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Role of hydrostatic pressure and wall effect in solidification of TC8 alloy

Role of hydrostatic pressure and wall effect in solidification of TC8 alloy www.nature.com/npjmgrav ARTICLE OPEN Role of hydrostatic pressure and wall effect in solidification of TC8 alloy 1 1 1 Xinghong Luo , Yaya Wang and Yang Li The solidification experiments of TC8 alloy under both microgravity and normal gravity were conducted using a drop tube. The solidification microstructure were found composed of fine equiaxed grains formed at early stage and bigger elongated grains formed at later stage. Between the two kinds of grains a curved transition interface was observed in 1g sample, while that in μg sample was almost flat. Generally, the amounts and aspect ratios of the grains are larger, and the grain sizes are smaller in 1g sample. Besides, no visible element macrosegregation occurred in both samples. The results suggest that the solidification velocities of the samples were rapid, and consequently the convection effect and solute transport effect caused by gravity had little influence on the solidification microstructure. Therefore, the solidification process was mainly controlled by thermal diffusion, and hydrostatic pressure and wall effect played a great role in it. npj Microgravity (2019) 5:23 ; https://doi.org/10.1038/s41526-019-0083-2 INTRODUCTION RESULTS With the continuous development of aerospace technology and Figure 1 shows the temperature vs. time of the sample tops during the experiments. Because the temperature during the falling advanced material preparation technology, people are looking process could not be monitored, only the temperature before for the perfect combination of the two technologies to serve the release of the μg sample was recorded. It can be seen that the future deep space exploration and interstellar navigation. Space curves of heating and melting phases of both samples are highly additive manufacturing technology is expected to be one of 1–3 coincident, indicating that μg sample and 1g sample have same them. With this technology, astronauts can make whatever heating history. Given that the external cooling environments of parts they need in situ, without having to spend precious the both were the same room temperature and vacuum payload resources to carry them directly from the ground. environment, it can be reasonably speculated that the cooling Currently, NASA, ESA, and other space agencies are working on environment of the both were basically the same, which ensured developing related technologies. As a kind of typical light metal, that the gravity level was the only variable during the titanium alloy has a series of excellent properties, such as high experiments. According to the complete temperature–time curve specific strength, good machining performance, strong corro- of 1g sample, it can be seen that the top temperature of the 4–6 sion resistance, etc., and is widely used in the aerospace field, sample dropped below the melting points after the power was so it is expected to be a candidate material for additive turned off for about 2.5 s (within 3.2 s), that is, the solidification of manufacturing in space. Microgravity effect exists in space the samples had finished before quenching in the silicon oil. environment, where buoyancy convection, hydrostatic pressure, Figure 2 shows the solidification microstructure in longitudinal sedimentation phenomena disappear basically, which will have sections of the samples. It can be seen that the solidification 7,8 important impacts on the solidification of alloys, resulting in structures of both 1g sample and μg sample are polycrystalline obvious changes in dendritic, eutectic, monotectic, and other structures, and the grain size increases with the increase of the 9–11 microstructure. Different from usual columnar solidification distance from the initial solidification interface. Morphologically, structure, the solidification structure of duplex titanium alloy is there are two kinds of grains, one is the equiaxed grain growing upwards from bottom at the early stage of solidification, and the usually equiaxed polycrystalline structure. However, few works other is the elongated grain growing with obvious orientation at were reported on the solidification behavior of titanium alloy the late stage of solidification. Between the two kinds of grains, a and polycrystalline structure in microgravity environment. In transition interface could be observed in each sample. In μg order to better understand the effect of space microgravity sample, the interface is almost flat; while in 1g sample, the environment on the solidification behavior of titanium alloy, so interface is curved, like a basin. Below the interface, the as to provide necessary technical support for its space additive solidification structure of both 1g sample and μg sample are fine manufacturing, it is necessary to conduct an experimental study equiaxed grains, no evident difference between them could be on it in advance. In view of this, one of commonly used titanium seen, except that the area below the transition interface is larger alloys, TC8, was selected in this paper to conduct a comparative in μg sample than in 1g sample. Above the interface however, the study on its solidification behavior in microgravity and gravity grain size, morphology, and orientation are significantly different 11,13–15 environments with a 50-m-high drop tube, so as to obtain between 1g sample and μg sample. In μg sample, the elongated the specificinfluence of microgravity effect on the solidification grains first grew upward along the direction perpendicular to the of titanium alloys. flat transition interface, especially in the center of the sample. CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China. *email: xhluo@imr.ac.cn Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; X. Luo et al. Then, with the solidification, the direction of grain growth of grain