to illus-


trate this way. As shown by Fig. 9(a), its dendrites are rela- tively developed and arm A is obviously longer than the others, i.e., its dendrites are not quite equiaxed. The distri- bution of eutectic, especially the eutectic β phase, is more


(a)


(b)


(c)


(d)


(e)


(f)


Figure 9. Microstructures of the not refined AZ91D alloys heated at 580°C (1076 ºF) for (a) 0 minutes, (b) 2 minutes, (c) 3 minutes, (d) 5 minutes, (e) 10 minutes and (f) 20 minutes and then water quenched.


50 International Journal of Metalcasting/Winter 2012


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