Influence of Longitudinal Static Magnetic Field on Microstructure and Microsegregation During Directional Solidification of DD98M Alloy
LIU Xiang1,2, WANG Yinghao1,2, ZHANG Xiaoxin1,2(), CHEN Chaoyue1,2, MENG Jie3, YU Jianbo1,2, WANG Jiang1,2(), REN Zhongming1,2
1 State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200444, China 2 School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China 3 Shi -changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
LIU Xiang, WANG Yinghao, ZHANG Xiaoxin, CHEN Chaoyue, MENG Jie, YU Jianbo, WANG Jiang, REN Zhongming. Influence of Longitudinal Static Magnetic Field on Microstructure and Microsegregation During Directional Solidification of DD98M Alloy. Acta Metall Sin, 2024, 60(12): 1595-1606.
Nickel-based superalloys have been widely used in gas turbines, aerospace, and other fields owing to their excellent high-temperature strength and creep resistance. Advanced directional-solidification techniques allow crystals to grow along specific directions, which can eliminate most or all of the transverse grain boundaries to obtain columnar- or single-crystal superalloys, which further improve the high-temperature mechanical properties. A strong magnetic field can modify the mass-transfer behavior during solidification via magnetic-damping or thermoelectromagnetic effect without contacting the material, thus improving the microstructure and microscopic segregation. In order to further refine the microstructure of nickel-based single crystal superalloys and improve the degree of homogenization of element distribution, the influence of longitudinal static magnetic field with a magnetic field intensity (B) that ranges from 0 to 4 T on the microstructure and microsegregation of liquid-metal-cooling directionally solidified nickel-based single-crystal superalloy DD98M was investigated. OM and SEM were applied to characterize the microstructure. Microsegregation was evaluated using a microsegregation coefficient and isoconcentration contour maps based on different data collection modes embedded in EDS. The results showed that with an increase in B, the primary dendrite spacing, average size of γ/γ' eutectic organization, and size of the γ' phases decreased. Meanwhile, the γ' phase in the interdendrite became more regularized. The microstructure refinement under static magnetic fields was attributed to the decrease in ΔT' / G (ratio of the temperature difference between the nonequilibrium solid-phase line and dendrite tip to the temperature gradient based on the Kurz-Fisher model) or the increase in subcooling of the melt surrounding the dendrites due to thermoelectric-magnetic convection. The relationship between ΔT' / G and B was revealed. The reduction in the γ' phase size was caused by the increase in the nucleation rate of the γ' phase due to the introduction of magnetic free energy difference (ΔGM) under a magnetic field. The magnetic field depressed the microsegregation of solutes, i.e., as B increased, the segregations of Al, Ta, Co, and W decreased. The effective partition coefficient (ke) of the dendritic scale and the average effective partition coefficients of the dendritic and interdendritic areas were obtained. It was found that the decrease in macrosegregation was essentially due to the effective distribution coefficient that approached 1 that due to the magnetic field.
Fund: National Key Research and Development Program of China(2019YFA0705300);National Major Research Instrument Development Project of China(52127807);Shanghai “Science and Technology Innovation Action-Yangfan Plan” Project(21YF1413000)
Corresponding Authors:
ZHANG Xiaoxin, associate professor, Tel: (021)66135623, E-mail: zhangxiaoxin@shu.edu.cn; WANG Jiang, professor, Tel: (021)66135585, E-mail: jiangwang@i.shu.edu.cn
Fig.1 Schematic of liquid metal cooling (LMC) Bridgman directional solidification apparatus with a high static magnetic field (1—temperature control system; 2—thermocouples; 3—heating element; 4—heating element protection cover; 5—double pass corundum tube; 6—alloy; 7—superconductor magnet; 8—LMC seal; 9—Ga-In-Sn liquid metal; 10—water cooling protection cover water outlet; 11—LMC water outlet; 12—drawbar; 13—water cooling protection cover water inlet; 14—bumper; 15—LMC water inlet)
Fig.2 Schematic of the sampling location (a), EDS analysis of spot scanning in interdendritic and dendritic areas (b), and EDS analysis of matrix pattern (c)
Fig.3 SEM images showing the dendritic morphologies of transverse sections of directionally solidified DD98M alloy under magnetic field intensities of B = 0 T (a), B = 0.5 T (b), B = 2 T (c), and B = 4 T (d)
Fig.4 Effect of B on the primary dendrite arm spacing (λ) under withdraw rate 100 μm/s
Fig.5 Sketches of generation mechanism of magnetic damping force (FEMB) (a) and thermoelectric-magnetic force(FTEMC)[32] (b); and variations of FEMB and FTEMC as a function of B[33] (c) (v—liquid velocity field, J—induced current, JTE—induced currents from the thermoelectric (TE) effect, F = FEMB + FTEMC, ΔF—difference between FEMB and FTEMC, Bmax—critical magnetic field intensity)
Fig.6 OM images showing the γ/γ' eutectic in transverse sections of directionally solidified DD98M alloy under B = 0 T (a), B = 0.5 T (b), B = 2 T (c), and B = 4 T (d) (Insets show the locally enlarged SEM images)
Fig.7 SEM images of γ' phase dendritic areas (a1-d1) and interdendritic areas (a2-d2) in transverse sections of directionally solidified DD98M alloy under B = 0 T (a1, a2), B = 0.5 T (b1, b2), B = 2 T (c1, c2), and B = 4 T (d1, d2)
Fig.8 Relationship between the size of γ' phase and B
Fig.9 Microsegregation coefficients of each alloying element under different B
Fig.10 Isoconcentration contour maps of Al (a1-a4), Ta (b1-b4), Co (c1-c4), and W (d1-d4) in the transverse section of directionally solidified DD98M alloy under B = 0 T (a1-d1), B = 0.5 T (a2-d2), B = 2 T (a3-d3), and B = 4 T (a4-d4) (Color bars in Figs.10a1-a4, b1-b4, c1-c4, and d1-d4 indicate the mass fractions of Al, Ta, Co, and W, respectively)
Fig.11 Concentration profiles of Al (a), Ta (b), Co (c), and W (d) of directionally solidified DD98M alloy under different B on dendritic scale (CS—solid composition, C0—original composition, CS / C0—relative solute concentration, fs—solid fraction)
Fig.12 Relationships between ln(CS / C0) and ln(1 - fs) under different B ( fcritical—critical solid fraction ratio)
B / T
Mall
Mdendritic
fcritical / %
0
400
146
0.365
0.5
400
135
0.338
2
400
150
0.375
4
400
139
0.348
Table 1 Statistics on the critical solid phase ratio at the dendritic boundary under different B
Fig.13 Effects of B on the average effective partition coefficients (ke) of solute elements in interdendritic (I) and dendritic (D) areas
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