温度梯度对Ti-45Al合金定向凝固组织演化影响的数值模拟

采用基于溶质扩散控制模型结合CA方法对Ti-45Al合金定向凝固过程中温度梯度对显微组织演化影响进行了数值模拟.模拟结果表明,随着温度梯度增加,凝固形态经历了树枝晶→胞/枝混合结构→粗胞晶→细胞晶→超细胞晶的转变,而一次胞/枝晶臂间距随着凝固组织变化不断减小.此外,温度梯度很高时,固液界面处溶质富集程度很小,界面处出现溶质截流.     

静磁场下热电磁效应及其对凝固组织的影响

由于塞贝克效应,当施加一个温度梯度时,在凝固界面将会产生一个热电流。在磁场下定向凝固过程中热电流和静磁场相互作用将会产生一个显著的热电磁力。此磁力将会诱发各种现象,比如液体搅拌、固相运动以及固相受力。由于在金属的凝固过程中常常存在温度梯度,这些效应将会普遍存在。在较小和适度的磁场下热电磁力将促进液体的流动,在较强的磁场下其将抑制液体的流动。另外,热电磁流有多尺度效应,即尺度越小,抑制液体流动所需的磁场越高。至今,已经完成了大量涉及多种合金的实验,所有的实验结果均表明在凝固前沿和糊状区均存在热电磁流。热电磁流动显著地影响凝固过程中微观和宏观偏析、凝固组织以及糊状区晶界结构。热电磁流动的方向以及相应的偏析可以通过改变磁场的方向而被控制。另一方面,作用于固相的热电磁力也将显著地影响固相凝固组织,表现为固相所受的热电磁力导致枝晶的断裂、枝晶碎片的运动和柱状晶-等轴晶转变等。近年来,同步辐射X射线成像技术被应用于实时观测在横向磁场下定向凝固过程中枝晶的生长行为,观察在磁场下枝晶发生断裂和枝晶碎片沿一定方向运动的行为,这进一步证实了磁场下热电磁效应在凝固过程中显著地影响凝固组织。     

凝固速率对IN718合金定向凝固组织和偏析行为的影响

采用LMC高梯度定向凝固和微观分析方法,研究了不同凝固速率下IN718合金的凝固组织和偏析行为。结果表明,随着凝固速率的提高,凝固界面由胞晶转为枝晶;一次胞/枝晶间距在凝固速率为20μm/s时提高到最大值257μm,随后逐渐降低,二次枝晶间距明显细化,溶质分配比也随凝固速率的增加而减小。同时,凝固速率增加使糊状区枝晶骨架的渗透性减小,增大了对液体流动的阻碍作用。     

凝固速度对奥氏体不锈钢定向凝固组织及其固液界面稳定性的影响

本文研究了凝固速率对1Cr18Ni9Ti不锈钢定向凝固组织及其固液界面稳定性转变规律的影响.结果表明,在某特一定的温度梯度下,随着凝固速度的增加,定向凝固的固液界面由平面转变为胞状晶,再转变为树枝晶.研究发现,随着凝固速率的增大,定向凝固组织枝晶形貌逐渐细化,枝晶间距减小.     

宽结晶间隔二元铝合金的定向凝固特性

本文用具有宽结晶间隔的Al—Cu合金在非平衡的稳态条件下进行定向凝固试验。观察了溶质再分布特性、固/液界而形态和定向凝固过程,通常均应认为是界面前沿液体中的温度梯度和晶体生长速率的比值的函数。结果表明,平面界面生长所需的G/R比值比枝晶生长所需的约高10~3倍。     

Nb基超高温合金的制备技术及定向凝固组织

Nb基超高温合金的制备技术主要包括粉末冶金、真空电弧熔炼和定向凝固等，其中定向凝固可以显著提高该合金的高低温力学性能。Nb基超高温合金的定向凝固组织主要由沿着定向凝固方向生长的初生Nbss枝晶、铌硅化物块或板条以及（Nbss＋铌硅化物）共晶等组成，定向凝固速率和固液界面前沿液相中的温度梯度会显著影响其组织形貌及尺寸。在其定向生长过程中，Nbss、铌硅化物及（Nbss＋铌硅化物）共晶各相之间竞争生长，形成不同的组织形貌及尺寸，但在合适的条件下，共晶两相可以实现平行耦合生长。     