size in the equiaxed grain zone in μg sample locates on gradually shifted from vertical upward to tilt toward the center. the right of that in 1g sample, indicating that most of the grains in Finally, near the top of the sample, the grain growth direction that zone in μg sample are larger than those in 1g sample, but the became radial or even slightly inclined downward. Nevertheless, difference is not big. In the elongated grain zone, there are fewer in 1g sample, the elongated grains basically grew towards the small grains (<800 μm) and more big grains (>800 μm) in μg normal direction of the basin-like transition interface, and then sample than in 1g sample, making the average grain size larger gradually shifted to radial direction, after that, this pattern kept on than that of 1g sample. On the other hand, the elongation of even in the vicinity of the top of the sample. grains in 1g sample is greater than that in μg sample, according to Considering that the morphology and the size of the grains the data shown in Table 1, implying a sign of stronger oriented above and below the transition interface are quite different, the growth. distributions of their diameters were statistically analyzed The axial element content distribution from the remelting separately by equal area circle method, and the results are shown interface upward to the vicinity of the shrinkage cavity in μg in Fig. 3. The statistical data on number, size and morphology of sample and 1g sample are shown in Fig. 4. No obvious change of the grains above the transition interface in both samples are also the element content with the distance can be seen in both listed in Table 1. samples, that is, no macroscopic segregation occurred. Besides, Obviously, the average sizes of the grains in the equiaxed grain there is no significant difference between the composition zones are much smaller than those in the elongated grain zones in distribution in μg sample and that in 1g sample. both μg and 1g samples, and the average grain sizes in both the equiaxed grain zone and the elongated grain zone in μg sample DISCUSSION are larger than those in 1g sample. In detail, the distribution peak Generally, one of the most important differences of normal gravity from microgravity environment is the existence of buoyancy convection, which is evoked by the uneven density inside melt due to temperature gradient or concentration gradient. It usually has a significant impact on solidification structure and solute 11,13,15–19 distribution. In this work, however, things were different. TC8 alloy is an alloy with high melting point. Due to the huge Fig. 1 Temperature–time profiles of μg sample and 1g sample. The solid blue curve with square symbol shows the temperature–time profile of μg sample. The dashed red curve with circle symbol shows the temperature–time profile of 1g sample Fig. 3 Grain size distribution frequencies of μg sample and 1g sample. The blue curve with square symbol shows the size distribution frequency of the grains below the transition interface in μg sample, the red curve with circle symbol shows the size distribution frequency of the grains below the transition interface in 1g sample; The navy curve with up triangle symbol shows the size distribution frequency of the grains above the transition interface in μg sample, the purple curve with down triangle symbol shows the size distribution frequency of the grains above the transition interface in 1g sample Table 1. Statistical data on grains in upper half part of μg and 1g samples Fig. 2 Longitudinal section microstructure of the samples. a μg Sample Grain number per Grain size (μm) Aspect ratio sample and b 1g sample. From the bottom up, the red dash dot lines −2 unit area (mm ) indicate the positions of the initial solid–liquid interfaces, the blue Ave. Max. Min. Ave. Max. Min. dashed lines indicate the positions and shapes of the transition interfaces, and the blue dashed lines with arrows indicate the μg 3.126 595 1512 187 2.385 6.177 1.054 growth directions of the grains. The diameter of the samples is 1g 3.807 543 1202 167 2.966 7.345 1.058 about 6 mm npj Microgravity (2019) 23 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; X. Luo et al. Fig. 4 Line scanning analysis results of the elements distribution along solidification direction. a μg sample and b 1g sample. From the top down, the black spectral lines represent the distributions of Ti element, the red spectral lines represent the distributions of Al element, the blue spectral lines represent the distributions of Mo element, and the pink spectral lines represent the distributions of Si element difference between the high heating temperature (around 1650 ° slowed down gradually. In this case, the effect of hydrostatic C) and low ambient temperature, as well as the existence of the pressure and wall was gradually highlighted. In another words, the unmelted cold end, the average solidification rate could be fast. It radial temperature gradient and wall nucleation came into play. was roughly estimated to be about 2.6 mm/s for the both samples, They let the nucleation rate and growth rate near the surface of 1g by measuring the distance from the initial solidification interface sample to be competitive with those at the center of the sample, to the top shrinkage cavity in combination with the solidification and the result of the competition between the radial inward growth time shown on the temperature–time curve. This velocity was from the wall and the upward growth along the axial was to bend beyond the average convection velocity and solute diffusion rate the macroscopic solid–liquid interface to form a basin-like structure in melt, which was reported in the order of 1 mm/s and shown in Fig. 2b. Different from this, such effect was minimal in μg −2 −1 20,21 10 –10 mm/s, respectively. As a result, the impact of sample, and the macroscopic solid–liquid interface remained flat. buoyancy convection on the solidification microstructure and When the solidification