高温合金具有优异的高温强度和良好的抗氧化性能，广泛应用于航空航天发动机和燃气轮机燃烧室的热端部件[1,2]。与多晶、柱状晶高温合金相比，单晶高温合金消除了横向和纵向晶界，使其耐温能力进一步提高[3]。单晶高温合金在制备过程中易受到不规则热溶质对流、复杂几何形状和温度场不断变化等因素影响，使其产生溶质分布不均匀、取向超差[4]、雀斑[5]、小角度晶界[6,7]和杂晶[8]等缺陷。其中，小角度晶界通常是指相邻枝晶取向差在2°~15°之间的界面；杂晶则是与基体枝晶取向差异较大的晶粒，其常出现在变截面平台区域[9]、选晶器出螺旋段区域[10,11]和籽晶重熔界面[12,13]附近。杂晶与基体枝晶存在明显晶界，破坏了单晶结构完整性，在高温服役条件下会成为薄弱环节，恶化了铸件的高温力学性能。
利用合金熔体的导电性，使用静磁场控制凝固组织已成为一种特殊有效手段。近年来，研究者使用静磁场控制高温合金的定向凝固组织和缺陷。例如，Zhang等[14]在高温合金DZ417G定向凝固阶段施加0~10 T的纵向静磁场，其一次枝晶间距随磁场强度增加先减小，在6 T达到最低值，随后上升。董建文等[15]在高温合金定向凝固中施加0~0.7 T横向磁场，一次枝晶间距随着磁场强度的增加而减小。施加纵向磁场降低了合金元素在枝晶间的偏析程度，如正偏析元素Ti和Ta、负偏析元素W和Mo的偏析系数均随磁场强度的增加逐渐逼近1；与无磁场下单晶样品相比，施加4 T磁场后，γ/γ'共晶组织含量降幅达到37.76%[16]。Xuan等[17]发现，5 T纵向磁场下单晶高温合金CMSX-4中缩孔含量较无磁场时降低0.04%，同时在980℃、250 MPa条件下合金的蠕变寿命也比不施加磁场时提高了37%。
上述关于磁场对高温合金组织的研究主要集中于凝固过程某一阶段，缺乏对整体组织演变过程的探索，这使得枝晶受磁场的影响表现出多种现象，枝晶被破坏[18~22]，或者没有被破坏[23~26]，也有文献不提及此影响[27,28]，因此，静磁场对枝晶生长的影响非常复杂，这可能与凝固中的温度梯度、抽拉速率与不同磁场强度耦合有关。保持单晶高温合金枝晶在凝固中的定向生长特性，对于其力学性能的改善至关重要。为了澄清纵向静磁场对单晶高温合金枝晶生长过程的影响，本工作采用籽晶法制备单晶高温合金，通过分析定向凝固中不同生长阶段的显微组织，研究纵向磁场下杂晶的形成及其后续演变规律，为利用静磁场优化单晶高温合金组织和缺陷提供理论支持。
1 实验方法
实验材料为第一代镍基单晶高温合金DD483，其化学成分(质量分数，%)为：Al 3.48，Ti 4.00，Ta 4.86，Cr 12.26，Co 9.19，W 3.76，Mo 1.99，C 0.07，Ni余量。采用籽晶法制备单晶高温合金试样，母合金棒尺寸为直径8.65 mm、长50 mm，籽晶尺寸为直径8.65 mm、长20 mm。
实验系统主要为纵向磁体和Bridgman定向凝固装置，实验装置示意图参见文献[25]。实验时，把加热炉、试样和拉杆装配好后放入磁场中，根据预先测得的磁场强度中心位置和液/固界面位置，在样品放置时确保2者位置重合。充入Ar气30 min后，启动加热系统，炉内温度到达目标值时，为保证样品内加热均匀，保温30 min，然后打开伺服抽拉系统，完成定向凝固实验。具体实验参数如下：(1) 在0、0.5和1 T磁场下以20 μm/s抽拉速率完成定向凝固，分析组织演变过程；(2) 在0.5 T磁场下，分别以20、50和100 μm/s抽拉速率完成定向凝固，分析组织演变过程。
图1
图1
籽晶法定向凝固单晶高温合金样品及其显微组织
Fig.1
Single-crystal (SC) superalloy prepared by the seed crystal (a) and the magnified image in the red box in Fig.1a (b) (20 and 50 mm indicate the lengths of the used seed crystal and the master alloy rods, respectively. 0, 5, 15, 25, and 35 mm indicate the observed positions of the across section. The red box indicates the area where the remelted zone of the seed crystal. The white- and green-dotted lines show the positions of the fully remelted interface and the partially remelted interface, respectively)