抽拉速度对Ti-48Al-8Nb合金定向凝固组织的影响

采用Bridgeman定向凝固设备成功制备了Ti-48Al-8Nb(原子分数,%)合金定向凝固试样,分析了不同抽拉速度对柱状晶形貌及过渡区、稳态区和淬火区凝固组织的影响。实验结果表明:在不同抽拉速度下,柱晶沿抽拉方向生长良好,柱状晶内部片层取向一致,与生长方向呈45°、90°夹角;当抽拉速度为8μm/s时,固液界面以胞状形貌向前推进,抽拉速度增大到15μm/s时固液界面开始向树枝状生长转化,随着抽拉速度进一步增大,二次枝晶间距逐渐变小,组织明显细化,片层间距显著减小;组织中与生长方向垂直的片层组织所占比例随抽拉速度增大而增加。     

DD6单晶精铸薄壁试样定向凝固过程数值模拟

针对单晶高温合金精铸薄壁试样制备困难的问题,建立了薄壁板形试样的有限元模型,采用ProCAST数值模拟的方法模拟DD6单晶精铸薄壁板形试样的定向凝固过程,研究了几何形状和工艺参数对定向凝固过程中温度场、温度梯度场及糊状区的影响。结果表明:薄壁板形试样中间部位工作端的温度梯度在60~65℃/cm范围内,糊状区固相线较为平直,液相线的位置在近炉壁一侧较低,远炉壁一侧较高。几何形状对单晶高温合金试样定向凝固过程有重要影响,提高浇注温度或降低抽拉速率有助于薄壁板形试样固液界面前沿液相温度梯度增大、糊状区宽度减小。单晶高温合金精铸薄壁试样定向凝固过程数值模拟结果与实际浇注结果吻合,凝固过程数值模拟为单晶精铸薄壁试样的制备提供了技术支持。     

基于三维LBM-CA模型模拟Al-4.7%Cu合金的枝晶形貌和成分分布

建立了三维格子玻尔兹曼方法(LBM)-元胞自动机(CA)耦合数值模型,并用该模型模拟研究了Al-4.7%Cu(质量分数)固溶体合金的凝固过程。该耦合模型采用元胞自动机方法模拟枝晶的生长,同时采用基于分子动力学理论的格子玻尔兹曼方法模拟合金凝固过程中的温度场、流场以及溶质场。模拟结果再现了合金凝固过程中的三维枝晶形貌变化以及溶质富集过程,并将三维流场因素考虑进去,定量研究了自然对流、过冷度对单枝晶形貌和成分分布的影响。研究表明,在纯扩散条件下,枝晶呈现对称的生长现象,模拟自由枝晶稳态生长的尖端速度、尖端半径和过冷度的关系与Lipton-Glicksman-Kurz(LGK)理论模型吻合得较好。在自然对流条件下,枝晶的生长形貌呈现不对称性,即枝晶生长在迎流方向上得到了促进,在顺流方向上受到了抑制。熔体过冷度对枝晶生长的影响较大,过冷度的增加导致枝晶生长加快,二次枝晶增多且呈现出粗化现象,枝晶尖端固液界面处的溶质浓度偏高,加重了溶质偏析。     

定向凝固过程中的溶质径向分凝

基于实际定向凝固系统的对称性,采用线性近似方法计算了柱坐标系中,扩散控制条件下二元合金定向凝固时微弯曲固液界面导致的溶质径向不均匀分布.     

