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本文利用连续切片技术研究了定向凝固Al-40%Cu过共晶合金中金属间化合物Al₂Cu 初生相的三维微观组织, 以及抽拉速率跃迁下, 三维共晶组织形态的演变和间距调整. 结果表明: 抽拉速率为5 µm/s时, 初生Al₂Cu三维组织沿生长方向存在棱面和棱角, 表现出明显的棱面相生长形态; 凝固过程中初生Al₂Cu相释放的结晶潜热使得生长界面发生局部重熔, 三维组织中出现孔洞, 形成拓扑缺陷. 在Al-40%Cu合金三维共晶组织中,  Al₂Cu 相和Al相的体积分数分别为56.8%和43.2%, 且Al₂Cu相的生长方向与试样轴向夹角为5.1^o. 当抽拉速率从2 μm/s突然跃迁到500 μm/s时, 三维共晶组织形态从层片向棒状转变, 这种转变不是由合金中两相体积分数变化造成的. 三维共晶组织间距调整机制不同于二维组织中的分叉、内凹和界面重新形核, 而是通过三维空间非同一平面连续的分叉、分枝进行, 在三维下并没有观察到二维下的重新形核共晶间距调整机制.

Three-dimensional (3D) microstructures can clearly reveal the size, shape and distribution of the phases, providing a novel way to deeply understand the formation mechanism of the solidified phases. In this paper, by using the serial sectioning technique, the 3D microstructure of the primary Al₂Cu phase was reconstructed and the eutectic spacing adjustment was investigated during directional solidification of Al-40%Cu hypereutectic alloy. The results show that the primary Al₂Cu phase pattern was observed faceting due to the growing faceted angle and plane parallel to the solidification direction at the pulling rate of 5 μm/s. Further, there existed some hopper-like cavities in the 3D microstructure of the primary Al₂Cu phase that caused by remelting owing to a large amount of the latent heat difficult to be extracted from the solidification interface. For the 3D eutectic microstructures of the Al-40%Cu alloy, the growth direction of the Al and Al₂Cu phases in the coupled eutectics had a deviation angle of 5.1^o with the heat flux direction and the phase volume fractions of the  Al and Al₂Cu phases were measured directly to be 56.8% and 43.2%, respectively. In addition, at the abrupt change in pulling rate from 2 μm/s to 500 μm/s, the lamellar-to-rod eutectic transition was observed and the continuously splitting and branching occurred in different planes responsible for the eutectic spacing adjustment in the 3D microstructure of the Al-40%Cu alloy. Meanwhile the renuleation mechanism in the 2D microstructure for the adjustment of the eutectic spacing did not work in the 3D eutectic microstructure of the Al-40%Cu alloy.