Cusp磁场对热电磁力破坏单晶高温合金枝晶形态的克服和其对枝晶生长行为的影响

When the single-direction longitudinal or transverse magnetic field is applied in the directional solidification of single-crystal superalloy, the thermoelectromagnetic force destroys the stability of the dendrite and liquid-solid interface，and the magneto-hydrodynamic-damping effect homogeneously distributes the alloying elements in the melt. The former effect would deteriorate the mechanical properties, and the latter would benefit the properties. How is the disadvantage effect eliminated and is the advantage role exerted? The applicant has investigated that the effect of Cusp magnetic field on the dendrite morphology and orientation of the single-crystal superalloy. The results show that the dendrite morphology grows more regularly and the dendrite orientation exhibits more less deviation, which indicates that the Cusp magnetic field could eliminate the disadvantage role of the single-direction magnetic field. The effect of Cusp magnetic field on the dendrite structure of single-crystal superalloy is achieved by its influence on the flow field. The change of flow field further produce the effect on the temperature and solute distribute, which finally modify the stability of the dendrite morphology and the growth behavior. Therefore, this proposal will focus on the revelation on the above mechanisms..The profound researches will be conducted on the basis of the following aspects: (1) The distribution of the thermoelectricmagnetic force near the dendrite interface in the Cusp magnetic field would be simulated for clarifying its role in the melt convection. (2) The effect of Cusp magnetic field on the flow field and temperature field during directional solidification will be investigated. This provides the clarification of heat and flow environment, which determines the change of solute field and further modify the stability dendrite and liquid-solid morphology. (3) The solute field will be explored based on the melt convection, the longitudinal macrosegregation, the transverse microsegregation, the effective partition coefficient, the equilibrium partition coefficient, and the composition of mushy zone. This can reveal the solute-field modification that affects the growth behavior of dendrties. (4) The morphology, arm spacing, distribution homogeneity, and orientation of dendrites with Cusp magnetic field in single-crystal superalloys will be studied. Finally, the relationship between the characteristic parameters of dendrite, the transverse gradient of temperature, and the composition distribution along the radial direction would be established during the Cusp magnetic-field-assisted solidification..The research of this proposal will shed light on the effect of Cusp magnetic field on the stability and growth behavior of the dendrties by virtue of the composition homogeneity, the temperature distribution, and the flow field in the different regions during the directional solidification of single-crystal nickel-base superalloy. The regions refers to the mushy zone, the liquid-solid interface, and the far-field fields. The mechanism revelation will provide the theoretical guidance to control the microstructure and properties of single-crystal superalloy. Meanwhile, a potential technology for preparing the high performance of single-crystal superalloy will be developed.

纵向和横向静磁场作用于单晶高温合金的定向凝固时，热电磁力破坏枝晶界面形态，而磁制动效应显著均匀合金元素在多尺度上的分布，前者恶化力学性能，而后者作用恰好相反，如何克服不利效应而保留有利作用？申请者前期预研证实Cusp结构的磁场规整了枝晶形态并降低其位向偏离度，表明该磁场能解决以上问题，其原因在于该特殊结构磁场对不同部位熔体流动产生不同的作用，进而改变温度和溶质环境，最终影响到枝晶形态和生长行为，申请项目拟对此问题进行深入研究。通过研究枝晶周围热电磁力分布和其对界面附近及远处熔体流动的影响，结合熔体不同部位和方向的温度变化、溶质在宏微观尺度上的分布，探明磁场致使的流动对枝晶生长环境的影响；研究枝晶及宏观液固界面形态、枝晶间距和分布均匀度变化，建立枝晶特征参数、横向温度梯度和成分均匀化之间的关联，最终揭示Cusp磁场增强枝晶形态稳定性和影响生长的机理，为用磁场控制单晶高温合金组织提供理论依据。

保持单晶高温合金枝晶的定向生长特性是发挥其良好高温服役性能的必要条件。纵向和横向静磁场作用于单晶高温合金的定向凝固时，热电磁力破坏枝晶界面形态，而 Cusp这种特殊结构的磁场规整了枝晶形态，其原因在于该特殊结构磁场对不同部位熔体流动产生不同的作用，进而改变温度和溶质环境，最终影响到枝晶形态和生长行为。该项目对此问题进行了深入研究，考察了等直径、变截面平台区域、螺旋通道内枝晶生长时受Cusp磁场影响行为，通过考察磁场对枝晶周围热电磁力分布和其对界面附近及远处熔体流动的影响，结合熔体不同部位和方向的温度变化、溶质在宏微观尺度上的分布，揭示了磁场致使的流动对枝晶生长环境的影响机理。得到以下重要结论：1）Cusp磁场不破坏单晶高温合金枝晶生长过程中的定向生长特性，这源于其独特的磁场结构在液固界面产生的热电磁力沿着枝晶尖端到根部逐渐减小；2) Cusp磁场减小了单晶高温合金枝晶间的大角度、小角度晶界比例；3）Cusp磁场减小了单晶高温合金平台杂晶的形成，这种减小随着温度梯度、抽拉速度、截面尺寸的增大而弱化，这是由于该磁场独特结构在液固界面形成微搅拌作用。4）在所研究的抽拉速度下Cusp磁场的施加使得螺旋通道内单晶形成位置高度增大，高度增加幅度随抽拉速度的提高而增大。项目取得的研究成果最终揭示了Cusp磁场增强枝晶形态稳定性和液固界面稳定性的机理，为用磁场控制生长高质量的单晶高温合金提供重要的理论指导。