定向凝固Cu-0.33Cr-0.1Ti亚共晶合金中带状组织的形成机理

考察了Cu-0.33Cr-0.1Ti亚共晶合金在凝固速率为20μm/s下的定向凝固组织,利用SEM和EDS对带状组织的形貌和成分进行了分析,并从动力学、形核条件等方面探讨了带状组织形成的原因和机理。结果表明,该带状组织是由单相胞状α-Cu相与(α-Cu)+(β-Cr)共晶组织交替生长形成,带状组织的产生与定向凝固界面前沿的溶质分布有关,即主要受溶质浓度、有效溶质系数和凝固速率等因素的影响。     

Effect of Solidification Interface Morphologies on Microstructures of a Ni-base Superalloy

A cast Ni-base superalloy K5 wasdirectionally solidified and various solidification in-terfaces including plane front,cellular,cellular-dendritic and dendritic were obtained in awider range of G/R ratio by using improved highwithdrawal device and liquid metal cooling experi-mental sets.The precipitation pattern of some prin-cipal phases of the alloy and correlation of the vari-ous interfaces with microstructure were studied sys-tematically.It was indicated that the morphology ofsolidification interface of superalloy K5 varied withG/R ratio and that the solidification interfacemorphologies have a considerable effect on the fea-tures of phases both precipitated duringsolidification and post-solidification.Plane frontand cellular directional solidification of superalloyK5 lead to a substantial decrease of MC carbideand elimination of γ-γ'eutectic,but makeneedle-shape M_6C carbide precipitate easily duringageing treatment.The finer dendritic structuressolidified under the condition of higher cooling ratehave less dendritic segregation and idealmicrostructure.     

Flow instabilities of liquid and mushy regions during alloy solidification and under high gravity environment induced by rotation

Linear flow instabilities of the liquid and mushy regions during directional solidification of a binary alloy are studied under a high gravity environment where the rotation axis is inclined with respect to the high gravity vector. Stability analysis and numerical computation are carried out to determine the results for the stationary disturbances at several values of the rotation rates and for given values of the other parameters. The results provide information about the effects of Coriolis force on various flow features in the liquid and mushy layers. The preferred structure of the mush-liquid interface is found to be that of longitudinal rolls. The main mode of convection is found to be able to generate double-cell structure in the vertical direction and is strengthened in the mushy layer by the Coriolis force. The Coriolis force appears to be generally stabilizing in the sense that the motion in the liquid zone is significantly weakened, the tendency for the chimney formation in the mushy zone is reduced and the critical values of the liquid and mush Rayleigh numbers and the wave number increase with increasing the rotation rate.     

Three dimensional dendritic morphology and orientation transition induced by high static magnetic field in directionally solidified Al-10 wt.%Zn alloy

Effect of high static magnetic field on the dendritic morphology and growth direction in directionally solidified Al-10 wt.%Zn alloy were studied by three-dimensional (3D) X-ray micro-computed tomography, Electron Back-scattered Diffraction (EBSD) and X-ray Diffraction (XRD). The application of high static axial magnetic field (5T) during directional solidification was found to destabilize the solid/liquid interface and cause the growth direction of dendrite deviate from thermal gradient, leading to irregular solid/liquid interfacial shape and cellular to dendritic morphology transition. The thermoelectric magnetic convection (TEMC) caused by the interaction of thermoelectric effect and magnetic field was supposed to be responsible for the transition. In addition, the EBSD and XRD results confirm that the preferred growth direction of α-Al was found to transform from the traditionally expected <100> to <110>. The dendrite orientation transition (DOT) in Al-10 wt.%Zn alloy can be attributed to the effect of applied magnetic field on the anisotropy of crystal during solidification. The result indicates the potential application of high static magnetic field in altering the morphology and preferred growth direction of dendrite during directional solidification.     

高温合金定向凝固技术研究进展

首先回顾了定向凝固的发展历史,重点分析了液态金属冷却定向凝固的技术特点。总结了高温度梯度下制备的定向凝固法单晶高温合金在组织和性能方面的研究现状,结合作者在本领域的研究,着重分析了定向凝固温度梯度、凝固速率、晶体取向、熔体超温处理、熔体对流控制对组织和性能的作用规律和机制,认为高温度梯度定向凝固是细化组织、减少缺陷、提高合金性能的重要途径。最后展望了高温合金定向凝固的发展趋势。     

Effect of transverse static magnetic field on radial microstructure of hypereutectic aluminum alloy during directional solidification