process was half over and the solute distribution could be neglected, which was partially solid–liquid interface was getting closer and closer to the upper testified from one side by the elements distribution analysis surface of the melt, the temperature of the solidified solid was results. Another result led by the high solidification velocity was getting higher and higher due to absorption of heat dissipation the formation of equiaxial polycrystalline structures instead of from melt and latent heat of solidification, and the axial common columnar structures. temperature gradient in the melt became smaller and smaller. In Actually, besides buoyancy convection, the difference between this situation, axial heat dissipation through the solidified solid was becoming difficult, and that through the outer surface of the microgravity environment and normal gravity environment includes hydrostatic pressure and wall effect as well. They both melt gradually occupied a dominant position, which made the have influence on the heat dissipation, especially through crucible heat dissipation direction in the melt deflect from vertical wall, during solidification. In addition, the wall effect may also downward to radial outward gradually. This was especially true induce heterogeneous nucleation. Therefore, under the solidi- in 1g sample, the occurrence of deflection was obviously earlier fication condition in this paper, the heat in the melt had two and the degree was higher than those in μg sample, respectively. dissipation channels, one was the axial dissipation through the As the nucleation rate and growth rate of the melt decreased solid, and the other was the radial dissipation through the crucible further with temperature gradient, the solidified structure wall. Concretely, according to the results, in initial solidification gradually changed from the fine equiaxial grains in the initial phase of the samples, because of the strong axial heat dissipation phase to the elongated grains growing in the opposite direction of effect of the unmelted cold end, the direction of heat flow in the heat dissipation. Comparatively, the grain size in μg sample was melt near the solid–liquid interface was downward perpendicular bigger and the aspect ratio of grain in 1g sample was larger, to the solid–liquid interface; at the same time, large temperature suggesting that the nucleation and growth rates were slower in μg difference also resulted in large temperature gradients near the sample and the heat flow in 1g sample was stronger. The reason solid–liquid interface, the combination of the both caused the for these differences was closely related to hydrostatic pressure. In melt to solidify upward at a very fast speed and form fine equiaxial μg sample, due to lack of hydrostatic pressure, the melt surface polycrystalline structures. In this phase, hydrostatic pressure was tension played a prominent role. The melt had a poor wetting on not inactive, although its effect was relatively weak. Under the crucible and the heat exchange with the surrounding area was hydrostatic pressure, the melt in 1g sample contacted with the weak. Therefore, the temperature gradient in the melt was lower, crucible wall more tightly, making it easier to dissipate heat via the leading to a lower nucleation rate and growth rate. In 1g sample, crucible wall and to nucleate heterogeneously on the crucible on the other hand, with the assistance of hydrostatic pressure, the wall, and consequently producing a higher nucleation rate and a melt and the crucible wall were wetting better, the heat in melt higher growth rate. In μg sample however, without hydrostatic diffused to the surrounding environment more easily, which pressure, the contact between the melt and the wall was weak or produced higher temperature gradient and promoted hetero- 23 25,26 even, to some extent, detached if the wettability between the geneous nucleation, leading to higher nucleation rate, melt and the wall was not good enough. That may explain why growth rate and more significantly oriented growth. This indicates the grain number density was higher and grain size was smaller in that, with the solidification rate slowing down, the change of 1g sample than in μg sample. After the rapid solidification in the solidification process caused by the difference of hydrostatic initial phase, the axial temperature gradient in front of the pressure was further highlighted. solid–liquid interface decreased gradually as the temperature of Until close to the end of solidification, under hydrostatic the solid end increased, and the nucleation and growth rate pressure, the elongated grains in 1g sample basically maintained Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2019) 23 X. Luo et al. the growth trend along the radial directions towards the sample height of 30 mm, and wall thickness of 1 mm, and placed in the center of the induction coil in the vacuum chamber at the top of the drop tube. A center. However, the grain growth direction in μg sample showed removable tray is used to support the crucible from its bottom. Adjust the a tendency of continuous upward deflection. At this time, without height of the sample in the coil so as to let only the upper end of the sample hydrostatic pressure, the surface of μg sample had most likely be heated to melt and keep the lower end of the sample as solid. Switch on detached from the crucible wall completely, due to volume the power supply to heat the sample for some seconds until its upper end is shrinkage during solidification and surface tension effect, and melted, then switch off the power supply and remove the tray became free surface. Then the grains grew in a direction simultaneously to release the sample. During its free fall in the tube, perpendicular to the sample surface towards the sample center. solidification under microgravity from the unmelted solid end of the sample To sum up, in addition to the influence of the buoyancy takes place, until it falls