脉冲电流对冷坩埚定向凝固TiAl基合金微观组织和性能的影响

图1 在连续凝固过程中加载脉冲电流的示意图

Fig.1 Schematic diagram of loading pulse current in continuous solidification process

将从定向凝固试棒上线切割的试样打磨,采用Philips X'pert X射线衍射仪(XRD)分析其物相组成,扫描角度为0°~120°。将金相试样打磨、抛光和化学腐蚀后,用TG-3金相显微镜(OM)和S-4700扫描电子显微镜(SEM)观察其微观组织,腐蚀剂的组成为10% HNO₃+10% HF+80% H₂O；用Instron 5569万能材料试验机测试试样的室温拉伸性能。

2 实验结果

2.1 脉冲电流对定向凝固温度场的影响

从图1可以看出,在实验过程中试棒接触陶瓷管引起侧向散热。图2给出了脉冲电流作用下试棒的温度场分布。可以看出,温度最高处都分布在感应线圈缠绕处的中间部分,施加脉冲方波电流,频率为200 Hz,占空比为0.5,电流密度为0 mA/mm²、17.6 mA/mm²、35.3 mA/mm²和52.9 mA/mm²时,对应最高温度为1711.4℃、1730.1℃、1753.1℃和1774.3℃,脉冲电流作用下最高温度与不加电流最大相差63℃。图3给出了试棒的径向温度分布,具体数值列于表1。可以看出,在不同脉冲电流作用下试棒表层的温度与径向温度最高点之间的温度不同。脉冲电流密度为0 mA/mm²时,熔体最外层与径向温度最高处的温度差为46.5℃。随着电流密度增加到17.6 mA/mm²、35.3 mA/mm²和52.9 mA/mm²时,其温度差分别为31.4℃、25.3℃和17.9℃。随着电流密度的增大熔体最外层与径向温度最高处的温度差逐渐减小,即径向温度差减小,侧向温度梯度减小,侧向散热减小。

图2

图2 在不同脉冲电流作用下TiAl基合金凝固过程中温度场的分布

Fig.2 Temperature field distribution of TiAl based alloy during solidification under different pulse current (a) 0 mA/mm²; (b) 17.6 mA/mm²; (c) 35.3 mA/mm²; (d) 52.9 mA/mm²

表1 在不同脉冲电流作用下熔体的径向温度差

Table 1 Radial temperature difference of melt under different pulse current

Current density /mA·mm^-2	Radial temperature difference of melt/℃
0	46.5
17.6	31.4
35.3	25.3
52.9	17.9

图3

图3 在不同脉冲电流作用下TiAl基合金的径向温度分布

Fig.3 Radial temperature distribution of TiAl based alloy under different pulse current

2.2 脉冲电流作用下TiAl基合金的宏观凝固组织

图4给出了在不同脉冲电流作用下Ti-45.5Al-4Cr-2.5Nb合金试样的宏观组织。可以看出,在脉冲电流作用下的定向凝固过程分为三个区域：初始凝固区、稳定生长区和最终凝固区。初始凝固区可分为两部分：下部分为未熔化区,具有初始的等轴晶组织；上部分为熔化竞争区,偏离角较小,具有粗大的柱状晶组织；进入稳定生长区后,侧向散热使晶粒生长发生了一定的偏离；最后为最终凝固区,下拉至预设长度后缓慢降低加热功率并关闭电源,以侧向为主的散热使柱状晶向侧向生长并最终凝固。

图4

图4 在不同脉冲电流作用下Ti-45.5Al-4Cr-2.5Nb合金试样的宏观凝固组织

Fig.4 Longitudinal section macrostructures of 45.5Al-4Cr-2.5Nb alloy with different pulse current (a) 0 mA/mm²; (b) 17.6 mA/mm²; (c) 35.3 mA/mm²; (d) 52.9 mA/mm²

图4a给出了电流密度为0 mA/ mm²的定向凝固试样。可以看出,试样总体上具有定向凝固组织特征,但是出现了很多细小的晶粒。由于侧向散热较为严重,晶体生长偏离角度较大。施加频率为200 Hz、电流密度为17.6 mA/mm²的脉冲电流后稳定生长区晶粒生长偏离角度明显下降(图4b),且随着脉冲电流密度的增大(图4c),晶粒生长偏离角度减小,柱状晶生长的稳定性和连续性提高。当电流密度增大到52.9 mA/mm²时,在过渡区域生成的晶粒数目增多且较为细小,进入稳定生长区后生成具有定向趋势的定向凝固组织,晶粒生长偏离角进一步减小。即随着电流密度的增大和作用时间的延长,焦耳热效应与集肤效应越明显,使试棒表层温度提高侧向散热减少,从而使柱状晶的偏离角减小。图5给出了不同脉冲电流作用下晶粒平均偏离角所示,具体数值列于表2。脉冲电流频率保持200 Hz,晶粒平均偏离角由电流密度为0 mA/mm²时的30.2°减小到电流密度为52.9 mA/mm²时的17°,与上述模拟结果一致。即随着电流密度的增大径向温度差减小、侧向温度梯度减小和侧向散热减小,使枝晶偏离角减小。