The effect of different scales thermoelectric magnetic convection(TEMC)on the radial solidification microstructure of hypereutectic Al alloy has been investigated under transverse static magnetic field during directional solidification,focusing on the formation of freckle.Our experimental and numerical simulation results indicate that the TEMC circulation at sample scale under transverse static magnetic field leads to the enrichment of solute Al on one side of the sample.The TEMC and the solute enrichment degree increase with the increase of magnetic field when the magnetic field increases to 0.5 T.The enrichment degree of solute elements under magnetic field is affected by temperature gradient and growth rate.The non-uniform distribution of solute Al in the radial direction of the sample results in the non-uniform distribution of primary dendrite arm spacing(PDAS).Moreover,the applied magnetic field can lead to freckle formation and its number increases with the increase of magnetic field.The change of freckle is consistent with the anisotropy TEMC caused by the anisotropy of primary dendrite or primary dendrite network under magnetic field.Finally,the mechanism of synergism effect of the anisotropy TEMC,the distribution of solute Al and the PDAS on freckle formation and evolution is studied during directional solidification under magnetic field.     

电场对定向凝固类包晶合金凝固组织的影响

为研究直流电流对亚包晶合金凝固过程的影响,选用与亚包晶合金都存在相似凝固过程的AMPD-4.1%SCN透明亚包晶模拟物为研究对象。使用显微镜感光器件(CCD)和智能通讯测温仪表对实验过程进行实时拍照和温度记录,研究了亚包晶透明模拟物在电场作用下的结晶过程和晶体生长规律。结果表明：在电场的作用下,由电迁移效应使定向结晶的亚包晶模拟物的初生β相颗粒逐渐向正极方向迁移,使凝固界面前沿的液相成分与包晶点的成分(0.05%SCN,原子分数)接近,从而促进包晶反应的进行;电场的作用使电流偏聚产生的焦耳热效应和溶质富集引起的成分过冷,使定向凝固的枝晶尖端产生特殊分裂的生长形貌,使枝晶尖端分裂,枝晶间距减小。     

温度梯度条件下糊状区的凝固行为研究

利用一个垂直的定向凝固装置,研究了Al-6wt%Cu合金在温度梯度约12K/mm条件下,糊状区在保温不同时间并激冷后得到的微观组织及成分分布特征.SEM/EDX分析表明,糊状区在保温时间内将发生自发的、以扩散机制为主的、类似TGZM效应的凝固过程.完全凝固后所形成的成分梯度与温度梯度的对应关系对于预测定向凝固过程中固液相界面温度以及相平衡测定具有重要意义.     

Phase-field simulation for the evolution of solid/liquid interface front in directional solidification process

In this study, the phase field method was used to study the multi-controlling factors of dendrite growth in directional solidification. The effects of temperature gradient, propelling velocity, thermal disturbance and growth orientation angle on the growth morphology of the dendritic growth in the solid/liquid interface were discussed. It is found that the redistribution of solute leads to multilevel cavity and multilevel fusion to form multistage solute segregation, and the increase of temperature gradient and propelling velocity can accelerate the dendrite growth of directional solidification, and also make the second dendrites more developed, which reduces the primary distance and the solute segregation. When the temperature gradient is large, the solid-liquid interface will move forward in a flat interface mode, and the thermal disturbance does not affect the steady state behavior of the directionally solidified dendrite tip. It only promotes the generation and growth of the second dendrites and forms the asymmetric dendrite. Meanwhile, it is found that the inclined dendrite is at a disadvantage in the competitive growth compared to the normal dendrite, and generally it will disappear. When the inclination angle is large, the initial primary dendrite may be eliminated by its secondary or third dendrite.     

Pb-Sn合金侧向凝固过程A偏析的数值模拟

基于二元系凝固过程热溶质的传输行为,建立了描述A偏析形成及演化的数学模型,给出了固相分数与温度场及浓度场的耦合关系.先用已有的实验结果验证了模型的正确性,然后模拟计算了Pb-Sn合金侧向凝固过程A偏析的形成及演化过程,并研究了浮力数对A偏析形成位置及偏析程度的影响.结果表明,在糊状区中双扩散对流引起的密度变化,导致局部流动,形成偏析通道;为了维持偏析通道中局部液体的流动,枝晶间的液体通过糊状区从液相区得到补充.在相同的凝固条件下,浮力数越小,A偏析的形成时间愈迟,偏析的程度也越小.