Study On Crystal Morphologies And Structures Of ZA27 Alloy

Feathery dendrite is an unusual grain and it is completely different from the common columnar crystal and dendrite. It will not only greatly enrich the theories of metal solidification and crystal growth, but also has great significance for eliminating the feathery grains and obtaining fine grains. In this thesis, the formation conditions and morphologies of the feathery crystals in ZA27 alloy were studied by analyzing the effects of casting parameters and Si and P elements on the microstructures. The results are shown as followings:The pouring temperature, chilled ability and casting ingot size have large effects on the morphologies of a-Al grains in the Zn-Al alloy. It was found that pouring temperature has greatest effect. The feathery dendrites were easy to form when the pouring temperature was at or above 650℃. The greater the chilled ability and the smaller the ingot size, the easier the feathery crystal formation. In addition, feathery dendrites in the edge of the cast specimen were easier to for than those in the center. Although the process parameters were different, all of the them reflected the role of temperature gradient during solidification:the greater the temperature gradient, the easier the feathery crystal formation.The addition of Si element suppressed the formation of feathery crystals, and as the content of Si increased, the influence was enhanced. The primary Si plates suspended in the liquid and obstructed the growth of the dendrites. However, after being modified by P element, the primary Si particles became smaller and more dispersive, and the hindering effect on the dendrite growth was decreased. At the same pouring temperature, the formation tendancy of feathery dendrites became weak as the sequence of ZA27, ZA27-Si-P and ZA27-Si.The feathery dendrite was in columnar dendrite morphologies with large ratio of length to diameter on two-dimension micrographs. But in three-dimension space, they were in three-dimensional developed dendrite morphologies with multiple-driection branches and in laminar structures in local zones. The feathery dendrite was composed of one primary trunk and six rows of secondary arms and all of them grew along<110> directions. The dendrite trunk grew along a<110> direction in the (111) twin plane and a pair of the secondary arms was in the (111) twin plane and symmetrically grew at 60°with respect to the trunk direction. At the both sides of the twinning plane, another pair of the secondary arms was at 60°with respect to the trunk direction and symmetrically grew at 70.5°with respect to the twin plane. The rest pair of the secondary arms was at 90°with respect to the trunk direction and symmetrically grew at 54.7°with respect to the twin plane.The formation of the feathery dendrites was attributed to the combined results of the preferred crystallographic growth direction and external forces. In addition,2-dimension feathery dendrites sometimes could be found and they should be a kind of special morphologies of feathery dendrites. They were composed of many parallel laminae.