图5

图5 在不同脉冲电流作用下试样晶粒的平均偏离角

Fig.5 Average deviation angle of grains with different pulse current

表2 在不同脉冲电流作用下试样晶粒的平均偏离角

Table 2 Average deviation angle of grains with different pulse current

Current density/mA·mm^-2	Average deviation angle of grains / (°)
0	30.2
17.6	21.5
35.3	18.6
52.9	17.0

2.3 在脉冲电流作用下定向凝固TiAl基合金的相组成

图6给出了脉冲电流密度不同的定向凝固Ti-Al基合金的金相照片。从图6a、b、c、d可以看出,TiAl合金试样的金相组织都是由带状组织和带状组织间的片层结构组成的。

图6

图6 在不同脉冲电流作用下定向凝固TiAl合金的OM像

Fig.6 OM images of directional solidified TiAl with different pulse current (a) 0 mA/mm²; (b) 17.6 mA/mm²; (c) 35.3 mA/mm²; (d) 52.9 mA/mm²

为了研究脉冲电流对Ti-Al基合金的影响,图7给出了脉冲电流密度不同的定向凝固试样的XRD谱。可以看出,与不加脉冲电流相比,脉冲电流密度不同的定向凝固Ti-Al基合金主要由γ-TiAl相、B2相及α₂-Ti₃Al相组成。衍射谱中的衍射峰及其强度表明,Ti-45.5Al-4Cr-2.5Nb合金中γ-TiAl相的含量较高。

图7

图7 在不同脉冲电流密度下定向凝固TiAl合金的XRD谱

Fig.7 XRD spectra of directional solidification region of TiAl alloys at different current intensities

2.4 脉冲电流对晶粒尺寸的影响

为了考察脉冲电流对晶粒尺寸的影响,切取脉冲密度不同的试样稳定生长区的横截面,其晶粒平均径向尺寸的统计结果列于表3。

表3 在不同脉冲电流作用下试样晶粒的平均径向尺寸

Table 3 Average grain width of samples with different pulse current

Current density/mA·mm^-2	Average grain width/μm
0	850.67
17.6	728.16
35.3	685.93
52.9	776.32

可以看出,随着脉冲电流密度的增大晶粒的平均径向尺寸逐渐减小,电流密度为35.3 mA/mm²的晶粒其平均径向尺寸最小为685.93 μm,脉冲电流密度提高到52.9 mA/mm²的晶粒其平均径向尺寸反而增大到776.32 μm。其原因是,脉冲电流降低了形核功和糊状区的温度梯度。脉冲电流引起的形核功为^[33]

Δ W = 4 \frac{J^{2} (r^{2} - d^{2})}{A^{2} r^{4}} δ V

(1)

式中ΔW为电流所引起的形核功变化,J为电流密度,r为熔体半径,d为洛伦兹力到晶核中心距离,A为常数, $δ$ 为与形核相关常数,V为晶核体积大小。可以看出,因为ΔW小于0,脉冲电流使形核功降低,不但细化了晶粒电流产生焦耳热效应和洛伦兹力还使枝晶重熔或断裂而提高了形核率,使晶粒细化。但是,电流密度过大间接影响糊状区的温度梯度。糊状区的温度梯度可表示为

G = Δ T / Δ d

(2)

式中G为温度梯度,ΔT为轴向温度增量,Δd为轴向距离增量。脉冲电流的焦耳热效应和洛伦兹力使定向凝固过程中的柱状晶枝晶发生重熔、断裂和晶粒游离,使柱状晶的晶粒细化。但是,随着电流密度的增大熔池长度变长,使糊状区的长度增大,即式中的Δd增大使温度梯度G减小。因此,电流密度增大至一定数值使温度梯度的减小,从而使晶粒的平均径向尺寸增大。