利用DM6000型光学显微镜(OM)观察合金的枝晶宏观形貌，腐蚀剂配比为30 g CuSO4 + 50 mL H2SO4 + 1000 mL H2O。利用3400N型扫描电镜(SEM)进行电子背散射衍射(EBSD)分析，EBSD面扫样品整个横截面表面，对整体取向信息进行重构，从而获得枝晶不同区域的晶界及其长度[29]。EBSD电解抛光液配比为10 mL HClO4 + 90 mL CH3COOH，电压40 V，电流4 A，样品浸入电解液中5 s。
2 实验结果
2.1 磁场强度对杂晶形成及演变的影响
不同磁场强度下各凝固阶段横截面宏观组织形貌、EBSD结果和枝晶取向如图2~4所示。0 mm处，0 T磁场下整个横截面上枝晶取向均在20°以内，而0.5和1 T磁场下枝晶最大取向分别为51°和53° (图4a)，这表明磁场的施加使凝固组织中出现大取向枝晶(杂晶)，且多分布于试样边缘(图2b1和c1，图3b1和c1)；随着磁场强度增加，杂晶数量增多，0.5和1 T磁场下取向值大于20°的杂晶比例分别为3.5%和10.4% (图4b)。当凝固进行到15 mm及以上位置，二次枝晶充分长大，呈现典型十字形。0 T不同位置处的枝晶取向均在15°以下(图4a)；0.5 T下，凝固距离5、15、25和35 mm处杂晶最大取向分别为27.3°、22.1°、18.2°和20° (图4a)，20°以上取向的杂晶比例分别为2.5%、1.5%、9%和0% (图4b)；磁场强度增大到1 T时，5、15、25和35 mm处杂晶最大取向分别为50.2°、35.3°、15.1°和15° (图4a)，20°以上取向的杂晶比例分别为8.3%、1.5%、0%和0% (图4b)。综上可知，磁场破坏了单晶组织的定向生长特性，使得凝固组织中出现杂晶，磁场强度增大使杂晶的最大取向和数量增加；随着凝固的进行，大取向杂晶数量迅速减小，表现为凝固至15 mm以上时横截面上枝晶取向均在20°以内；随磁场强度的加大，大取向杂晶消失的凝固长度有所增加。15 mm时，0.5和1 T下杂晶的最大取向值分别为21°和35°，当凝固至25 mm时，0.5和1 T下最大杂晶的取向值接近一致。
图2
图2
抽拉速率为20 μm/s时，不同磁场强度下合金经定向凝固后在不同凝固长度处横截面的宏观形貌
Fig.2
Macroscopic morphologies of cross sections at the positions of 0 mm (a1-c1), 15 mm (a2-c2), and 35 mm (a3-c3) prepared by directional solidification under different magnetic fields at the pulling rate of 20 μm/s (B —magnetic field) (a1-a3) 0 T (b1-b3) 0.5 T (c1-c3) 1 T
图3
图3
抽拉速率为20 μm/s时，不同磁场强度下合金经定向凝固后在不同凝固长度处EBSD反极图
Fig.3
EBSD inverse pole figures (IPFs) of cross sections at the positions of 0 mm (a1-c1), 15 mm (a2-c2), and 35 mm (a3-c3) prepared by directional solidification under the magnetic fields of 0 T (a1-a3), 0.5 T (b1-b3), and 1 T (c1-c3) at the pulling rate of 20 μm/s (Red, green, and blue curves in the figures represent the grain boundaries of 2°-5°, 5°-15°, and 15°-65°, respectively)
图4
图4
不同凝固距离处枝晶最大取向及枝晶取向偏离<001>角度大于20°的比例
Fig.4
Maximum dendrite orientations at different solidification distances (a) and the percentages of dendrite orientation deviation from <001> more than 20° (b)