down to the bottom of the tube, where a container full of silicon oil is placed to quench and collect the sample. For comparison convection and sedimentation on the concentration field, purpose, same experiment on same sample is conducted without free fall. temperature field and flow field in melt during solidification Specifically, the sample is not released immediately after melting but kept process, and thus on the solidification structure of alloys, the still on the tray for 3.2 s, then the tray is removed and the sample was hydrostatic pressure caused by gravity will also change the heat allowed to quench in silicon oil just below it. In this case, solidification under diffusion in melt due to the influence on interface effect, so as to normal gravity from the unmelted solid end of the sample takes place. For have a significant impact on solidification structure. Furthermore, the convenience of description, in this paper the drop sample is referred to as μg sample, and the still sample is referred to as 1g sample. even when the solidification velocity exceeds the convective The experimental samples were cut longitudinally along their central velocity, rendering it ineffective, the hydrostatic pressure can still axes. The microstructure on the longitudinal sections of the samples were affect the solidification structure. Therefore, the effect of hydro- observed and photographed using a MEF4A metallographic microscope static pressure should not be ignored when analyzing the after sample grinding, polishing, and etching with 2%HF+ 5%HNO + 93% influence of gravity on the solidification structure of materials. H O (vol%) solvent. Since the grain structure in the solidified micro- structure was not clearly displayed, the grain boundaries in microstructure were highlighted by image editing software. Then the Image pro plus 6.0 METHODS analysis software was used for statistical analysis of the processed images, Rod samples with diameter of 6 mm and length of 28 mm were machined and the size distributions, numbers, and aspect ratios of the grains were from extruded rod of TC8 alloy with chemical composition of Ti–6.33% obtained. In addition, the composition distributions along the central axes Al–3.46%Mo–0.27Si(wt%). The experimental facility is a 50-m-high drop on the longitudinal sections of the samples were analyzed by line scanning tube with a diameter of 150 mm, which can be evacuated down to 1 × with electron probe X-ray microanalysis (EPMA). −4 10 Pa, and therefore, supply a microgravity environment at an −6 acceleration level down to 10 g for about 3.2 s. A schematic diagram Reporting summary of the experimental setup is shown in Fig. 5. At the top of the drop tube, an Further information on research design is available in the Nature Research induction coil and a 10 kW semiconductor high-frequency induction power Reporting Summary linked to this article. supply are equipped. Besides, a monochrome infrared pyrometer is installed at the top of the drop tube to monitor the temperature of samples from their top ends. The experimental procedures are as follows: The sample is loaded into a corundum crucible with inner diameter of 6 mm, DATA AVAILABILITY The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request. Received: 25 June 2019; Accepted: 18 September 2019; REFERENCES 1. Neal, C. R. The moon 35 years after Apollo: what’s left to learn? Chem. Erde- Geochem 69,3–43 (2009). 2. Prater, T. et al. 3D printing in Zero G Technology Demonstration Mission: com- plete experimental results and summary of related material modeling efforts. Int. J. Adv. Manuf. 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Investigation of microgravity effect on solidification of medium-low- AUTHOR CONTRIBUTIONS melting-point alloy by drop tube experiment. Sci. China Ser. E 51, 1370–1379 (2008). X.L. designed the microgravity experiments. Y.W. and Y.L. carried out the microgravity 14. Chen, L. & Luo, X. H. A new way to explore microgravity effect by drop tube experiments. Y.W. analyzed the solidification microstructure and determined the experiment. Acta Met. Sin. 43, 769–774 (2007). composition distributions of the samples. X.L. and Y.W. discussed the results and 15. Feng, S. B. & Luo, X. H. in International Symposium on Physical Sciences in Space, conclusions, and wrote the manuscript. Vol. 327, Journal of Physics Conference Series (eds Meyer, A. & Egry, I.) (Iop Pub- lishing Ltd, 2011). 16. Yu, J. et al. Homogeneous InGaSb crystal grown under microgravity using Chi- COMPETING INTERESTS nese recovery satellite SJ-10. NPJ Microgravity 5, 8 (2019). The authors declare no competing interests. 17. 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A numerical Reprints and permission information is available at http://www.nature.com/ and experimental study of natural convection and interface shape in crystal reprints growth. J. Cryst. Growth 173, 492–502 (1997). 21. Blacha, L., Golak, S., Jakovics, A. & Tucs, A. Kinetic analysis of aluminium eva- Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims poration from the Ti–6Al–7Nb Alloy. Arch. Met. Mater. 59, 275–279 (2014). in published maps and institutional affiliations. 22. Hunt, J. D. Steady state columnar and equiaxed growth of dendrites and eutectic. Mater. Sci. Eng. 65,75–83 (1984). 23. Kurz, W. & Fisher, D. J. Fundamentals of Solidification, 4th rev. edn (Trans Tech Publications, 1998). 24. Nagai, H. et al. Thermal conductivity measurement of liquid materials by a hot- Open Access This article is licensed under a Creative Commons disk method in short-duration microgravity environments. Mater. Sci. Eng. A 276, Attribution 4.0 International License, which permits use, sharing, 117–123 (2000). adaptation, distribution and reproduction in any medium or format, as long as you give 25. Schmelzer, J. W. P., Abyzov, A. S., Fokin, V. M., Schick, C. & Zanotto, E. D. Crys- appropriate credit to the original author(s) and the source, provide a link to the Creative tallization of glass-forming liquids: maxima of nucleation, growth, and overall Commons license, and indicate if changes were made. The images or other third party crystallization rates. J. Non-Cryst. Solids 429,24–32 (2015). material in this article are included in the article’s Creative Commons license, unless 26. Kegley, D. R., Wittig, J. E., Hofmeister, W. H., Bayuzick, R. J. & Rowe, R. G. Mater. Sci. indicated otherwise in a credit line to the material. If material is not included in the Forum 50, 129–136 (1989). article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons. org/licenses/by/4.0/. ACKNOWLEDGEMENTS This research was supported by the Chinese manned space flight pre-research project (030302). © The Author(s) 2019 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2019) 23 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png npj Microgravity Springer Journals

