Progress in Research on Materials Under Microgravity in China

Research on materials under microgravity in China began in the 1980s, sparked by Prof. Lanying Lin (academician of CAS), Prof. Xiji Wang (academician of CAS), Prof. Guirong Min (academician of CAS), and Prof. Huabao Lin (academician of CAS), and others. The first semiconductor crystal, first optical crystal, and first alloys were grown in space on board a recoverable satellite in 1987. Since then, microgravity materials science became a new scientific branch in China.Scientific and technical activities on space crystal growth and solidification are carried out through two major programs: ground-based studies and orbital experiments. The main results obtained during 2001-2003 are reported below.     

金属材料快速凝固激光加工与成形

报道近年来在先进金属材料快速凝固激光表面改性、金属间化合物高温耐磨耐蚀涂层新材料快速凝固激光熔覆制备技术、钛合金及高温合金等高性能金属零件快速凝固激光成形技术等领域的研究进展, 主要内容包括:钛合金激光表面合金化及激光熔覆表面改性、激光熔覆高温耐磨耐蚀多功能金属间化合物涂层、小面相非平衡凝固液固界面结构及生长机制、钛合金及高温合金高性能零部件激光快速成形、难熔高活性金属材料及定向生长柱状晶钛合金激光约束熔铸成形技术.      

Progress of microgravity material research during the period of 2007—2009

Orbital experimental researches on crystal growth of Mn-doped GaSb and Bi2Se0.21Te2.79 are briefly summarized. The space experiments were completed in September of 2007 on broad the Foton-M3 satellite of Russia. Ground-based researches on the solidification behaviors of Al-Al3Ni, Al-Al2Cu, Ag-Cu eutectic, Al-Pb monotectic and Cu-Co peritectic alloys in a 50-meter-high drop tube were investigated. New experimental results on the ultrasonic field and the temperature recycling induced to chiral symmetry breaking of NaClO3 crystal also were reported in the present paper.      

Gravitational influence on binary alloy melt equilibria

Micro- and macrosegregation, the nonuniform redistribution of solute during solidification, are common to both casting and welding, processes of fundamental importance in materials engineering. In multicomponent crystal growth where solid/liquid density differences are appreciable gravity-induced separation can lead to significant spatial variations in resulting ingot composition. In fact, this phenomenon is also operative in liquid/liquid systems such as monotectic alloys exhibiting a liquid miscibility gap, where buoyancy-driven flows can result in a sometimes unwanted separation of phases upon cooling through the miscibility dome.     

微重力与电磁场对凝固过程的影响

欧洲空间局两项材料领域课题MICAST和CETSOL均涉及微重力科学相关研究.通过等轴晶柱状晶转变（CET）分析了微重力环境对材料凝固过程的影响;介绍了一项在地面施加行波电磁场的标准（Benchmark）凝固实验,其温度场演化及微观凝固组织和浓度偏析均展示了磁控受迫对流对金属材料凝固过程的显著作用.      

Electromagnetic containerless undercooling facility and experiments for the shuttle

An electromagnetic furnace is being prepared for flights aboard the space shuttle. This apparatus is capable of melting metals and alloys up to 1400°C melting point by induction heating with subsequent solidification of the freely levitated melt without contact with any container. The solidification can be carried out with greatly reduced fields resulting in minimal heating and stirring of the free melt. Sequential specimens can be processed during flight. Several experiments are planned for a series of flights, beginning in 1985 with an undercooling experiment on NiSn alloys. These will be interspersed with detailed studies of fluid flow caused by low and high field levels in order to quantify the corresponding effect upon the solidification process.     

Space Materials Science in China: I. Experiment Studies under Microgravity

The virtual absence of gravity-dependent phenomena in microgravity allows an in-depth understanding of fundamental events that are normally obscured and therefore are difficult to study quantitatively on Earth. Of particular interest is that the low-gravity environment aboard space provides a unique platform to synthesize alloys of semiconductors with homogeneous composition distributions, on both the macroscopic and microscopic scales, due to the much reduced buoyancy-driven convection. On the other hand, the easy realization of detached solidification in microgravity suppresses the formation of defects such as dislocations and twins, and thereby the crystallographic perfection is greatly increased. Moreover, the microgravity condition offers the possibilities to elucidate the liquid/solid interfacial structures, as well as clarify the microstructure evolution path of the metal alloys (or composites) during the solidification process. Motivated by these facts, growths of compound semiconductors and metal alloys were carried out under microgravity by using the drop tube, or on the scientific platforms of Tiangong-2 and SJ-10. The following illustrates the main results.      