单晶高温合金中，不同取向枝晶凝固相接产生的界面被定义为晶界。图5为磁场下各凝固距离处不同角度的晶界长度。0 T下， 0、5、15、25和35 mm处横截面组织中，2°~5°晶界长度分别为12.23、27.35、27.57、26.24和46.60 mm；5°~15°晶界长度分别为32.97、27.33、24.26、13.63和8.73 mm (图5a)。0.5 T条件下，0、5、15、25和35 mm处，2°~5°晶界长度分别为33.66、40.11、57.99、41.83和56.90 mm；5°~15°晶界长度分别为19.19、14.95、25.39、24.50和18.12 mm；15°~65°晶界长度分别为84.90、29.40、2.70、1.10和0.50 mm (图5b)。可以看到，(1) 磁场促使15°~65°大角度晶界出现，且随着凝固距离增加其晶界长度逐渐降低，在0~15 mm之间急剧降低，而15 mm之后减小程度减弱；大角度晶界的出现及其长度随凝固距离的变化与图3和4反映的大取向杂晶形成和数量随凝固距离的变化是一致的；(2) 磁场的施加也使得2°~5°晶界长度增加，其长度随凝固距离的增加而加大，这表明随着凝固的进行，5°~15°和15°~65°的晶界逐渐演变为2°~5°晶界，即大的晶界总是逐渐演变成小的晶界，此规律与无磁场下是一致的。与无磁场相比，凝固到15 mm后，0.5 T磁场使得5°~15°晶界长度随凝固进行向更小角度演变速率减缓。在15~25和25~35 mm凝固范围，0 T下5°~15°晶界减少的幅度分别为43.82%和35.95%，在0.5 T磁场下分别为3.51%和25.00%。
图5
图5
磁场下各凝固距离处不同角度范围段的晶界长度
Fig.5
Grain boundary lengths of different angles at different solidification distances under the magnetic fields of 0 T (a), 0.5 T(b), and 1 T (c) (For clarity, the grain boundary length data in the figures are represented by integers)
1 T条件下，0、5、15、25和35 mm处，2°~5°晶界长度分别为34.49、47.08、58.10、49.62和72.70 mm；5°~15°晶界长度分别为67.30、109.50、106.60、103.40和80.90 mm；15°~65°晶界长度分别为142.00、105.23、25.06、15.79和12.00 mm (图5c)。可以看到，磁场强度的提高加大各个角度的晶界长度，且各个角度的晶界长度随凝固距离的变化规律与0.5 T磁场下一致。与0.5 T相比，1 T下15°~65°和5°~15°晶界长度随着凝固距离的变化向更小晶界演变速率减慢，在15~25和25~35 mm范围，5°~15°晶界长度减少幅度在1 T磁场下分别为3%和21.76% (0.5 T下分别为3.51%和25.00%)，15°~65°晶界长度减少幅度分别为36.99%和24% (0.5 T下分别为59.26%和54.55%)。
2.2 抽拉速率对杂晶形成及演变的影响
为了进一步研究磁场下单晶高温合金籽晶回熔区附近杂晶形成的原因，考察了相同磁场强度下不同抽拉速率对定向凝固组织的影响。图6和7给出了0.5 T磁场不同抽拉速率下各凝固位置处横截面宏观形貌和EBSD观察结果。可以看到，0 mm处不同抽拉速率下均形成杂晶(图6a1~c1和7a1~c1)，且杂晶位于样品边缘处的数量较多。图8为0.5 T磁场下各角度的晶界长度统计。0 mm处，在20、50和100 μm/s拉速下，2°~5°晶界长度分别为33.66、83.60和57.60 mm；5°~15°晶界长度分别为19.19、79.00和57.42 mm；15°~65°晶界长度分别为84.90、161.50和168.40 mm。与20 μm/s相比，50 μm/s下2°~5°、5°~15°、15°~65°增大幅度分别达到49.94、59.81和76.60 mm；抽拉速率从50 μm/s增大到100 μm/s时，各个角度的晶界长度无明显增大。
图6
图6
0.5 T磁场强度下不同抽拉速率时合金经定向凝固后在不同凝固长度处横截面宏观形貌
Fig.6
Macroscopic morphologies of cross section at the directional-solidification lengths of 0 mm (a1-c1), 15 mm (a2-c2), and 35 mm (a3-c3) under 0.5 T magnetic field at the pilling rates of 20 μm/s (a1-a3), 50 μm/s (b1-b3), and 100 μm/s (c1-c3)
图7
图7
0.5 T磁场强度下不同抽拉速率时合金经定向凝固后在不同凝固长度处横截面EBSD反极图
Fig.7
EBSD IPFs of cross sections at the directional-solidification lengths of 0 mm (a1-c1), 15 mm (a2-c2), and 35 mm (a3-c3) under 0.5 T magnetic field (The red, green, and blue curves in the figures represent the grain boundaries of 2°-5°, 5°-15°, and 15°-65°, respectively) (a1-a3) 20 μm/s (b1-b3) 50 μm/s (c1-c3) 100 μm/s
