定向凝固Al-40%Cu合金三维微观组织重构及共晶间距演变

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Al₂ Cu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5μm/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°.当抽拉速率从2μm/s突然跃迁到500μm/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 MICROSTRUCTURE RECONSTRUCTION AND THE EUTECTIC SPACING ADJUSTMENT DURING DIRECTIONAL SOLIDIFICATION OF Al-40%Cu HYPEREUTECTIC ALLOY

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 5μm/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°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 2μm/s to 500μm/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.