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采用静电悬浮技术实现了液态Nb81.7Si17.3Hf合金在不同过冷度下的快速凝固, 实验最大过冷度达404 K (0.19TL). 实验测定出其液态超过冷临界过冷度为527 K (0.24TL). 当液相过冷度超过194 K时, 合金相组成由(Nb)和αNb5Si3转变为(Nb)和Nb3Si. 若过冷度小于此临界值, 凝固过程中(Nb)相优先形核生长, 剩余液相形成了(Nb)+αNb5Si3层片共晶. 高速摄影实验发现初生相的生长速度可达89.4 mm/s. 在194 K以上深过冷条件下, (Nb)初生相将消失, (Nb)+Nb3Si非规则共晶在过冷熔体中直接生长. 非规则共晶生长速度随过冷度呈幂函数增大, 最高可达115.9 mm/s. 由于共晶间距减小和(Nb)相体积分数增大, 在194 K过冷度下合金断裂韧性可达21.9 MPa·m1/2, 提升至小过冷条件的3.4倍.The phase selection mechanism and eutectic growth kinetics of Nb81.7Si17.3Hf alloy are investigated by electrostatic levitation technique. The maximum undercooling of this alloy reaches 404 K (0.19TL). By analyzing the cooling curves, its hypercooling limit is obtained to be 527 K (0.24TL). A critical undercooling of 194 K is determined for the transition of solidification path. Below this undercooling threshold, (Nb) phase firstly nucleates and grows into primary dendrites, resulting in the enrichment of Si and Hf in the residual melt, which is conducive to the formation of the (Nb)+αNb5Si3 eutectics. Therefore, (Nb)+αNb5Si3 lamellar eutectics form in interdendritic space. With the increase of undercooling, the growth velocity of primary (Nb) dendritic follows a power function, while the eutectic growth velocity increases slowly. The maximum values of (Nb) dendritic reaches 89.4 mm/s. A modified LKT/BCT model is used to calculate the growth velocity of (Nb) dendrites. The results are in good agreement with the experimental values, indicating that after the LKT model is modified slightly, it can be used to describe the rapid dendrite growth behavior of the (Nb) phase in the Nb81.7Si17.3Hf alloy melt. Meanwhile, the lamellar spacing of (Nb)+αNb5Si3 eutectics notably decreases to 360 nm at 194 K undercooling. Above the critical threshold, the primary (Nb) dendrites disappear, whereas (Nb) phase and Nb3Si phase nucleate independently in the undercooled liquid and grow into anomalous eutectics. The growth velocity of anomalous eutectic exhibits a power function relationship with the increase of undercooling, with a maximum value of 115.9 mm/s. The interphase spacing of (Nb)+Nb3Si anomalous eutectics is larger than that of (Nb)+αNb5Si3 lamellar eutectics. Owing to the formation of nanosized eutectics and the increase of volume fraction of (Nb) phase, the alloy fracture toughness at 194 K reaches 21.9 MPa·m1/2, which is 3.4 times as large as that under small undercooling condition.
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Keywords:
- electrostatic levitation /
- rapid solidification /
- Nb-Si based alloy /
- microstructure control /
- mechanical properties
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图 1 Nb81.7Si17.3Hf合金的相组成分析 (a) 合金成分选择 (原子百分比); (b) XRD图谱
Fig. 1. Phase constitution analysis of Nb81.7Si17.3Hf alloy: (a) Selection of alloy composition (atomic percent); (b) XRD patterns.
图 2 静电悬浮条件下Nb81.7Si17.3Hf合金的快速凝固过程 (a) 最大过冷度404 K时温度曲线; (b) 结晶平台持续时间与过冷度的关系
Fig. 2. Rapid solidification process of Nb81.7Si17.3Hf alloy under ESL condition: (a) Temperature curve at the maximum undercooling of 404 K; (b) thermal arrest time versus undercooling.
图 3 Nb81.7Si17.3Hf合金再辉过程中原位高速摄影图像 (a) ΔT = 161 K; (b) ΔT = 271 K
Fig. 3. In situ high-speed photography observation of recalescence processes for Nb81.7Si17.3Hf alloy: (a) ΔT = 161 K; (b) ΔT = 271 K.
图 4 枝晶与共晶生长速度随过冷度变化规律 (a) 初生(Nb)枝晶; (b) 共晶组织
Fig. 4. Dendrite and eutectic growth velocities versus undercooling: (a) Primary (Nb) dendrite; (b) eutectic.
图 5 Nb81.7Si17.3Hf合金凝固组织演变规律 (a) 母合金; (b) ΔT = 194 K; (c) ΔT = 219 K; (d) ΔT = 404 K
Fig. 5. Solidification microstructures of Nb81.7Si17.3Hf alloy at different undercoolings: (a) Master alloy; (b) ΔT = 194 K; (c) ΔT = 219 K; (d) ΔT = 404 K.
图 6 Nb81.7Si17.3Hf合金凝固组织特征随过冷度变化规律 (a) (Nb)相体积分数; (b) 共晶间距
Fig. 6. Microstructure features of Nb81.7Si17.3Hf versus undercooling: (a) Volume fraction of (Nb) phase; (b) eutectic spacings.
图 7 快速凝固Nb81.7Si17.3Hf合金微观力学性能 (a) 裂纹长度l和压痕半宽a与过冷度的关系, 插图(a1) 维氏硬度仪在样品表面产生的压痕形貌; (b)—(d) 合金显微硬度、弹性模量和断裂韧性与过冷度的关系
Fig. 7. Micromechanical properties of rapidly solidified Nb81.7Si17.3Hf alloy at different undercoolings: (a) Length of the crack a and the half width of indentation l versus undercooling, where the inset (a1) is indentation morphology produced by Vickers hardness tester on sample surface; (b)–(d) Vickers hardness, elastic modulus and fracture toughness versus undercooling.