图8
图8
0.5 T磁场不同抽拉速率下合金在各位置处不同角度的晶界长度
Fig.8
Length distribution statistics of dendrite grain boundaries at different locations after 0.5 T magnetic field solidification at the pulling rates of 20 μm/s (a), 50 μm/s (b), and 100 μm/s (c) (For clarity, the grain boundary length data in the figures are represented by integers)
当凝固界面向前推进，大角度晶界被迅速淘汰，与20 μm/s相比，50和100 μm/s下，大角度晶界降低幅度增大，如在15 mm处，在20、50、100 μm/s拉速下，15°~65°大角度晶界长度分别为2.70、12.01和3.03 mm，与0 mm相比，其长度降低分别为82.20、149.49和165.37 mm。凝固继续推进到35 mm处，同15 mm处相比，大角度晶界长度进一步降低。综上所述，抽拉速率的增大使得起始凝固段的杂晶数量增加，也使得杂晶在生长过程中被淘汰的速率加大。
3 分析讨论
实验结果表明，磁场下籽晶重熔区界面上会出现杂晶，其主要分布于试样边缘；随磁场强度加大，杂晶个数和大角度晶界长度均增加；随着凝固的进行，大角度晶界逐渐向小角度晶界演化，晶界角度越大，演化速率越快。在相同磁场强度下，抽拉速率的增大使得起始凝固段的杂晶数量增加，也使得杂晶在生长过程中被淘汰速率加大。
3.1 磁场下回熔区凝固起始界面上杂晶的形成
籽晶法定向凝固中杂晶的产生有以下3种原因：一是源于籽晶重熔区界面温度场随着抽拉系统突然启动而发生改变。由于热量从炉体辐射进入熔体需要一定的时间，抽拉系统起动后籽晶重熔界面等温线由上凸急剧变为下凹，此时在下凹界面前沿熔体过冷，促进独立形核，形成杂晶[7]。这类杂晶通常取向随机，与籽晶母体取向无关联，2者之间易形成大角度晶界。二是在枝晶定向生长过程中糊状区热溶质对流冲刷枝晶臂，导致枝晶臂的折断形成雀斑[5]。枝晶碎片在流动中发生较大的位移与转动，因此与基体枝晶形成大角度晶界。三是糊状区内枝晶在收缩应力作用下收缩受阻或强度受损从而被机械性撕裂，被撕裂后的枝晶受到周围枝晶的支撑，不会发生大幅度的位移与偏转，因此与基体枝晶常形成小角度晶界[30]。
上述讨论可知，抽拉系统突然启动导致液/固界面前沿温度场改变，熔体过冷形核是杂晶产生的原因之一。对0.5 T磁场下样品在启动抽拉之前和启动下拉5 mm后的液/固界面进行淬火，观察其形态(图9)。可以看到，启动抽拉系统前，重熔区液/固界面保持平直状态(图9a)，下拉5 mm后液/固界面仍然保持平直(图9b)，因此磁场下籽晶重熔区杂晶的产生不是源于界面前沿温度场的改变，而应该是磁场效应作用于糊状区枝晶导致枝晶臂断裂。根据已有报道[31]，静磁场对定向凝固枝晶的影响效应主要有磁制动(magneto-hydrodynamic damping)、热电磁对流(thermoelectro-magnetic convection，TEMC)和热电磁力(thermoelectro-magnetic force，TEMF)。磁制动效应是指在磁场中熔体的流动产生感应电流与外加磁场作用而产生与流动方向相反的Lorentz力，总是趋向于使对流运动强度减弱，其可以稳定均匀化流场，均匀化成分和组织分布。而热电磁对流和热电磁力是近年来强磁场便捷使用后受到关注的一对共生效应，它们是液/固界面前沿的热电流(液/固相不同电势和温度梯度的复合产生)和磁场相互作用产生的，驱动界面附近熔体流动形成热电磁对流，而在界面刚凝固的固相中形成热电磁力。热电磁效应更多地发挥在液/固界面附近区域，在远离界面微米尺度范围内迅速衰减为零，过大的热电磁对流能够干扰界面前沿流场结构，从而破坏枝晶定向生长特性，过大的热电磁力也能拧断枝晶臂产生杂晶[32]。
图9
图9
0.5 T磁场强度下淬火界面形貌
Fig.9
Quenching interface morphologies under 0.5 T magnetic field
(a) 0.5 T remelted quenching interface
(b) 0.5 T quenched after pulling down at 20 μm/s for 5 mm
式中，σS、σL、fL、fS、SS、SL、G、B分别为固相(S)和液相(L)的电导率、体积分数、绝对热电势及固/液界面处温度梯度和磁场强度。