2.5 脉冲电流对片层取向的影响

控制定向凝固TiAl合金片层的取向,是提高其力学性能的途径之一,片层取向与轴向夹角越小其综合力学性能越高^[34]。图8给出了脉冲电流不同的TiAl基合金片层的取向OM照片。根据TiAl合金的相图,由于所选合金为亚包晶TiAl合金,凝固的初生相为β相,其择优取向为<001>,β与α为<110>β// $\{0001\}$ α关系,而<110>β与<001>为0°或45°关系,即α相 $\{0001\}$ 与<001>β轴向为0°或45°^[35]。实验中实验条件和实验误差,使片层的取向与理论值不完全相符。

图8

图8 在不同脉冲电流作用下TiAl基合金是片层取向

Fig.8 Effect of different pulse current on lamellar orientation of TiAl based alloy (a) 0 mA/mm²; (b) 17.6 mA/mm²; (c) 35.3 mA/mm²; (d) 52.9 mA/mm²

图9给出了脉冲电流不同的TiAl基片层取向小于45°的占比统计。可以看出,随着脉冲电流密度的增大0°~45°片层所占区域的比例比较接近,为75%~83%。这些结果表明,脉冲电流对片层取向的影响较小,电流密度对各片层取向所占比例影响不大,片层取向主要取决于凝固参数,即下拉速度^[35]。

图9

图9 在不同脉冲电流作用下TiAl基合金的片层角度小于45°的片层区域占比

Fig.9 Proportion of lamellar area with lamellar angle less than 45° with different pulse current

2.6 脉冲电流对微观组织的影响

为了进一步分析脉冲电流对TiAl基合金微观组织的影响,图10给出了脉冲电流密度不同的定向凝固TiAl合金的SEM照片。可以看出,微观组织与其宏观组织都由带状组织和其间的片层组织构成,带状组织内有明显的黑色和灰白色两相。为了分辨两相的具体成分,对各试样的两相进行了能谱分析,其结果列于表4。

图10

图10 在不同脉冲电流作用下定向凝固TiAl合金的SEM照片

Fig.10 SEM images of directionally solidified TiAl at current intensities (a) 0 mA/mm²; (b) 17.6 mA/mm²; (c) 35.3 mA/mm²; (d) 52.9 mA/mm²

表4 在不同脉冲电流作用下TiAl基合金的能谱分析结果

Table 4 EDS analysis of TiAl based alloy with different pulse current

Sample	Analysis location	Ti	Al	Cr	Nb
0 mA/mm²	1	45.87	48.88	2.58	2.67
0 mA/mm²	2	51.80	35.07	10.2	2.93
17.6 mA/mm²	1	48.37	46.16	2.71	2.76
17.6 mA/mm²	2	49.69	37.16	10.27	2.88
35.3 mA/mm²	1	47.49	46.74	2.82	2.95
35.3 mA/mm²	2	51.54	35.85	9.51	3.10
52.9 mA/mm²	1	46.21	47.71	2.92	3.16
52.9 mA/mm²	2	52.26	35.25	9.47	3.02

EDS统计结果表明,带状组织中黑色相Al元素的含量高,而灰白色相Al元素含量较低,Cr元素含量高。综合XRD衍射图谱和原材料的二元TiAl合金当量成分,虽然在XRD衍射谱中有α₂相的衍射峰,但是在EDS中很难分辨。其原因是,α₂相含量较低或其分布介于γ与B2相间,衍射峰不明显而难以分辨。带状组织中的黑色组织为富Al 的γ-TiAl相,灰白色组织为B2相。EDS结果表明,带状的黑色相Al含量较高,为γ-TiAl相。根据TiAl合金相图及其凝固的特点可以推断,长条状组织之间主要是B2相与γ相相互交叠的片层组织。

多数TiAl基合金的片层都为γ/α₂片层,但是Ti-45.5Al-4Cr-2.5Nb却是γ/B2片层,是此成分TiAl合金的凝固路径与固态相变过程造成的。即Ti-45.5Al-4Cr-2.5Nb为富Cr相TiAl合金,由Ti-Al-Cr合金相图^[36]。由此可以推断,其平衡凝固过程为L→β+L→β→α+β→α→α+γ→α+γ+B2→α₂+γ+B2,即随着熔体温度的降低首在液相中析出初生β相,随后剩余液相转变为单相β,随着温度的进一步降低β相析出α相,随着温度的继续降低γ相从α相中析出。γ相Al的含量较高和Cr含量较低,使其周围的Al含量低、Cr含量高,恰恰对应EDS结果B2相的特征。随着γ相的析出和长大,Al含量的降低Cr含量的提高还促进α相直接转变为B2相,最终形成了γ相与B2相的片层组织。

2.7 脉冲电流作用下TiAl基合金的力学性能的影响

材料的强度与其组织密切相关,材料的晶粒尺寸越小其强度越高^[37]。图11给出了脉冲电流不同的TiAl基合金的拉伸曲线,具体性能列于表5。可以看出,随着脉冲电流密度的增大TiAl基合金的抗拉强度先提高后降低,延伸率提高。

