【作者】 许菁菁；

【导师】 王同敏；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2010， 硕士

【摘要】 同步辐射光源具有高能量、高亮度、高分辨率、单色性好等优点,其配合高读写速度及高分辨率的CCD成像系统,可实现对金属合金凝固过程的枝晶生长动态行为进行实时成像观察。本研究利用国内的北京同步辐射光源和上海同步辐射光源,对低熔点的Sn-12wt.%Bi合金的凝固进程中的枝晶生长、枝晶粗化以及在直流电场作用下的枝晶生长进行实时成像研究,成功地获得一系列丰富的等轴晶、柱状晶生长行为,如等轴晶-柱状晶转变、枝晶竞争生长、枝晶臂的断裂上浮等视频图像。通过得到的100gm厚的合金样品凝固的视频图像,解释了枝晶演变的动态过程,可以利用其对枝晶生长的动力学行为进行定量分析。实验发现不同冷却速率和温度梯度下,发生的粗化机制不同。粗化主要发生在凝固的开始阶段,因此二次枝晶间距也决定于这个阶段。枝晶的合并主要发生在凝固的后期,而枝晶根部的重熔断裂多发生在低冷却速率下,1.5℃／min。在不同的温度梯度相同冷却速率下,从枝晶最初生成到达到最终二次枝晶间距的时间不同,但其最终的二次枝晶间距大致相同。合金枝晶生长的实时成像为验证枝晶粗化模型提供了有利的证据。利用同步辐射成像对直流电场下Sn-Bi合金凝固时的枝晶生长过程进行实时观察。施加直流电场(电流密度7-32 A／cm2)后,枝晶生长受到抑制,枝晶尖端发生钝化成圆形或平直,并且没有发现三次枝晶的生成。随着电流密度的增加,枝晶形态发生了由柱状枝晶到胞状等轴晶再到枝晶状等轴晶的变化。特别地,当电流强度增大时,一次枝晶尖端发生分裂。这些成像结果为验证或完善已有的金属合金凝固理论提供了直接的第一手实验数据。同步辐射X射线实时成像技术为研究金属合金凝固理论提供了崭新的实验手段和研究思路。更多还原

【Abstract】 Dendrite growth in metallic alloy during solidification was observed by synchrotron radiation technique, which has high energy, high brightness, good monochromaticity, worked with high read and write speed and high resolution CCD system. Dendrite growth, dendrite coarsening, and dendrite growth under DC of Sn-12wt.%Bi alloy were studied at Beijing synchrotron radiation facility and Shanghai synchrotron radiation facility with an updated synchrotron radiation imaging technique. A series of growth behavior and morphology evolution of dendrite have been in situ observed, such as columnar-to-equiaxed transition, dendrite competition, and dendrite fragmentation & floating etc. The images obtained in 100μm thick samples reveal the dynamic process of dendrite growth and allow quantitative measurement and analysis.It is shown by in situ observations that different coarsening mechanisms of secondary dendrite arm might operate under different cooling conditions or solidification stages. Coarsening mainly happened in early solidification stage realized by the competitions, and therefore, secondary dendrite arm spacing (DAS) is mainly determined in this stage. Coalescence was the dominant coarsening mechanism in late solidification stage. In particular, at lower cooling rate, e.g.1.5℃/min, dendrite fragmentation frequently occurred, and played an important role during dendrites coarsening. When alloy solidified at the same cooling rate with different temperature gradients, the time taken from the beginning of solidification to forming the final secondary arm spacing was different, but the final secondary arm spacing were almost same. The real time observations in real alloy can give direct proofs to verify the dendrite arm coarsening models.Synchrotron radiation imaging technique was used to in situ observe the dendrite growth of a solidifying Sn-Bi binary alloy under a direct current (DC) electric field. By applying a DC (7-32 A/cm2), the dendrite branching was suppressed, the dendrite tip was modified to be round or flat, and no tertiary dendrite was found. With increasing DC density, the dendrite morphology was changed from columnar dendritic to equiaxed cellular to equiaxed dendritic. In particular, the primary dendrite branched following a tip-split manner in a higher intensity DC. These results can offer the direct proofs to verify or improve the solidification theories of metallic alloy. This research opens a novel window to the study of alloy solidification and enables the unambiguous understanding of solidification processes in optically opaque, metallic alloys.更多还原