Role of hydrostatic pressure and wall effect in solidification of TC8 alloy

npj Microgravity , Volume 5 (1) – Oct 11, 2019

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Life Sciences; Life Sciences, general; Classical and Continuum Physics; Biotechnology; Immunology; Space Sciences (including Extraterrestrial Physics, Space Exploration and Astronautics) ; Applied Microbiology
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Abstract

www.nature.com/npjmgrav ARTICLE OPEN Role of hydrostatic pressure and wall effect in solidification of TC8 alloy 1 1 1 Xinghong Luo , Yaya Wang and Yang Li The solidification experiments of TC8 alloy under both microgravity and normal gravity were conducted using a drop tube. The solidification microstructure were found composed of fine equiaxed grains formed at early stage and bigger elongated grains formed at later stage. Between the two kinds of grains a curved transition interface was observed in 1g sample, while that in μg sample was almost flat. Generally, the amounts and aspect ratios of the grains are larger, and the grain sizes are smaller in 1g sample. Besides, no visible element macrosegregation occurred in both samples. The results suggest that the solidification velocities of the samples were rapid, and consequently the convection effect and solute transport effect caused by gravity had little influence on the solidification microstructure. Therefore, the solidification process was mainly controlled by thermal diffusion, and hydrostatic pressure and wall effect played a great role in it. npj Microgravity (2019) 5:23 ; https://doi.org/10.1038/s41526-019-0083-2 INTRODUCTION RESULTS With the continuous development of aerospace technology and Figure 1 shows the temperature vs. time of the sample tops during the experiments. Because the temperature during the falling advanced material preparation technology, people are looking process could not be monitored, only the temperature before for the perfect combination of the two technologies to serve the release of the μg sample was recorded. It can be seen that the future deep space exploration and interstellar navigation. Space curves of heating and melting phases of both samples are highly additive manufacturing technology is expected to be one of 1–3 coincident, indicating that μg sample and 1g sample have same them. With this technology, astronauts can make whatever heating history. Given that the external cooling environments of parts they need in situ, without having to spend precious the both were the same room temperature and vacuum payload resources to carry them directly from the ground. environment, it can be reasonably speculated that the cooling Currently, NASA, ESA, and other space agencies are working on environment of the both were basically the same, which ensured developing related technologies. As a kind of typical light metal, that the gravity level was the only variable during the titanium alloy has a series of excellent properties, such as high experiments. According to the complete temperature–time curve specific strength, good machining performance, strong corro- of 1g sample, it can be seen that the top temperature of the 4–6 sion resistance, etc., and is widely used in the aerospace field, sample dropped below the melting points after the power was so it is expected to be a candidate material for additive turned off for about 2.5 s (within 3.2 s), that is, the solidification of manufacturing in space. Microgravity effect exists in space the samples had finished before quenching in the silicon oil. environment, where buoyancy convection, hydrostatic pressure, Figure 2 shows the solidification microstructure in longitudinal sedimentation phenomena disappear basically, which will have sections of the samples. It can be seen that the solidification 7,8 important impacts on the solidification of alloys, resulting in structures of both 1g sample and μg sample are polycrystalline obvious changes in dendritic, eutectic, monotectic, and other structures, and the grain size increases with the increase of the 9–11 microstructure. Different from usual columnar solidification distance from the initial solidification interface. Morphologically, structure, the solidification structure of duplex titanium alloy is there are two kinds of grains, one is the equiaxed grain growing upwards from bottom at the early stage of solidification, and the usually equiaxed polycrystalline structure. However, few works other is the elongated grain growing with obvious orientation at were reported on the solidification behavior of titanium alloy the late stage of solidification. Between the two kinds of grains, a and polycrystalline structure in microgravity environment. In transition interface could be observed in each sample. In μg order to better understand the effect of space microgravity sample, the interface is almost flat; while in 1g sample, the environment on the solidification behavior of titanium alloy, so interface is curved, like a basin. Below the interface, the as to provide necessary technical support for its space additive solidification structure of both 1g sample and