图10
图10
计算域和其网格划分：包括液相、固相和胞状液/固界面的计算域及其横截面
Fig.10
Computing domain and its meshing
(a) computational domains including liquid, solid, and cellular liquid-solid interfaces
(b) cross section, taken from the white curve area in Fig.10a
图11
图11
不同磁场强度下胞晶液/固界面附近热电流与热电磁力分布
Fig.11
Distribution of thermoelectric current (a1, b1) and thermoelectro-magnetic force (a2, b2) at the liquid-solid interface of the cellar crystal under different magnetic fields (The red arrows represent the directions of the current) (a1, a2) 0.5 T (b2, b2) 1 T
0.5和1 T纵向磁场下，液/固界面附近热电磁力分布如图11a2和b2所示。可以看到，热电磁力在胞晶尖端大小相同，方向相反，形成力矩。计算结果表明，0.5和1 T磁场下枝晶尖端附近热电磁力最大值分别为5.68 × 106和1.14 × 107 N/m3。当热电磁力大于此温度下枝晶强度时，该力可以扭断枝晶臂产生大取向杂晶。Dahle和Arnberg[33]测定了不同温度下AlSi7Mg合金的屈服强度，其在接近液相线温度时断裂强度为1 × 10-2~1.5 × 10-2 MPa。在本工作中，该合金为一种强度较高的高温合金，其在高温下强度应该与AlSi7Mg合金处于同一数量级。假设枝晶臂是半径为R、高度为h的圆柱体，其体积V = πR2h，枝晶根部面积A = πR2，枝晶所受剪切应力τ= FV / A = Fh[20]。如果枝晶高度为15 mm，将上述f值带入可得0.5和1 T磁场下剪切应力分别为8.52 × 10-2和1.71 × 10-1 MPa。因此，热电磁力是引起枝晶破碎的主要原因。
胞界面前沿熔体中的热电磁力驱动熔体流动形成热电磁对流，同时该处熔体也受到磁制动力的影响，2者表现为相互竞争作用，Khine和Walker[34]研究表明，当Hartmann数(Ha)为10时，TEMC效应达到最大。Ha可以表达为：
式中，σ为熔体电导率，η为动力黏度，L为枝晶特征长度。本工作中，当磁场强度为0.5 T、特征长度为400 μm时，Ha为263。因此，此时熔体液/固界面前沿应以磁制动效应为主，热电磁对流效应不会给枝晶造成冲击力，但是从图12中磁场下液/固界面前方的流动结构看到，热电磁对流使得界面前沿熔体流动结构发生变化。无磁场下，胞界面前方形成垂直方向的流胞，而施加磁场后，胞界面前方的流动表现为围绕胞周围的环流。
图12
图12
不同磁场强度下样品熔体内流场结构
Fig.12
Flow field structures of the transverse (a1-c1) and longitudinal (a2-c2) sections in the melt under the magnetic fields of 0 T (a1, a2), 0.5 T (b1, b2), and 1 T (c1, c2) (The cross section was taken at 4.5 mm from the tip of the cell)
所以，本工作中磁场下单晶高温合金定向凝固中籽晶回熔附近产生的大取向杂晶可以归结为热电磁力对枝晶的拧断，热电磁力随磁场强度的增大而增大，所以杂晶数量也随磁场强度增大而增加。
3.2 杂晶在回熔区域的分布位置
重熔界面上杂晶多分布于样品边缘，这显然不是热电磁力效应引起的，应该是热电磁对流作用改变了液/固界面前方的流动结构造成的。从图12磁场下胞界面前沿流动结构可以看出，各个晶胞周围的流动耦合导致界面前方形成样品尺度的大环流，而且在样品周围的流动速率要大于中心。热电磁力拧断枝晶后，破碎枝晶被快速运动的环流所挟持，定向凝固过程后，杂晶多分布于样品边缘。马德新[35]利用高速旋转离心机制备出定向凝固试样糊状区熔体，发现陶瓷管内壁接触的金属液几乎全部甩出，而试样内部仅暴露出枝晶尖端，糊状区的流通性在贴近壳壁处要比铸件内部高出1个数量级以上。所以，热电磁对流对样品尺度流动结构的改变以及在枝晶定向凝固中样品边缘具有更大的流速，导致杂晶更多分布于样品边缘处。在同一磁场强度下，随着抽拉速率的提高，回熔界面区域边缘处的杂晶数量也增加，这应该是启动抽拉速率的增大导致的液/固界面前沿实际温度梯度加大，产生更大的热电磁力，且启动抽拉速率的瞬间对熔池熔体流动造成的冲击也会增强，使得重熔界面上产生更多的杂晶。
