合金在长落微重力与重力环境下的凝固行为研究

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	合金在长落微重力与重力环境下的凝固行为研究
	封少波
学位类型	博士
导师	李殿中 ; 罗兴宏
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料加工工程
关键词	微重力长落管凝固 Al-cu合金高温合金 Microgravity Long Drop Tube Solidification Al-cu Alloy Superalloys
摘要	"摘　要绝大多数金属材料在其成为最终产品前都要经历一次甚至多次凝固过程，在凝固过程中形成的微观组织和成分分布对合金的使用性能具有重要影响。因此，控制凝固过程中的显微组织形成是人们长期追求的目标。在金属的凝固中形核和长大过程都因与流体相关联而直接或间接地受到重力的作用，这阻碍了人们对凝固过程物理本质的进一步认识。近年来，利用微重力环境来研究重力在凝固过程中的作用成为一条新的途径。长落管是地面上模拟空间微重力环境的主要手段之一，但合金熔体在长落管的短时微重力环境下难以凝固，这极大地限制了其在微重力材料科学研究方面的应用。因此，本文首先设计一种新的实验方法解决了上述问题，并利用该方法开展了Al-Cu合金及SRR99单晶在长落管的微重力环境与重力环境下的凝固实验，对比分析了重力对凝固组织和成分偏析的影响。此外，还研究了偏析对DZ483高温合金糊状区内液相密度的影响。本文取得了以下主要研究成果： 1. 总结了传统长落管微重力材料实验方法的特点，分析认为熔体在下落过程中难以实现凝固的主要原因是缺乏有效形核质点和良好的冷却途径。为此提出了利用母材作为籽晶来克服这些问题，并设计了新的实验方法——籽晶法。利用几种典型金属及合金对新方法进行了实验验证。结果表明，籽晶不仅为熔体提供了凝固生长基底，还可作为冷端促进熔体冷却，从而可充分利用长落管的短时微重力环境。 2. 利用籽晶法开展了Al-4.5wt%Cu合金在微重力与重力环境下的凝固试验。结果表明：重熔部位的凝固组织由从籽晶外延生长的柱状枝晶及等轴晶粒构成。与重力试样相比，微重力试样外延生长枝晶一次及二次臂间距减小。此外，微重力试样中等轴晶粒不仅分布在试样顶部，还分布在外延生长区域内及靠近坩埚壁处，而重力试样中等轴晶粒集中分布在试样顶部。分析认为，宏观浮力对流使溶质原子在中心区域的外延生长枝晶凝固前沿聚集，从而导致重力试样一次枝晶间距增加。此外，枝晶间的微观浮力对流也增大了一次枝晶间距。宏观浮力对流和微观浮力对流还促进了二次枝晶粗化，导致二次枝晶间距增大。重力导致的对流和浮力对等轴晶粒的分布也存在影响。 3. Al-20wt%Cu合金在微重力与重力环境下的凝固研究表明，合金重熔部分的凝固组织主要为等轴晶粒。微重力试样中等轴晶粒尺寸细小且较均匀，枝晶形貌较为发达，二次枝晶臂细小。而重力试样中等轴晶粒尺寸较大但不均匀，枝晶形貌较破碎，二次枝晶臂粗大。分析认为，等轴晶主要来源于未熔化的枝晶碎片和异质形核。宏观浮力对流造成部分晶粒长大速度减慢，使得晶粒尺寸不均。此外，枝晶间的微观浮力对流加速了二次枝晶臂的粗化，甚至可导致枝晶破碎。 4. 利用等温凝固结合水淬的方法研究了偏析对DZ483镍基高温合金糊状区内液相密度的影响，并分析了不同合金元素的偏析对液相密度变化的贡献。结果表明：液相密度随凝固的进行而减小，合金糊状区内出现密度反转；Mo、Ta的偏析导致液相密度增大，而Ti和W的偏析导致液相密度减小。合金元素的偏析对液相密度影响由大到小的顺序为: Ti＞Ta＞W＞Cr＞Mo＞Al＞Co。碳化物的析出消耗了大量的Ta和Ti，使得液相密度在1325至1315℃之间随温度的降低而有所增加。 5. SRR99镍基单晶高温合金在微重力与重力环境下的凝固研究表明，合金重熔部分的凝固组织主要为从籽晶外延生长的柱状枝晶和从器壁形核生长的枝晶。与重力试样相比，微重力试样外延生长枝晶一次及二次枝晶间距均变小。此外，微重力试样和重力试样中合金元素的偏析行为未发生明显变化，但偏析程度却有所不同。与重力试样相比，微重力试样中Al、Ti和Co的偏析更为显著，而W的偏析有所减轻，但Ta的偏析未发生变化。Al、Ti的偏析程度不同导致微重力试样枝晶干中g¢相尺寸较大，分布不均，而重力试样中g¢相尺寸较小，分布较均匀。分析认为，宏观浮力对流使合金的一次枝晶间距增大，但微观浮力对流导致一次枝晶间距减小，两者综合作用导致一次枝晶间距有所增加。此外，宏观和微观浮力对流还促进了二次枝晶粗化，导致二次枝晶间距增大。微观浮力对流还影响了一些合金元素的偏析程度，从而导致枝晶干析出的g¢相有所变化。关键词：微重力，长落管，凝固，Al-Cu合金，高温合金"
其他摘要	"ABSTRACT Solidification behavior of alloys under microgravity and normal gravity conditions in long drop tube Shaobo Feng (Materials Processing Engineering) Supervised by Prof. Dianzhong Li and Prof. Xinghong Luo In practice the manufacture of almost every metal product involves solidification at some stage. The microstructures formed and redistributions of alloy compositions during solidification processing have a great effect on the properties of metals and alloys. Therefore, good control of the solidification microstructures is of utmost importance. Since nucleation and growth during metals solidification are associated with fluid, gravity has some direct or indirect influence on these processes. The effects of gravity hinder further understanding of the nature of solidification. In recent years, utilization of microgravity enviroment has become a promising way to study the effect of gravity on solidification. A long drop tube is one of the effective methods to simulate the microgravity conditions on the ground. However, molten alloy droplets are difficult to solidify during the short duration of microgravity conditions in the long drop tube, which greatly limits its application to microgravity materials science research. Thereby, in this work a new experimental