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As an important and promising experimental method of simulating the containerless state in outer space, acoustic levitation provides excellent contact-free condition for investigating solidification process. Meanwhile, the radiation pressure and acoustic streaming caused by nonlinear effects bring various kinds of novel phenomena to crystallization kinetics. In this work, high-speed charge coupled device (CCD), low-speed camera and infrared thermal imager are used simultaneously to observe the crystallization process of acoustically levitated SCN-DC transparent alloys. The undercooling ability and solidification process of alloy droplets with different aspect ratios are explored in acoustic levitation state. For hypoeutectic SCN-10%DC, eutectic SCN-23.6%DC and hypereutectic SCN-40%DC alloys, the experimental maximum undercoolings reach 22.5 K (0.07TL), 16 K (0.05TE) and 32.5 K (0.1TL) and the corresponding crystal growth velocities are 27.91, 0.21 and 0.45 mm/s, respectively. In SCN-10%DC hypoeutectic alloy, the nucleation mode of SCN dendrite changes from edge nucleation into random nucleation with the increase of undercooling. For SCN-23.6%DC eutectic alloy, when the undercooling exceeds 12.6 K, DC dendrites preferentially nucleate and grow, and then the (SCN+DC) eutecticadheres to and grows on DC dendrites. Moreover, the growth interface of DC dendrites gradually changes from sharp into smooth within SCN-40%DC hypereutectic alloy as the undercooling degree rises. The undercooling distribution curve and nucleation probability variation trend versus aspect ratio are analyzed. It is found that as the aspect ratio increases, undercooling of alloy droplet first increases, then decreases, and finally remains almost unchanged. Further analysis shows that with the increase of aspect ratio, the cooling rate will rise and thus enhance the undercooling. However, the increase in surface nucleation rate and the droplet oscillation inhibits deep undercooling of alloy droplet. Therefore, the coupled effects of cooling rate, surface nucleation rate, and droplet oscillation determine the undercooling of the alloy. In the case of SCN-40% DC hypereutectic alloy, the acoustic streaming and surface oscillation arising from acoustic field are the main factors intensifying surface nucleation.
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Keywords:
- acoustic levitation /
- SCN-DC alloy /
- nucleation /
- crystal growth
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图 1 三种不同成分液态SCN-DC合金达到的过冷度
Figure 1. Maximum undercoolings achieved by three liquid SCN-DC alloys.
图 2 超声悬浮条件下合金液滴的过冷与凝固 (a) 实验装置示意图; (b) SCN-DC合金的冷却曲线
Figure 2. Undercooling and solidification of acoustically levitated alloy: (a) Schematic diagram of experimental setup; (b) cooling curves of SCN-DC alloys.
图 3 超声悬浮条件下合金熔体的形状与受力情况 (a) 液滴上下表面受到的声辐射压; (b) 声压与液滴形状的关系
Figure 3. The acoustic pressure distribution of levitated droplet: (a) Acoustic radiation pressure on surface; (b) acoustic pressure versus droplet aspect ratio.
图 4 合金液滴过冷度随变形程度的变化关系(a) SCN-10%DC亚共晶; (b) SCN-23.6%DC共晶; (c) SCN-40%DC过共晶
Figure 4. Relationship between undercooling and aspect ratio of alloy melt: (a) SCN-10%DC hypoeutectic; (b) SCN-23.6%DC eutectic; (c) SCN-40%DC hypereutectic.
图 5 超声悬浮条件下不同变形程度合金液滴形核规律 (a) 冷却速率; (b) 形核孕育时间
Figure 5. The nucleation characteristics of various deformed alloy melt: (a) Cooling rates; (b) nucleation gestation time.
图 6 SCN-40%DC过共晶合金形核概率与过冷度的关系 (a) γ = 2.3; (b) γ = 3.2; (c) γ = 4.1
Figure 6. Relationship between undercooling and nucleation probability within hypereutectic SCN-40%DC alloy: (a) γ = 2.3; (b) γ = 3.2; (c) γ = 4.1.
图 7 合金液滴温度场与流场分布 (a) 液滴红外热像仪照片; (b) 合金液滴纵截面沿x方向上的温度分布; (c)合金熔体内外部流场
Figure 7. Temperature and flow field of alloy: (a) Infrared thermography of alloy droplet; (b) temperature distribution of liquid alloy in x orientation on the longitudinal section; (c) internal and external flow field of alloy.
图 8 SCN-DC合金液滴凝固过程 (a) SCN-10%DC亚共晶; (b) SCN-23.6%DC共晶; (c) SCN-40%DC过共晶
Figure 8. Solidification process of SCN-DC alloy melt: (a) SCN-10%DC hypoeutectic; (b) SCN-23.6%DC eutectic; (c) SCN-40%DC hypereutectic.
图 9 枝晶和共晶生长速度与过冷度的关系 (a) SCN枝晶; (b) (SCN+DC)共晶; (c) DC枝晶
Figure 9. Dendritic and eutectic growth velocities versus undercooling: (a) SCN dendritic; (b) (SCN+DC) binary eutectic; (c) DC dendritic.
表 1 声场计算所需物理参数
Table 1. Physical parameters used for calculation
参数 单位 数值 超声频率 f kHz 22 发射端振幅 A μm 15 等效半径 $ {{R}}_{\text{s}} $ $ \text{mm} $ 4.15 重力加速度 g $ \text{m/}{\text{s}}^{2} $ 9.8 介质密度 $ {\rho }_{0} $ $ \text{kg/}{\text{m}}^{3} $ 1.29 介质黏度 $ {\eta}_{0} $ $ {10}^{-5}\text{}\text{Pa∙s} $ 1.81 声速 $ {{c}}_{0} $ $ \text{m/s} $ 340 合金密度 $ {\rho }_{\text{s}} $ $ {10}^{3}\text{kg/}{\text{m}}^{3} $ 1.02 合金表面张力 $ \sigma $ $ {10}^{-2}\text{}\text{N/m} $ 3.75 合金黏度 $ {\eta}_{\text{L}} $ $ {10}^{-3}\text{}\text{Pa∙s} $ 3.22 温度T K 293 
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