3.3 大位向杂晶的消失和小位向杂晶的竞争生长：晶界演变
随着凝固进行，0 T下5°~15°晶界逐渐演化成2°~5°晶界，而施加磁场后，15°~65°大角度晶界出现，且凝固起始后迅速衰减演变成5°~15°的晶界，5°~15°的晶界也逐渐演化成2°~5°晶界，大角度晶界总是向小角度晶界演化，晶界角度越大，淘汰越快。杂晶取向演变的过程就是其晶界演变过程，可以用枝晶竞争生长模型来阐释此演变过程。
一般来讲，镍基单晶高温合金为fcc结构，其<001>方向具有最大的生长优势，在定向凝固中能够淘汰其他取向的晶粒。Walton和Chalmers[36]指出择优取向(与<001>取向偏离度较小)的枝晶总是淘汰非择优取向(与<001>取向偏离度较大)枝晶；而Zhou等[37]研究发现，在某些特殊情况下，非择优枝晶会反常淘汰择优枝晶。任何2个枝晶的相对取向均可以用各种方位的双晶模型来描述，其分为汇聚型和发散型(图13)，图13右面蓝色的枝晶固定方位，左边枝晶呈现不同方位。发散型(图13b1~b3)中，择优枝晶在生长过程中总会淘汰掉非择优枝晶，且随着非择优枝晶与<001>取向的偏离程度增加，淘汰速率增大[38]；而在汇聚型竞争生长中，通常也是择优枝晶淘汰非择优枝晶。然而，在图13a2和a3排列方式中，某些特殊条件下非择优枝晶会反常淘汰择优枝晶。在图13a2中，当非择优枝晶偏离<001>取向小于20°时，反常淘汰现象会发生[39]，这是由于2个枝晶尖端前沿溶质场相互重叠(图14a)，导致择优枝晶尖端溶质富集，生长滞后，从而被非择优枝晶淘汰。然而当非择优枝晶偏离择优枝晶取向大于20°时，尖端过冷度增大，使其尖端高度将远低于择优枝晶，因此择优枝晶有充足的生长空间淘汰非择优枝晶。对于图13a3，2个枝晶前沿溶质场相互影响程度加大，任何一个枝晶生长优势均无法形成，2个枝晶共同生长[40]，直至凝固结束。可以看到，在竞争生长过程中，枝晶偏离<001>取向越大，就越容易被淘汰，其中就会伴随着大角度晶界逐渐向小角度晶界演变。所以随着凝固的进行，回熔区附近产生的大取向杂晶很快被淘汰，15°~65°大角度晶界迅速演变成5°~15°的晶界，使得5°~15°的晶界比例增加；而当2个枝晶偏离<001>取向均不大时，部分会呈现为图13a2和a3的排列方式，此时非择优枝晶会反常胜存或者2者并存生长，这会导致枝晶淘汰速率减小，晶界演变速率也减慢。所以，当凝固进行到一定程度后，枝晶由大取向演变成小取向的速率降低，晶界演变过程也减缓。施加磁场后，由于磁制动效应的存在，使得远场熔体中对流强度减弱，液/固界面前沿溶质富集层变厚，枝晶间溶质层相互影响的程度增加，反常淘汰几率增加，即非择优取向的枝晶生长优势加大，所以磁场下杂晶被淘汰速率和大角度晶界演变过程减缓。
图13
图13
偏离<001>方向不同角度的双晶竞争生长示意图
Fig.13
Schematics of competitive growth of bi-crystal including converging (a1-a3) and diverging (b1-b3) at different angles deviating from the direction of <001> (The red and blue dendrites represent the favorable dendrites (A small deviation from the <001> direction) and the unfavorable dendrites (a large deviation from the <001> direction), respectively)(a1, b1 and a3, b3) the favorable dendrites are in the same or the opposite orientation as the unfavorable dendrite, respectively
(a2, b2) the favorable dendrite is parallel to the direction of heat flow
图14
图14
反常淘汰的汇聚型双晶模型中枝晶前沿溶质富集层相互作用示意图
Fig.14
Schematics of solute-enrichment-layer interaction in dendrite front in unusual overgrowth of converging bi-crystal model (Different color curves represent the isoconcentration contour)
(a) converging type shown in Fig.13a2 (b) converging type shown in Fig.13a3
3.4 抽拉速率对竞争生长和晶界演化的影响