μg sample are fine manufacturing, it is necessary to conduct an experimental study equiaxed grains, no evident difference between them could be on it in advance. In view of this, one of commonly used titanium seen, except that the area below the transition interface is larger alloys, TC8, was selected in this paper to conduct a comparative in μg sample than in 1g sample. Above the interface however, the study on its solidification behavior in microgravity and gravity grain size, morphology, and orientation are significantly different 11,13–15 environments with a 50-m-high drop tube, so as to obtain between 1g sample and μg sample. In μg sample, the elongated the specificinfluence of microgravity effect on the solidification grains first grew upward along the direction perpendicular to the of titanium alloys. flat transition interface, especially in the center of the sample. CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China. *email: xhluo@imr.ac.cn Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; X. Luo et al. Then, with the solidification, the direction of grain growth of grain size in the equiaxed grain zone in μg sample locates on gradually shifted from vertical upward to tilt toward the center. the right of that in 1g sample, indicating that most of the grains in Finally, near the top of the sample, the grain growth direction that zone in μg sample are larger than those in 1g sample, but the became radial or even slightly inclined downward. Nevertheless, difference is not big. In the elongated grain zone, there are fewer in 1g sample, the elongated grains basically grew towards the small grains (<800 μm) and more big grains (>800 μm) in μg normal direction of the basin-like transition interface, and then sample than in 1g sample, making the average grain size larger gradually shifted to radial direction, after that, this pattern kept on than that of 1g sample. On the other hand, the elongation of even in the vicinity of the top of the sample. grains in 1g sample is greater than that in μg sample, according to Considering that the morphology and the size of the grains the data shown in Table 1, implying a sign of stronger oriented above and below the transition interface are quite different, the growth. distributions of their diameters were statistically analyzed The axial element content distribution from the remelting separately by equal area circle method, and the results are shown interface upward to the vicinity of the shrinkage cavity in μg in Fig. 3. The statistical data on number, size and morphology of sample and 1g sample are shown in Fig. 4. No obvious change of the grains above the transition interface in both samples are also the element content with the distance can be seen in both listed in Table 1. samples, that is, no macroscopic segregation occurred. Besides, Obviously, the average sizes of the grains in the equiaxed grain there is no significant difference between the composition zones are much smaller than those in the elongated grain zones in distribution in μg sample and that in 1g sample. both μg and 1g samples, and the average grain sizes in both the equiaxed grain zone and the elongated grain zone in μg sample DISCUSSION are larger than those in 1g sample. In detail, the distribution peak Generally, one of the most important differences of normal gravity from microgravity environment is the existence of buoyancy convection, which is evoked by the uneven density inside melt due to temperature gradient or concentration gradient. It usually has a significant impact on solidification structure and solute 11,13,15–19 distribution. In this work, however, things were different. TC8 alloy is an alloy with high melting point. Due to the huge Fig. 1 Temperature–time profiles of μg sample and 1g sample. The solid blue curve with square symbol shows the temperature–time profile of μg sample. The dashed red curve with circle symbol shows the temperature–time profile of 1g sample Fig. 3 Grain size distribution frequencies of μg sample and 1g sample. The blue curve with square symbol shows the size distribution frequency of the grains below the transition interface in μg sample, the red curve with circle symbol shows the size distribution frequency of the grains below the transition interface in 1g sample; The navy curve with up triangle symbol shows the size distribution frequency of the grains above the transition interface in μg sample, the purple curve with down triangle symbol shows the size distribution frequency of the grains above the transition interface in 1g sample Table 1. Statistical data on grains in upper half part of μg and 1g samples Fig. 2 Longitudinal section microstructure of the samples. a μg Sample Grain number per Grain size (μm) Aspect ratio sample and b 1g sample. From the bottom up, the red dash dot lines −2 unit area (mm ) indicate the positions of the initial solid–liquid interfaces, the blue Ave. Max. Min. Ave. Max. Min. dashed lines indicate the positions and shapes of the transition interfaces, and the blue dashed lines with arrows indicate the μg 3.126 595 1512 187 2.385 6.177 1.054 growth directions of the grains. The diameter of the samples is 1g 3.807 543 1202 167 2.966 7.345 1.058 about 6 mm npj Microgravity (2019) 23 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA 1234567890():,; X. Luo et al. Fig. 4 Line scanning analysis results of the elements distribution along solidification direction. a μg sample and b 1g sample. From the top down, the black spectral lines represent the distributions of Ti element, the red spectral lines represent the distributions of Al element, the blue spectral lines represent the distributions of Mo element, and the pink spectral lines represent the distributions of Si element difference between the high heating temperature (around 1650 ° slowed down gradually. In this case, the effect of hydrostatic C) and low ambient temperature, as well as the existence of the pressure and wall was gradually highlighted. In another words, the