Solidification and Crystal Growth on the SJ-10 Recoverable Scientific Experiment Satellite

The low-gravity environment aboard the space provides a unique platform for understanding crystal-growth-related phenomena that are masked by gravity on the Earth and for exploring new crystal growth techniques. We have characterized the wetting behavior of metal alloys and carried out melt growth of compound semiconductors under the support of materials science program in the SJ-10 recoverable satellite. We found that interfacial reaction plays a significant role in the interfacial evolution of Sn-based alloys. Detached growth of InAsSb was realized under microgravity, whereas during the terrestrial experiment the crystal and the crucible wall contact with each other. Moreover, the suppression of buoyancy-driven convection results in a more uniform composition distribution in the InGaSb and Bi₂Te₃-based semiconductor alloys.      

A review of long drop tubes as a supplement/alternative to space experiments

The 100 meter high drop tube at the Marshall Space Flight Center has proven to be a viable facility for studies of containerless solidification. Advantages are that experiments are inexpensive and large numbers of specimens can be processed rapidly. It would not be unusual to run ten specimens in a day. Another significant advantage is that the undercooling behavior can be followed with sufficient sensitivity to easily detect the onset of recalescence and subsequent events.Disadvantages are the restrictions on specimen sizes and types of alloys that can be run in a microgravity environment. Practical specimen sizes range between 50 mg and 500 mg depending on the type of furnace being used. Refractory alloys can be processed in a vacuum (about 10^?5 torr) and therefore at microgravity. Non-refractory alloys demand an atmosphere (about 200 torr) to obtain appreciable undercooling before impact at the bottom of the tube. Under these conditions significant g forces result.Because of the present limitations of the 100 meter drop tube, the most definitive work has been done on niobium based alloys. Large amounts of undercooling have been observed routinely and the effects of undercooling on microstructure have been characterized in detail. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy have been used to determine types of phases, amounts of phases, and compositions of phases. It is clear, as would be expected, that the results bear some resemblance to rapid solidification processing by quenching. However, there are dissimilarities due to the uniqueness of solidification by deep undercooling without quenching in long drop tubes and accompanying recalescence effects.     

The influence of microgravity on the growth of metals and alloys—The project of Czechoslovak experiment

The crystal growth under near-zero gravity conditions may lead to materials of better crystalline and compositional perfection [1]. Unidirectional solidification of metals is a part of Czechoslovak programme on space research within the framework of Interkosmos [2]. On the model-like systems of metals grown in the space we want to study the effect of foreign atoms on the surface tension and the lattice defects density. The objectives of our ground-based and space experiments are discussed.     

Mass transport in the near vicinity of solidification fronts under conditions of microgravity

Cellular solidification has been known and discussed since a long time. The appearance of cellular and dendritic microstructure closely resembles Benard cells known from fluid physics. Similar generation mechanismus may possibly be assumed. Both g-dependent and g-independent convective phenomena may probably be linked to the occurrence of instabilities at solidification fronts. It is to be expected that normal freezing of model alloys (advantageously such with no volume change at the freezing point) in a defined temperature gradient (gradient furnace) and quenching them may help to quantity g-influence on solidification.     

Eutectic solidification of Al-Cu alloys influenced by convection

Directional solidification of eutectic Al-Cu alloys has been studied to demonstrate the influence of both impurity concentration and convective conditions. The λ/R relations show that higher iron concentration and intensive stirring coarsen the microstructure. The formation and growth of new iron-containing phases at the solidification front restrict impurity influence. Kinetic data follow a dependence λ.Rⁿ=const with n-value close to 0,5.     

Experimental study of morphological and convective instabilities: The MEPHISTO space program

It is now well established that the morphological instability, i.e. the transition during solidification from a planar L/S interface to a cellular one, is strongly influenced by convection. The most recent theories on this topic, which are very advanced, suffer from the lack of experimental tests because uncontrolled convective effects cannot be avoid on the ground. Moreover the check of all the pertinent solidification parameters are not controlled in the same time or measured in real time. After a review of the main 1g experiments and their own limitations, we describe a new apparatus (MEPHISTO) which allows :

• •in-situ measurements of the main solidification parameters and in particular the undercooling at the solidification front in real time by a non perturbative method.
• •real time supervision of convective motions influence on crystal growth. First results obtained under 1g condition are presented :
  • ◦hydrodynamic scaling laws testing,
  • ◦instabilities detection,
  • ◦transient phenomena.

Main space results are then anticipated including the role of g-jitters.     