磁场下回熔区界面上大于20°的杂晶和15°~65°大角度晶界在凝固的起始阶段很快被淘汰，相同磁场强度下抽拉速率的增大强化了这种现象，在随后的凝固中，5°~15°的晶界也逐渐演化成2°~5°，抽拉速率的增大也强化了演变过程。这是由于高抽拉速率下，过冷度增大，择优枝晶生长的优势加大，且枝晶间距的减小也使得非择优枝晶演变缺乏生长空间；另外，高抽拉速率下界面前沿的溶质富集层厚度减薄，枝晶间溶质层相互影响的程度减弱，降低了反常淘汰几率，间接加大了正常竞争机制，即择优取向的枝晶生长优势加大，其数量增加。所以大取向杂晶和大角度晶界在高抽拉速率下被淘汰过程加快，晶界演变速率也增大。
4 结论
(1) 磁场的施加使得单晶高温合金定向凝固组织中重熔区界面上组织中出现大取向杂晶，其大多分布于边缘；也使得15°~65°大角度晶界出现，磁场强度的增大使杂晶数量增加，大角度晶界长度增加。
(2) 凝固起始阶段，大取向杂晶和大角度晶界生长后，很快被淘汰，分别演化成小取向枝晶和小角度晶界；随着凝固继续进行，枝晶取向和晶界角度进一步减小，但演化速率急剧降低；磁场的施加也降低了其演变速率。
(3) 在相同磁场强度下，抽拉速率的增大使得起始凝固段的杂晶和大角度晶界数量增加，也使得杂晶和大角度晶界在起始生长过程中被淘汰速率加大，也强化了枝晶取向和晶界角度减小的演化过程。
(4) 磁场下单晶高温合金在定向凝固籽晶回熔区界面上杂晶的形成可以归结为热电磁力对枝晶的扭断，而宏观尺度上的热电磁环流在凝固过程挟制着扭断碎晶，使得碎晶演化成较多分布于样品边缘的杂晶。
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New study and development on electromagnetic field technology in metallurgical processes
[J].Electromagnetic metallurgy technology is an essential method of high quality steel production. This article reviews the development of electromagnetic metallurgy technology in recent years, focusing on the whole process of continuous casting, including electromagnetic purification of steel in tundish, nozzle flow control, mould electromagnetic stirring and electromagnetic brake, flow field control via magnetic field, electromagnetic soft contact electromagnetic continuous casting, electromagnetic field regulation of solidification structure, solid phase transformation and microstructure control under electromagnetic field, the mechanism of electromagnetic field action is explained, the principle and characteristics of electromagnetic field technology are analyzed, and the concept of multi-mode magnetic field is proposed in the field of flow field control by using electromagnetic field to meet the requirements of complex states in high quality steel continuous casting. In the field of static magnetic field control solidification structure, a new principle of applying high thermal electromagnetic force is proposed, and it is presented that the development of electromagnetic metallurgy technology needs to combine the artificial intelligence of big data to play a better role.