method was designed to solve these problems, and by means of this new method, the solidification behaviors of Al-Cu alloys and SRR99 single crystal alloys were studied under microgravity and normal gravity conditions in the long drop tube. By comparing solidification microstructures and compositions distribution of alloys under these two situations, the influence of gravity is investigated. In addition, the effect of microsegregation on the liquid density variation in DZ483 Ni-based superalloys was investigated. The main results are summarized as follows: 1. The characteristics of traditional experimental methods of the long drop tube for microgravity materials science were summarized. The main reasons that droplets cannot be solidified during free falling were analyzed, which are lack of effective nucleation sites and good cooling method. Therefore, an idea was proposed that utilization of base materials acting as a seed crystal to overcome these problems. Thus, a new experimental method, seed crystal method, of long drop tube for microgravity materials science was designed. Solidification of some typical metals and alloys were carried out to test the new method. Results show that seed crystal method can not only supply seed base for solidification, but also provide cooling part. Therefore, the short-time microgravity condition of long drop tube is thoroughly exploited. 2. By means of the seed crystal method, solidification of Al-4.5wt%Cu alloy was carried out under microgravity and normal gravity conditions. Results show that solidification microstructures of the molten part of both microgravity samples (mg samples) and unit gravity samples (1g samples) are mainly consisted of columnar dendrites epitaxially growing from master alloys, and partly of equiaxed grains. Compared to the 1g samples, primary and secondary arm spacings of epitaxially growing columnar dendrites are smaller in the mg samples. Besides, in the mg samples equiaxed grains are distributed not only at the top of the sample, but also in the area of epitaxially growing columnar dendrites and in the area near the crucible, while in the 1g samples equiaxed grains mostly appear at the top part of the sample. It is suggested that macroscopic buoyancy convection causes the accumulation of Cu solute at the front of epitaxially growing columnar dendrites in the central part of the 1g samples, which leads to an increase of the primary arm spacing. Moreover, microscopic thermosolutal convection has similar influence on the primary arm spacing as macroscopic buoyancy convection does. In addition, macroscopic and microscopic buoyancy convections enhance the coarsening rate, and therefore increase the secondary arm spacing. Additionaly, macroscopic buoyancy convection and buoyancy force influence the distribution of equiaxed grains in the 1g samples. 