表 1 计算采用的Nb81.7Si17.3Hf合金物理参数
Table 1. Physical parameters of Nb81.7Si17.3Hf alloy
参数 符号/单位 数值 文献 液相线温度 TL/K 2172 This work 熔化焓 ΔHm/(J·mol–1) 32943 [24] 熔体比热 CPL/(J·mol–1·K–1) 31.88 [24] (Nb)相界面能 σ0/(J·m–2) 0.28 [25] 热扩散系数 Dt/(m2·s–1) 3.1×10–5 [22] 液相线斜率(Si) m1/(K·%–1) 94.32 [12] 液相线斜率(Hf) m2/(K·%–1) 5.51 [12] 平衡分配系数(Si) k1 0.17 [12] 平衡分配系数(Hf) k2 0.52 [12] 溶质扩散系数(Si) D1/(m2·s–1) 1.8×10–9 [24] 溶质扩散系数(Hf) D2/(m2·s–1) 1.2×10–9 [24] 溶质扩散特征长度 a0/m 2.0×10–9 [24] 
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[1] Wang J W, Chen H H, Zhang Z G, Wang B, Ma H T, Song M Q, Zhai J H, Ding L F 2021 J. Appl. Phys. 130 135104
Google Scholar
[2] Tsakiropoulos P 2022 Prog. Mater. Sci. 123 100714
Google Scholar
[3] Wang Q, Wang H P 2023 J. Phys. Condes. Matter 35 105401
Google Scholar
[4] Bewlay B P, Jackson M R, Zhao J C, Subramanian P R, Mendiratta M G, Lewandowski J J 2003 MRS Bull. 28 646
Google Scholar
[5] Guo Y L, Liang Y J, Lu W J, Jia L N, Li Z M, Peng H, Zhang H 2019 Appl. Surf. Sci. 486 22
Google Scholar
[6] Vellios N, Tsakiropoulos P 2007 Intermetallics 15 1518
Google Scholar
[7] Mendiratta M G, Dimiduk D M 1991 Scr. Metall. Mater. 25 237
Google Scholar
[8] Chen Y, Kolmogorov A N, Pettifor D G, Shang J X, Zhang Y 2010 Phys. Rev. B 82 184104
Google Scholar
[9] Sala K, Morankar S, Mitra R 2021 Metall. Mater. Trans. 52 1185
Google Scholar
[10] Chen Y, Shang J X, Zhang Y 2007 Phys. Rev. B 76 184204
Google Scholar
[11] Grammenos I, Tsakiropoulos P 2010 Intermetallics 18 242
Google Scholar
[12] Yang Y, Chang Y A, Zhao J C, Bewlay B P 2003 Intermetallics 11 407
Google Scholar
[13] Becker S, Devijver E, Molinier R, Jakse N 2020 Phys. Rev. B 102 104205
Google Scholar
[14] 阮莹, 胡亮, 闫娜, 解文军, 魏炳波 2020 中国科学: 技术科学 50 603
Google Scholar
Ruan Y, Hu L, Yan N, Xie W J, Wei B 2020 Sci. Sin. Tech. 50 603
Google Scholar
[15] Warrender J M, Mathews J, Recht D, Smith M, Gradecak S, Aziz M J 2014 J. Appl. Phys. 115 163516
Google Scholar
[16] 魏绍楼, 黄陆军, 常健, 杨尚京, 耿林 2016 物理学报 65 096101
Google Scholar
Wei S L, Huang L J, Chang J, Yang S J, Geng L 2016 Acta. Phys. Sin. 65 096101
Google Scholar
[17] Clopet C R, Cochrane R F, Mullis A M 2013 Appl. Phys. Lett. 102 031906
Google Scholar
[18] 黄起森, 刘礼, 韦修勋, 李金富 2012 物理学报 61 166401
Google Scholar
Huang Q S, Liu L, Wei X X, Li J F 2012 Acta. Phys. Sin. 61 166401
Google Scholar
[19] Bertero G A, Hofmeister W H, Robinson M B, Bayuzick R J 1991 Metall Trans. A 22 2713
Google Scholar
[20] Wang Q, Zheng C H, Li M X, Hu L, Wang H P, Wei B 2023 Appl. Phys. Lett. 122 234102
Google Scholar
[21] Wang H P, Liao H, Hu L, Zheng C H, Chang J, Liu D N, Li M X, Zhao J F, Xie W J, Wei. B 2024 Adv. Mater. 36 2313162
Google Scholar
[22] Li M X, Wang H P, Lin M J, Zheng C H, Wei B 2022 Acta. Mater. 237 118157
Google Scholar
[23] Mohan D, Phanikumar G 2019 Phil. Trans. R. Soc. A 377 20180208.
Google Scholar
[24] Gale W, Totemeier T C 2004 Smithells Metals Reference Book (8th Ed.) (Amsterdam: Elsevier Butterworth-Heinemann Publications) p8-1
[25] Vinet B, Magnusson L, Fredriksson H, Desre P J 2002 J. Colloid. Sci. 255 363
Google Scholar
[26] Wang H P, Liao H, Chang J, Liu D N, Wang Q, Li M X, Zheng C H, Hu L, Wei B 2024 Mater. Today 75 386
Google Scholar
[27] Sekido N, Kimura Y, Miura S, Wei F G, Mishima Y 2006 J. Alloys. Compd. 425 223
Google Scholar
[28] Laugier M T 1987 J. Mater. Sci. Lett. 6 897
Google Scholar
[29] Chen D Z, Wang Q, Chen R R, Wang S, Su Y Q, Fu H Z 2022 J. Alloys. Compd. 928 167124
Google Scholar
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