电磁冶金技术研究新进展
[J].电磁冶金技术是高品质钢生产的必备手段。本文综述了近年来电磁冶金技术的发展，围绕连铸的全流程，包括中间包电磁净化钢液、水口控流、结晶器内电磁搅拌和电磁制动等磁场控制流场、电磁软接触结晶器连铸、电磁场调控凝固组织、电磁场下固态相变及组织控制在内各方面，阐述了电磁场作用的机理，分析了应用电磁场技术的原理和特点，在电磁场控制流场领域提出了多模式定制磁场的概念，以满足高品质钢连铸中复杂状态的要求。在静磁场控制凝固组织领域提出应用强磁场热电磁力的新原理，并指出电磁冶金技术的发展需结合大数据的人工智能以更好发挥作用。
Thermoelectric magnetic force acting on the solid during directional solidification under a static magnetic field
[J].Thermoelectric magnetic force (TEMF), which is induced by the interaction between the thermoelectric current and the applied magnetic field, acting on the solid during directional solidification under a static magnetic field was derived. Equipping the derived equation, an analytical calculation of the velocity of a solid spherical particle submitted to the TEMF was carried out. The experiment with corresponding phenomenon was performed and recorded by the in situ synchrotron X-ray imaging, which permitted a direct measurement of the velocity of the TEMF-driven motion of detached fragments. The measurement of the velocities showed a reasonable agreement with the calculation results.
Development of strength in solidifying aluminium alloys
[J].
Thermoelectric magnetohydrodynamic effects during Bridgman semiconductor crystal growth with a uniform axial magnetic field
[J].
Freckle formation during directional solidification of complex castings of superalloys
[J].
定向凝固的复杂形状高温合金铸件中的雀斑形成
[J].通过实施工业条件下的定向凝固实验, 对复杂形状的高温合金铸件中的雀斑进行了检测和分析, 揭示了这种凝固缺陷产生的几个新特点. 结果表明, 雀斑易于产生在铸件的棱角部位而不是平滑表面上, 称之为棱角效应; 铸件外形台阶式地突然扩张和缩小会分别抑制和促进雀斑形成, 称之为台阶效应; 雀斑易于产生在向内倾斜而不是向外倾斜的铸件表面上, 称之为斜面效应; 叶片曲率为正的外凸曲面出现严重的雀斑缺陷, 而曲率为负的内凹曲面却毫无雀斑, 此现象称之为曲率效应. 实验中还发现雀斑不但出现在铸件的外表面, 插入型芯也会诱导铸件内部雀斑的产生, 这说明糊状区的液体流动具有强烈的附壁效应, 不管这种壁面是处在铸件外部还是内部. 通过分析可以确认, 正是这种附壁效应在各种具体形状特征下发挥作用, 导致了上述关于雀斑生成的各种效应.
The origin of the preferred orientation in the columnar zone of ingots
[J].
Mechanism of competitive grain growth in directional solidification of a nickel-base superalloy
[J].
Converging competitive growth in bi-crystal of Ni-based superalloy during directional solidification
[J].
镍基高温合金定向凝固过程中的汇聚型双晶竞争生长
[J].采用籽晶法控制晶体取向, 研究了不同抽拉速率下镍基高温合金汇聚型双晶的竞争生长. 结果表明, 低抽拉速率下, 非择优枝晶能够穿插进入择优枝晶间的液相通道, 抑制择优枝晶的生长, 使晶界向择优晶粒方向偏移. 高抽拉速率下, 非择优枝晶几乎全部被晶界择优枝晶阻挡, 晶界与择优枝晶干平行. 非择优枝晶进入择优枝晶间的液相通道使择优枝晶萎缩消失是非择优晶粒淘汰择优晶粒的主要因素, 并以此提出了抽拉速率对竞争生长的影响机制.
Overgrowth behavior at converging grain boundaries during competitive grain growth: A two-dimensional phase-field study
[J].
Structure evolution in directionally solidified bicrystals of nickel base superalloys
[J].
双晶镍基高温合金定向凝固过程的结构演化
[J].研究了双晶镍基高温合金IN792定向凝固过程的结构演化. 对于汇聚生长晶粒, 非择优取向枝晶在晶界处能够阻挡择优取向枝晶的生长, 从而导致晶界从非择优取向晶粒向择优取向晶粒倾斜, 竞争长大的结果是择优取向晶粒消失, 非择优取向晶粒获胜; 对于发散生长晶粒, 晶界处会形成新的枝晶, 从而导致晶界从择优取向晶粒向非择优取向晶粒倾斜, 竞争长大的结果是择优取向晶粒获胜, 非择优取向晶粒消失.
图1