unmelted cold end, the average solidification rate could be fast. It radial temperature gradient and wall nucleation came into play. was roughly estimated to be about 2.6 mm/s for the both samples, They let the nucleation rate and growth rate near the surface of 1g by measuring the distance from the initial solidification interface sample to be competitive with those at the center of the sample, to the top shrinkage cavity in combination with the solidification and the result of the competition between the radial inward growth time shown on the temperature–time curve. This velocity was from the wall and the upward growth along the axial was to bend beyond the average convection velocity and solute diffusion rate the macroscopic solid–liquid interface to form a basin-like structure in melt, which was reported in the order of 1 mm/s and shown in Fig. 2b. Different from this, such effect was minimal in μg −2 −1 20,21 10 –10 mm/s, respectively. As a result, the impact of sample, and the macroscopic solid–liquid interface remained flat. buoyancy convection on the solidification microstructure and When the solidification process was half over and the solute distribution could be neglected, which was partially solid–liquid interface was getting closer and closer to the upper testified from one side by the elements distribution analysis surface of the melt, the temperature of the solidified solid was results. Another result led by the high solidification velocity was getting higher and higher due to absorption of heat dissipation the formation of equiaxial polycrystalline structures instead of from melt and latent heat of solidification, and the axial common columnar structures. temperature gradient in the melt became smaller and smaller. In Actually, besides buoyancy convection, the difference between this situation, axial heat dissipation through the solidified solid was becoming difficult, and that through the outer surface of the microgravity environment and normal gravity environment includes hydrostatic pressure and wall effect as well. They both melt gradually occupied a dominant position, which made the have influence on the heat dissipation, especially through crucible heat dissipation direction in the melt deflect from vertical wall, during solidification. In addition, the wall effect may also downward to radial outward gradually. This was especially true induce heterogeneous nucleation. Therefore, under the solidi- in 1g sample, the occurrence of deflection was obviously earlier fication condition in this paper, the heat in the melt had two and the degree was higher than those in μg sample, respectively. dissipation channels, one was the axial dissipation through the As the nucleation rate and growth rate of the melt decreased solid, and the other was the radial dissipation through the crucible further with temperature gradient, the solidified structure wall. Concretely, according to the results, in initial solidification gradually changed from the fine equiaxial grains in the initial phase of the samples, because of the strong axial heat dissipation phase to the elongated grains growing in the opposite direction of effect of the unmelted cold end, the direction of heat flow in the heat dissipation. Comparatively, the grain size in μg sample was melt near the solid–liquid interface was downward perpendicular bigger and the aspect ratio of grain in 1g sample was larger, to the solid–liquid interface; at the same time, large temperature suggesting that the nucleation and growth rates were slower in μg difference also resulted in large temperature gradients near the sample and the heat flow in 1g sample was stronger. The reason solid–liquid interface, the combination of the both caused the for these differences was closely related to hydrostatic pressure. In melt to solidify upward at a very fast speed and form fine equiaxial μg sample, due to lack of hydrostatic pressure, the melt surface polycrystalline structures. In this phase, hydrostatic pressure was tension played a prominent role. The melt had a poor wetting on not inactive, although its effect was relatively weak. Under the crucible and the heat exchange with the surrounding area was hydrostatic pressure, the melt in 1g sample contacted with the weak. Therefore, the temperature gradient in the melt was lower, crucible wall more tightly, making it easier to dissipate heat via the leading to a lower nucleation rate and growth rate. In 1g sample, crucible wall and to nucleate heterogeneously on the crucible on the other hand, with the assistance of hydrostatic pressure, the wall, and consequently producing a higher nucleation rate and a melt and the crucible wall were wetting better, the heat in melt higher growth rate. In μg sample however, without hydrostatic diffused to the surrounding environment more easily, which pressure, the contact between the melt and the wall was weak or produced higher temperature gradient and promoted hetero- 23 25,26 even, to some extent, detached if the wettability between the geneous nucleation, leading to higher nucleation rate, melt and the wall was not good enough. That may explain why growth rate and more significantly oriented growth. This indicates the grain number density was higher and grain size was smaller in that, with the solidification rate slowing down, the change of 1g sample than in μg sample. After the rapid solidification in the solidification process caused by the difference of hydrostatic initial phase, the axial temperature gradient in front of the pressure was further highlighted. solid–liquid interface decreased gradually as the temperature of Until close to the end of solidification, under hydrostatic the solid end increased, and the nucleation and growth rate pressure, the elongated grains in 1g sample basically maintained Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2019) 23 X. Luo et al. the growth trend along the radial directions towards the sample height of 30 mm, and wall thickness of 1 mm, and