【作者】 李喜；

【导师】 任忠鸣；

【作者基本信息】 上海大学 ， 钢铁冶金， 2009， 博士

【摘要】 强静磁场下的金属凝固是新发展的研究方向,本文采用定向凝固等手段,以Al Cu、Al Ni和Bi Mn合金为对象,实验研究了强静磁场(文中称为强磁场)对二元合金凝固的影响;深入考察了强磁场作用下二元合金单相生长界面稳定性、胞晶和枝晶生长,共晶生长、晶体取向和相变等规律,建立模型分析了强磁场的多种物理效应,得到有价值的结论。在Al-Cu二元单相合金的生长中,发现强磁场导致晶体平界面生长变得不稳定和不规则,同时也促使胞晶和枝晶生长不稳定与分枝甚或混乱。在较低的磁场下的胞晶生长中,产生环状组织。随磁场强度的增加,枝晶的一次间距增加,高次枝晶间距减小。在合金定向凝固中发现存在多尺度的热电磁效应,此效应引起热电磁流动和作用于固相上的热电磁力。强磁场下定向凝固界面稳定性降低和胞晶/枝晶分枝与热电磁流动和热电磁力相关。固相中的热电磁力和不规则性热电磁流动可以引发界面和枝/胞晶的不稳定性和分枝。建立了理论和数值模型,对定向凝固热电磁流动和热电磁力进行了分析,发现随着磁场强度的增加,热电磁流动逐渐达到最大值;之后,热电磁流动逐渐减小,而作用在固相上的热电磁力则随着磁场强度和温度梯度的增加呈线性增加。提出在强磁场下定向凝固过程中界面扩散边界层因磁化使溶质富积,进而促进界面和枝/胞晶不稳定生长的观点。在Al-Cu单相合金枝晶生长中,发现强磁场使得α-Al枝晶趋于以<111>方向转向磁场方向,即使枝晶因热电磁力的作用而生长混乱的情况下,仍以<111>方向转向磁场方向,形成取向组织。在Al-Ni过共晶合金定向凝固中,强磁场使得条状Al3Ni初生晶生长偏离磁场方向,在足够强的磁场中形成规则的层状组织。分析表明,上述现象产生的原因是,具磁各向异性的晶体在磁场中受力的作用而发生重新取向。.在Al Al2Cu和Al Al3Ni共晶的生长中,发现强磁场减小片状和纤维状共晶间距,导致带状组织的形成;并可以引发片状共晶的退化。进而,提出了在磁场下共晶生长过程中扩散边界层磁化和溶质富积模型,认为由于在固液界面处溶质富积,因而改变磁化率,从而导致磁力线发生弯曲,产生横向磁场梯度,影响了原子的扩散,进而改变共晶间距和导致带状组织的形成。发现强磁场可以使常规条件下无取向特性的Al Al2Cu共晶具备特定取向,强制Al2Cu共晶相以[001]晶向沿磁场方向取向;进而,提出了强磁场作用下片状共晶取向模型和生长模型。研究表明,强磁场在共晶固相中将诱生热电磁应力,该应力可以促使片状共晶形貌的不稳定和退化。研究了强磁场对凝固相变的影响,提出了利用物质在梯度强磁场中的受力变化测定相变点的方法。其原理为,当相变发生时,物质磁化率改变,从而其在梯度磁场中的受力相应发生变化,通过测量该力的变化可判定物质发生相变的温度点,从而测得相变温度的变化。利用这一方法测定了Bi Mn合金包晶相变温度随磁场强度的变化,结果发现随着磁场强度的增大,该相变温度逐渐增大,在10T磁场下相变温度提高20℃左右。观察了该合金在包晶相变过程中组织的变化,发现在强磁场下,随着相变的进行,生成相的形态发生沿垂直磁场方向分裂,平行磁场方向聚合的变化。建立模型分析了磁场对相变温度的影响,结果与实验一致。发现强磁场使Bi Mn合金的BiMn相以<001>方向沿磁场方向取向排列,从而导致强的磁各向异性,剩磁成倍提高;随着磁场强度和作用时间的增加,磁各向异性逐步增强。更多还原

