以新型Co-Al-W基高温合金为基础,进行了外加静磁场下定向凝固实验,考察了不同磁场强度对凝固组织形貌和偏析的影响。结果显示：在抽拉速率为5 μm/s时,施加纵向强磁场,诱发熔体流动,造成界面失稳,形成“斑状”偏析和游离碎晶;磁场不变(2 T),进一步增加抽拉速率时,边部的游离碎晶和“斑状”偏析组织减少,凝固界面变得平直;施加横向磁场时,诱发更强的界面前沿流动,偏析加剧,碎晶增多;增加偏析合金元素Ta时,偏析进一步加剧,造成过冷形核,诱发柱状晶向等轴晶转变(CET)。磁场下热电磁对流形成偏析,是造成CET的根本原因。

Recently, a new Co-Al-W-based alloy with ordered L1₂ structure has been attracted much attention of researchers, these alloys have higher melting point than Ni-base superalloys with morphologically identical microstructure, but grain defect formation caused by thermosolutal convection has become an important problem for its application. Magnetic field is always applied to damp the convection which reduces the formation of defects. However, there are hitherto few papers to investigate the effect of magnetic field on grain defects during Co-Al-W-based alloy directional solidification. In this work, The effect of high magnetic field on the solidification structure and macrosegregation in directionally solidified Co-Al-W-based alloy was investigated. The results showed that the application of longitudinal magnetic field can induce convection and cause deformation of the solid-liquid interface shape, forming the macrosegregation and the stray grains in the mushy zone at the pulling rate of 5 μm/s. With the increase of pulling rate, the macrosegregation and the stray grains disappeared gradually at 2 T magnetic field. While the transverse magnetic field was applied, the macrosegregation became serious and the number of the stray grains increased. The macrosegregation further became more serious and the columnar-to-equiaxed transition was induced after adding the Ta element. The main reason of undercooling nucleation and columnar-to-equiaxed transition (CET) was the microsegregation induced by thermoelectric magnetic convention.