1 State Key Laboratory of Advanced Special Steel & Shanghai Key Laboratory of Advanced Ferrometallurgy, Shanghai University, Shanghai 200072, China 2 School of Metallurgical and Materials Engineering, Zhangjiagang Campus of Jiangsu University of Science and Technology, Zhangjiagang 215600, China
Cite this article:
Jianbo YU, Yuan HOU, Chao ZHANG, Zhibin YANG, Jiang WANG, Zhongming REN. Effect of High Magnetic Field on the Microstructure in Directionally Solidified Co-Al-W Alloy. Acta Metall Sin, 2017, 53(12): 1620-1626.
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.
Fund: Supported by National Natural Science Foundation of China (Nos.51404148, 51690162 and U1560202), United Innovation Program of Shanghai Commercial Aircraft Engine (Nos.AR910 and AR911) and fund of the State Key Laboratory of Solidification Processing (No.SKLSP201602)
Table 1 Chemical compositions of Co-Al-W-based alloys(mass fraction / %)
Fig.1 Schematic of Bridgman directional solidification apparatus in a static magnetic field (1—water output, 2—thermal insulation, 3—Al2O3 crucible, 4—heating elements, 5—specimen, 6—superconductor magnet, 7—refractory disc, 8—Ga-In-Sn liquid metal, 9—water input, 10—drawing rod, 11—water input, 12—water output, B—magnetic field strength)
Fig.2 Longitudinal microstructures near solid/liquid interface in directionally solidified Co-Al-W alloy at the pulling rate of 5 μm/s under the longitudinal magnetic fields of 0 T (a), 0.5 T (b), 1 T (c), 2 T (d) and 4 T (e)
Fig.3 Longitudinal microstructures near solid/liquid interface in directionally solidified Co-Al-W alloy under 2 T longitudinal magnetic field at the pulling rates of 5 μm/s (a), 10 μm/s (b), 50 μm/s (c) and 100 μm/s (d)
Fig.4 Longitudinal microstructures near solid/liquid interface in directionally solidified Co-Al-W alloy at the pulling rate of 5 μm/s under the transverse magnetic fields of 0.05 T (a), 0.1 T (b), 0.3 T (c) and 0.7 T (d)
Fig.5 Longitudinal microstructures near solid/liquid interface in directionally solidified Co-Al-W-Ta alloy at the pulling rate of 5 μm/s under the longitudinal magnetic fields of 0 T (a), 0.5 T (b), 1 T (c), 2 T (d) and 4 T (e)
Fig.6 Longitudinal microstructures near solid/liquid interface in directionally solidified Co-Al-W-Ta alloy at the pulling rate of 100 μm/s under the longitudinal magnetic fields of 0 T (a), 0.5 T (b), 1 T (c), 2 T (d) and 4 T (e)
Fig.7 Mass fractions of Co, Al and W solutes in Co-base single crystal superalloy (a), Al (b) and W (c) solutes in Co-Al-W alloy at the pulling rate of 100 μm/s, and Ta solute (d) in Co-Al-W-Ta alloy at the pulling rate of 20 μm/s under various magnetic field strengths
Fig.8 Schematic for the CET during directional solidification under a static magnetic field (G—temperature gradient, TEMC—thermoelectromagnetic convection, TE—thermo electromagnetic)
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