【Abstract】 Metal solidification under high static magnetic field is a new research direction. Thiswork has investigated on the effect of a high magnetic field on the solidification behavior inthe binary alloy experimentally, theory and numerically. The stability of the interface, cell anddendrite during the solidification of signal phase, the eutectic growth, crystal orientation andphase transformation in binary alloy has been studied experimentally and the model is built toanalyze the many kinds of physical effects. Some valuable conclusions have been gained.In the growth of the Al Cu binary single phase alloy, it has been found that an applicationof a high magnetic field has caused the instability and irregularity of the interface, the cell andthe dendrite. Indeed, the field has branched and broken the cell and dendrite. With theincrease of the magnetic field intensity, the primary dendritic spacing increases and the highorder dendritic spacing decreases. Moreover, during the cellular growth under a lowermagnetic field, the ring like structure has formed. The thermoelectric effects on various scalesduring the directional solidification have been found and these effects will induce thethermoelectric magnetic convection (TEMC) in the liquid and the thermoelectric magneticforce (TEMF) on the solid. The irregularity and instability of the interface and the branch andthe irregularity of the cell and dendrite should be attributed to the irregular thermoelectricmagnetic convection (TEMC) in the liquid and the thermoelectric magnetic force (TEMF) onthe solid. Further, the theoretical and numerical models have been built to analyze thethermoelectric magnetic convection (TEMC) in the liquid and the thermoelectric magneticforce (TEMF) on the solid and it has been found that with the increase of the magnetic fieldintensity, the velocity of the TEMC increases and reaches the maximum value in order of 0.1T;and then with the increase of the magnetic field intensity, the value of the TEMC decreases.However, the TEMF imposed on the solid increases lineally with the increase of the magneticfield intensity. Furthermore, a theory of the magnetization and solute buildup in the diffusionboundary layer under a high magnetic field for a binary alloy has been proposed; and the magnetization and solute buildup in the diffusion boundary layer could be partly responsibleto the instability of the planar interface and cell/dendrite. Moreover, in the dendrite growth ofsingle phase in the Al Cu alloy, it has been observed that the <111> crystal direction ofαAldendrite tends to align along the magnetic field. In the dendrite growth of single phase in theAl Ni alloy, it has been observed that the Al3Ni dendrite deflects from the solidificationdirection during the directional solidification under a magnetic field and the lay like structureforms finally under an enough high magnetic field. This should be attributed to themagnetocrystalline anisotropy of theAl3Ni crystal.. In the Al Al2Cu and Al Al3Ni eutectic growth, it has been found that an application of ahigh magnetic field has caused the decrease of the lamellar and fiber eutectic spacing and theformation of the land like structure. Moreover, the high magnetic field has degenerated thegrowth of the lamellar eutectic. Further, a theory of the magnetization and solute buildup inthe diffusion boundary layer during the eutectic growth under a high magnetic field has beenproposed; and the magnetization and solute buildup in the diffusion boundary layer willchange the eutectic spacing and cause the formation of the land like structure. Moreover, anapplication of a high magnetic field is capable of theAl Al2Cu eutectic with the <001> crystaldirection of the eutectic Al2Cu phase. Further, an alignment model of lamellar eutectics undera high magnetic field during directional solidification has been proposed. Moreover, anapplication of the high magnetic field during the eutectic growth will induce thethermoelectric magnetic force (TEMF) on the eutectic lamellae. This force may cause theinstability and the degeneration of the eutectic.Effect of a high magnetic field on the phase transformation has been investigated and amethod to measure the phase transformation temperature in a gradient magnetic field has beenproposed by relating the change of magnetic levitation force to the phase transformation dueto the change in magnetic susceptibility while the transformation occurs. By measuring thetemperature at which the magnetizing force changes abruptly, the phase transformation can bedetected. Through this method, the phase transformation BiMn1.08+L→BiMn in the Mn Bialloy has been investigated and the results has shown that with the increase of magneticfield, the temperature of the phase transformation increased significantly, and in a 10Tmagnetic field the temperature increase was about 20℃. Moreover, it has been found that along with the phase transformation, the high magnetic field has split the BiMn grains in thedirection perpendicular to the magnetic field and split grains further aggregate along themagnetic field direction. Further, a model has been built to analyze the effect of the magneticfield on the phase transformation temperature and it is well consistent with the experimentalresults. Moreover, it has been found that the magnetic field has aligned the MnBi phase withthe <001> crystal direction along the magnetic field direction. Along with the alignment ofthe MnBi phase, the remarkable magnetic anisotropy of the samples has formed; and with theincrease of the time imposed of the magnetic field and the magnetic field intensity, magneticanisotropy of the samples is enhanced.更多还原