凝固枝晶课程.ppt

Formation Of Dendrites When regual cells,such as those of Figs.3-2 and 3-9, form and grow at relatively low rates,they grow perpendicular to The liquid-solid interface regardless of crystal orientation. When,however,growth rate is increased,crystallographic effects bagin to exert an influence and the cell-growth direction deviates toward the preferred crystallographic growth direction (for example,100 for cubicmetals).sim- ultaneously, the cross section of the cell generally begins to deviate from its previously circular geometry owing to effects of crystallography.This structure has been describ- ed variously as a flanged structure,or maltess cross,in cu- bic materials;it is shown In Fig. 3-17.As growth rate increases still further, the cros strycture first becomes more apparent and then serrations begin to appear in the flanges of the cross; that is,secondary dendrite arms become discernible.32-35 The point at which the cellular structure in Fig.3-17a bec- omes dendritic is a matter of terminology,employed in the liter- ature is not consistent .Some writers prefer to think of the stru- cture as dendritic when it is growing in or near its crystallogra- phic orientation,as in Fig.3-17b.Others prefer to describe it as dendritic only when secondary branches can be discerned, as in Fig.3-17d.The latter terminology is employed in this text. Figure 3-18,which depicts a transparent organic liquid,sh- ows one example of solidification where secondary arms are just discernible.Closely comparable structures have described for a large number of metal alloys,including Pb-Sb alloys (Mor- ris and Winegard,34) and aluminum alloys(Biloni et al.36). Figure 3-19 shows a rangeof structures in Fe-10% Ni alloy from cellu- lar to dendritic. These structures were obtained in a unidirecti- onally solidified ingot, and so rate of heat removed decreased with the square of the distance from the chill, the strycture is cellular although the shape of the cross se