采用连续切片技术重构了定向凝固Cu-10.25%Mg过共晶合金中初生Laves相Cu₂Mg的三维枝晶形貌. 结果表明,Cu₂Mg二次枝晶的三维生长形态在不同凝固环境中呈现“八面体”、“棱面”或“板条”状, 侧枝生长形态不对称,二维截面组织中沿生长方向不连续的枝晶实质上是枝晶二次侧枝的横断面组织, 部分沿生长方向分叉的枝晶仅是某个初生枝晶一次轴的局部, 而非二维截面组织推断出的初生枝晶分叉现象.20 μm/s抽拉速率下合金的一次和二次枝晶间距分别为112.53和40.78 μm, 该值与凝固分数f_s=0.6时侧枝呈现“板条”枝晶的一次和二次枝晶间距测量值123.30和44.82 μm相近, 比f_s=0.8时侧枝呈现“八面体”枝晶的值198.00和47.66 μm要小. 三维初生Cu₂Mg枝晶“八面体”、“棱面”和“板条”侧枝的生长速率数值分别为1.40, 1.94和2.66 μm/s, 比一次枝晶主干生长速率要小近一个数量级.

Dendrite, which plays a key role in determining the mechanical properties of the casting parts, is the most important growth form in cast ingot and its pattern has received a wide range of investigations. In this work, the three-dimensional (3D) dendritic morphology of primary phase Cu₂Mg in Cu-10.25%Mg hypereutectic alloy has been studied using the serial sectioning technique during directional solidification, due to the fact that the Cu₂Mg phase is a typical Laves phase existing in most intermetallic compounds. The experimental results showed that the secondary dendrite profile of primary phase Cu₂Mg had different morphologies at the pulling rate of 20 μm/s, which included plate-like, octahedral and faceted patterns. The occurrence of these patterns was related to alloy macro--segregation and the thermo--solutal convection during directional solidification. The growth dendrites and their side-branching morphologies in the three-dimensional section were clearly interpreted differently from those in the two-dimensional section. The two-dimensionally discontinuous dendrites in growth direction were the secondary dendrites of primary phase Cu₂Mg and the branching dendrites in two-dimensional section were part of the primary dendrites. Furthermore, the primary and secondary dendrite arm spacing were calculated as 112.53 and 40.78 μm, close to those of the plate-like dendrites measured at the solidified fraction of 0.6 with the values of 123.30 and 44.82 μm, less than those of the octahedral dendrites measured at the solidified fraction of 0.8, which were 198.00 and 47.66 μm. In addition, the growth rates of secondary dendrites with octahedral, faceted and plate-like patterns were calculated as 1.40, 1.94 and 2.66 μm/s, respectively, an order of magnitude less than the pulling rate of 20 μm/s.