泡沫铜/低熔点合金复合相变材料凝固放热分析

为研究泡沫铜/低熔点合金(LMPA)复合相变材料在间歇放热工作环境下恢复至初始状态的能力及不同孔隙率泡沫铜的添加对其凝固放热过程的影响，通过数值模拟对比分析了47合金、正二十三烷与泡沫铜复合前后的凝固放热过程，并调节泡沫铜/47合金复合材料孔隙率计算模拟芯片温度在凝固放热过程中温度随时间变化曲线。结果表明:泡沫铜的添加对2类材料凝固过程均有促进作用，模拟芯片恢复至目标温度所需时间分别被缩短6.6%和47.7%。因体积潜热值的差距，泡沫铜/47合金凝固时需放出更多热量，恢复至目标温度的时间较长，是正二十三烷复合相变材料的1.52倍。随着孔隙率的增大，复合相变材料恢复至室温状态所用时长变化不大，考虑到孔隙率对相变热控过程中的影响，实际使用时应综合考虑。      

A space instrument for fundamental materials science problems: The MEPHISTO program

Keeping planar the L/S interface during solidification and controling heat and mass transfers in the liquid phase are 2 important constraints which need to be satisfied to grow suitable single crystals. In the first part of this paper we describe performances of the MEPHISTO instrument which allows :

— On line in-situ measurements of the real undercooling of a solidification front by a non-perturbative thermoelectric method.

— On line supervision of convective motions influence on crystal growth. In the second part, we present the first main results obtained during the ground testing of the space mock-up and we quantify the scientific results accuracy concerning both the onset of the morphological stability and the interface response to unsteady perturbations.     

热冲击作用下钨钒合金的表面开裂及熔化行为

本文研究了机械合金化+热压（HP）烧结制备的钨钒（W-V）合金在热冲击作用下的表面损伤行为。以合金中钒的质量分数作为变量,探究钒质量分数的变化（1%~10%）对钨钒合金抗热冲击性能的影响。利用光学显微镜、扫描电镜、能谱仪、纳米压痕仪等多种测试方式,分析表征了HP烧结钨钒合金的组织结构特征及其经过热冲击测试后的表面开裂及熔化行为特征。结果表明:在1 800℃、20 MPa的压力条件下保温2 h可以制备出致密度高、合金化程度高的钨钒合金,且随着钒质量分数的增加,合金样品的致密度有所提高;合金样品中钨基体硬度大于富钒相,在高能电子束模拟的国际热核聚变实验堆（ITER）边界局域模（ELMs）热冲击作用下,钨基体对裂纹扩展的阻碍作用明显强于富钒相;随着钒质量分数的升高,合金的开裂阈值和熔化阈值均降低,本文对相关机理进行了讨论。      

THE DGW-I FURNACE A MATERIALS PROCESSING FACILITY IN "SZ-2" SPACECRAFT

The DGW-I is a new material processing facility developed in China, which was firstly carried into orbit in November 1999 by the SZ-1 spacecraft and then in January 2001 carried by SZ-2 into space again, and successfully processed 6 samples of materials, including 3 samples of alloys, 2 of semiconductors and 1 sample of oxide crystal.     

Direct observation of processes in a solidifying dispersion

The evolution of a dispersion under the action of temperature gradients and solidification was followed optically in a transparent molten salt (CsCl) with inclusions of Pb-droplets and gas bubbles. This system is believed to model a solidifying metallic alloy. Rejection of Pb-particles by the solidification front was observed, while large gas bubbles were incorporated. Thermocapillary convection at the gas bubbles considerably distorted the temperature field and even caused local remelting. Marangoni migration of bubbles was not observed, contrary to expectations.     

空间科学实验通用地面检测系统研制

分析了地面检测设备在空间有效载荷研制过程中的作用,提出一种用于对多种空间微重力科学实验设备（载荷）进行地面测试的通用地面检测设备设计方法.通过载荷特性分析,对载荷中的控制对象进行分类.针对不同控制对象使用不同的操作进行控制.将载荷实验过程分解为一系列固定时刻执行的操作,通过配置静态配置表、动作配置表和动态配置表,实现对载荷实验过程的控制.地面检测设备由计算机、电源和RS422通信接口构成.针对不同载荷,使用规格一致的电缆和通信接口,保证地面监测设备的通用性.地面检测设备配合多功能炉、骨髓培养箱、辐射基因箱、煤燃烧箱、蒸发对流箱、导线特性箱及胶体材料箱7台载荷开展研制工作,在各载荷试验参数确定、空间试验流程确定、设备性能测试、环境模拟实验、电磁兼容实验、地面匹配实验以及载荷设备验收等过程中发挥了重要作用.      

A new module for the CSK-1 crystallizer: Thermographic DTA probe for the study of metastable phase equilibria under microgravity conditions

A special thermographic DTA probe was designed to complet the CSK-1 crystallizer for materials research in space. This equipment is desirable for a deeper understanding of the nature of the solidification under gravity-less conditions. It is shown that the new DTA probe in combination with a specially adapted measuring device ARP (GDR) is suitable for this purpose and the results of measurements are comparable with those obtained with commercial apparatures.