placed in the center of the induction coil in the vacuum chamber at the top of the drop tube. A center. However, the grain growth direction in μg sample showed removable tray is used to support the crucible from its bottom. Adjust the a tendency of continuous upward deflection. At this time, without height of the sample in the coil so as to let only the upper end of the sample hydrostatic pressure, the surface of μg sample had most likely be heated to melt and keep the lower end of the sample as solid. Switch on detached from the crucible wall completely, due to volume the power supply to heat the sample for some seconds until its upper end is shrinkage during solidification and surface tension effect, and melted, then switch off the power supply and remove the tray became free surface. Then the grains grew in a direction simultaneously to release the sample. During its free fall in the tube, perpendicular to the sample surface towards the sample center. solidification under microgravity from the unmelted solid end of the sample To sum up, in addition to the influence of the buoyancy takes place, until it falls down to the bottom of the tube, where a container full of silicon oil is placed to quench and collect the sample. For comparison convection and sedimentation on the concentration field, purpose, same experiment on same sample is conducted without free fall. temperature field and flow field in melt during solidification Specifically, the sample is not released immediately after melting but kept process, and thus on the solidification structure of alloys, the still on the tray for 3.2 s, then the tray is removed and the sample was hydrostatic pressure caused by gravity will also change the heat allowed to quench in silicon oil just below it. In this case, solidification under diffusion in melt due to the influence on interface effect, so as to normal gravity from the unmelted solid end of the sample takes place. For have a significant impact on solidification structure. Furthermore, the convenience of description, in this paper the drop sample is referred to as μg sample, and the still sample is referred to as 1g sample. even when the solidification velocity exceeds the convective The experimental samples were cut longitudinally along their central velocity, rendering it ineffective, the hydrostatic pressure can still axes. The microstructure on the longitudinal sections of the samples were affect the solidification structure. Therefore, the effect of hydro- observed and photographed using a MEF4A metallographic microscope static pressure should not be ignored when analyzing the after sample grinding, polishing, and etching with 2%HF+ 5%HNO + 93% influence of gravity on the solidification structure of materials. H O (vol%) solvent. Since the grain structure in the solidified micro- structure was not clearly displayed, the grain boundaries in microstructure were highlighted by image editing software. Then the Image pro plus 6.0 METHODS analysis software was used for statistical analysis of the processed images, Rod samples with diameter of 6 mm and length of 28 mm were machined and the size distributions, numbers, and aspect ratios of the grains were from extruded rod of TC8 alloy with chemical composition of Ti–6.33% obtained. In addition, the composition distributions along the central axes Al–3.46%Mo–0.27Si(wt%). The experimental facility is a 50-m-high drop on the longitudinal sections of the samples were analyzed by line scanning tube with a diameter of 150 mm, which can be evacuated down to 1 × with electron probe X-ray microanalysis (EPMA). −4 10 Pa, and therefore, supply a microgravity environment at an −6 acceleration level down to 10 g for about 3.2 s. A schematic diagram Reporting summary of the experimental setup is shown in Fig. 5. At the top of the drop tube, an Further information on research design is available in the Nature Research induction coil and a 10 kW semiconductor high-frequency induction power Reporting Summary linked to this article. supply are equipped. Besides, a monochrome infrared pyrometer is installed at the top of the drop tube to monitor the temperature of samples from their top ends. The experimental procedures are as follows: The sample is loaded into a corundum crucible with inner diameter of 6 mm, DATA AVAILABILITY The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request. Received: 25 June 2019; Accepted: 18 September 2019; REFERENCES 1. Neal, C. R. The moon 35 years after Apollo: what’s left to learn? Chem. Erde- Geochem 69,3–43 (2009). 2. Prater, T. et al. 3D printing in Zero G Technology Demonstration Mission: com- plete experimental results and summary of related material modeling efforts. Int. J. Adv. Manuf. 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A 276, Attribution 4.0 International License, which permits use, sharing, 117–123 (2000). adaptation, distribution and reproduction in any medium or format, as long as you give 25. Schmelzer, J. W. P., Abyzov, A. S., Fokin, V. M., Schick, C. & Zanotto, E. D. Crys- appropriate credit to the original author(s) and the source, provide a link to the Creative tallization of glass-forming liquids: maxima of nucleation, growth, and overall Commons license, and indicate if changes were made. The images or other third party crystallization rates. J. Non-Cryst. Solids 429,24–32 (2015). material in this article are included in the article’s Creative Commons license, unless 26. Kegley, D. R., Wittig, J. E., Hofmeister, W. H., Bayuzick, R. J. & Rowe, R. G. Mater. Sci. indicated otherwise in a credit line to the material. If material is not included in the Forum 50, 129–136 (1989). article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons. org/licenses/by/4.0/. ACKNOWLEDGEMENTS This research was supported by the Chinese manned space flight pre-research project (030302). © The Author(s) 2019 Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA npj Microgravity (2019) 23

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