3. Solidification of Al-20wt%Cu alloy was carried out under microgravity and normal gravity conditions. Results show that solidification microstructures of the molten part mainly consist of equiaxed grains. In the mg samples the equiaxed grains are smaller, and grain sizes are more homogeneous. Additionally, dendrites are well-developed and have smaller secondary dendrite arms in the mg samples. While in the 1g samples the equiaxed grains are larger but less homogeneous. Moreover, the morphologies of dendrites are broken and secondary arms are coarsened in the 1g samples. It is suggested that the origin of equiaxed grains are unmelted dendrite fragments and heterogeneous nucleation. Macroscopic buoyancy convection slows down the growth of some grains, which is responsible for the inhomogeneous grain sizes. On the other hand, microscopic buoyancy convection increases the coarsening rate of secondary arm, and their tendency of fragemnt. 4. The effect of microsegregation on the liquid density variation in DZ483 Ni-based superalloys was investigated by isothermal solidification together with liquid quenching method. Besides, contribution of each element to the variation of liquid density was analyzed. Results show that liquid density in the mushy zone decreases as the proceeding of alloy solidification, and a density inversion appears in the mushy zone. The segregations of Mo and Ta lead to the increase of density, but the segregations of Ti and W present an opposite effect. The sequence of contribution of each element to the variation of the liquid density is Ti＞Ta＞W＞Cr＞Mo＞Al＞Co. The formation of MC-type carbides consumes an large amount of Ti and Ta atoms, resulting in the increase of liquid density between 1325 and 1315℃. 5. Solidification of SRR99 nickel base single crystal superalloy was carried out under microgravity and normal gravity conditions. Results show that solidification microstructures of the molten part consist of epitaxially growing columnar dendrites from master alloys and dendrites nucleating on the crucible wall. Compared to the 1g samples, primary and secondary arm spacings of epitaxially growing columnar dendrites are smaller in the mg samples. The segregation behaviors of elements are almost unchanged during solidification under microgravity conditions, but the extents of elements segregation are altered. In the mg samples the microsegregations levels of Al, Ti and Co elements are more serious, while the microsegregation level of W is reduced, but the microsegregation level of Ta is not altered. The differences in the microsegregation levels of Al and Ti lead to larger sizes and inhomogeneous distributions of g¢ phase precipitating in dendrite arms of the mg samples, while in the 1g samples the size of precipitated g¢ phase is smaller, and g¢ phase are distributed more homogeneously. It is suggested that in the 1g samples macroscopic buoyancy convection increases the primary arm spacing, but microscopic buoyancy convection decreases the spacing. As a result of these two effects, a little increase of primary arm spacing was observed in the 1g samples. In addition, macroscopic buoyancy convection increases the local solidification time, whereas microscopic buoyancy convection enhances the solute diffusion rate in the interdendritic fluid. Consequently the secondary arm spacing is increased. Microscopic buoyancy convection also affects the levels of some elements microsegregation and therefore influences the precipitation of g¢ phase. Key words: microgravity, long drop tube, solidification, Al-Cu alloy, superalloys"
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64484
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	封少波. 合金在长落微重力与重力环境下的凝固行为研究[D]. 北京. 中国科学院金属研究所,2012.