图11

图11 在不同脉冲电流作用下TiAl基合金的室温拉伸曲线

Fig.11 Tensile curves of TiAl based alloy with different pulse current

表5 在不同脉冲电流作用下TiAl基合金的拉伸性能

Table 5 Ttensile properties of TiAl based alloy under different pulse current

Sample	Tensile strength/MPa	Elongation/%
Master alloy	260.6	0.122
0 mA/mm²	264.3	0.153
17.6 mA/mm²	304.7	0.186
35.3 mA/mm²	404.1	0.217
52.9 mA/mm²	322.7	0.289

这种定向凝固合金的抗拉强度,主要受晶粒平均径向尺寸、晶粒尺寸以及片层宽度的影响。随着电流密度的增大晶粒平均径向尺寸先减小,当电流密度增大至52.9 mA/mm²时晶粒平均径向尺寸受电流热效应和集肤效应的影响,温度梯度降低使晶粒的平均径向尺寸增大,因此其抗拉强度小幅降低。延伸率主要受片层取向与轴向夹角的影响,夹角越小其综合力学性能越好,随着电流密度的增大其夹角逐渐减小,延伸率提高,即受力方向与片层取向的夹角越小其延伸率越高。

3 结论

(1) 施加脉冲电流进行连续凝固制备定向凝固Ti-45.5Al-4Cr-2.5Nb合金并进行模拟仿真,脉冲电流使凝固和相变路径改变,凝固组织主要有带状组织中黑色富Al的γ-TiAl相,灰白色B2相以及其间的γ/B2片层组织。

(2) 脉冲电流的焦耳热效应和电磁搅拌使枝晶重熔或破碎而提高形核率,减小了枝晶的平均宽度,增至一定值的脉冲电流密度引起熔池长度增大和温度梯度减小,使晶粒的平均径向尺寸增大；脉冲电流的集肤效应使熔体的表层电流偏聚,焦耳热效应使侧向散热减小,从而使枝晶的平均偏离角减小。

(3) 脉冲电流的频率不变时细小的晶粒、密集的片层区域有利于提高试样的抗拉强度,因此随着脉冲电流密度的增大晶粒平均尺寸先减小后增大,合金试样抗拉强度先提高后降低。

(4) 施加脉冲电流制备的定向凝固TiAl基合金其延伸率主要受片层取向与轴向夹角的影响,夹角越小其综合力学性能越高。

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研究了高密度脉冲电流对A356铝合金低温熔体凝固组织的影响.实验中对680和630℃的低温熔体施加脉冲电流处理,对比试验和凝固曲线测试结果表明.低温熔体中施加脉冲电流有利于增大凝固过冷度.提高冷却速率.缩短凝固时间.细化凝固组织.同时.经脉冲电流处理后.随熔体的保温时间延长.凝固组织逐渐粗化.分析认为.低温熔体结构比凝固过程本身更能影响最终凝固组织.

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DOI [本文引用: 2]

The effect of Al content on the microstructure and mechanical properties of directionally solidified Ti-(43-48)Al-2Cr-2Nb alloy prepared by cold crucible was investigated. The results show that the directional solidification structure is mainly composed of α2-phase and γ-phase. With the increase of Al content, the α2-phase decreases and the γ-phase content increases, while the direction of microstructural lamellae changed from parallel lamellae and 45° lamellae to vertical lamellae; For the alloy with 48%Al (atomic fraction), the orientation of lamellae is perpendicular to the direction of stress, its compressive strength is high, but plasticity is low; For the alloy with 45%Al (atomic fraction), the orientation of lamellae is parallel to the direction of stress, while maintaining high strength, the room temperature elongation is also high, in other word, the comprehensive properties are good.

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研究了Al含量对冷坩埚定向凝固Ti-(43-48)Al-2Cr-2Nb合金的显微组织和力学性能的影响。结果表明,Ti-(43-48)Al-2Cr-2Nb合金的定向凝固组织主要由α<sub>2</sub>相和γ相组成,随着Al含量的提高α<sub>2</sub>相逐渐减少而γ相含量提高,且其显微组织片层由以平行片层和45°片层为主的混合片层向垂直混合片层转变；Al含量(48%,原子分数)较高的Ti-(43-48)Al-2Cr-2Nb合金其片层取向与应力方向垂直,具有较高的抗压强度和较低的塑性；Al含量(45%,原子分数)适中的合金其片层取向与应力方向平行,具有较高的强度、室温延伸率和综合性能。

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