Solidification In Microgravity Research Articles

Wrinkled Shrinkage on Surface Monophasic Structure of Floating Refractory Alloy Droplets Solidified in Space Environment

AbstractThe surface‐wrinkled shrinkage structure of spheres is very common and also quite intriguing to explore and reveal the analogous droplet solidification kinetics in the space environment. Here, a liquid‐to‐solid phase transition experiment of floating refractory alloy droplets is designed to study the surface wrinkled shrinkage structures aboard China Space Station with a 10−5 gravity level. The experimental Ti‐Ni‐V alloy droplets undergo an extremely high overheating temperature beyond 1800 K and then achieve a supercooling of 307 K, displaying a strong thermodynamic metastability. The spherical surface deformation structures of this alloy under microgravity coupled with extreme metastable solidification are explored. Moreover, through the active control of metastable rapid solidification, a surface monophasic structure and a B2 structure capable of stress‐induced martensitic transformation are facilitated by wrinkled shrinkage dynamics under microgravity. Their findings shed further light on the surface deformation structures during the microgravity solidification process.

Freezing Shrinkage Dynamics and Surface Dendritic Growth of Floating Refractory Alloy Droplets in Outer Space.

The freezing shrinkage and dendritic growth are of great importance for various alloys solidified from high-temperature liquids to solids since they dominate microstructure patterns and follow-up processing. However, the microgravity freezing shrinkage dynamics is scarcely explored on the ground as it is hard to suppress the strong natural convection inside liquid alloys. Here, a series of in-orbit solidification experiments is conducted aboard the China Space Station with a long-term stable 10-5 g0 microgravity condition. The highest temperature up to 2265K together with substantial liquid undercoolings far from a thermodynamically stable state are attained for both Nb82.7Si17.3 and Zr64V36 refractory alloys. Furthermore, the solidification under microgravity of a droplet is simulated to reveal the liquid-solid interface migration, temperature gradient, and flow field. The microgravity solidification process leads to freezing shrinkage cavities and distinctive surface dendritic microstructure patterns. The combined effects of shrinkage dynamics and liquid surface flow in outer space result in the dendrites growing not only along the tangential direction but also along the normal direction to the droplet surface. These space experimental results contribute to a further understanding of the solidification behavior of liquid alloys under a weaker convection condition, which is often masked by gravity on the ground.

Microstructure Analysis of Al-7 wt% Si Alloy Solidified on Earth Compared to Similar Experiments in Microgravity

During ground-based solidification, buoyancy flow can develop by the density difference in the hypoeutectic type of the alloys, such as Al-7 wt% Si alloy. Buoyancy flow can affect the thermal field, solute distribution in the melt, and the position and amount of the new grains. As solidification is a very complex process, it is not very easy to separate the different effects. Under microgravity conditions, natural convection does not exist or is strongly damped due to the absence of the buoyancy force. Therefore, experiments in microgravity conditions provide unique benchmark data for pure diffusive solidification conditions. Compared to the results of the ground-based and microgravity experiments, it is possible to get information on the effect of gravity (buoyancy force). In the framework of the CETSOL project, four microgravity solidification experiments were performed on grain refined (GF) and non-grain refined Al-7 wt% Si alloy onboard the International Space Station in the Materials Science Laboratory. These experiments aimed to study the effect of the solidification parameters (solid/liquid front velocity vSL, temperature gradient GSL) on the grain structure and dendritic microstructures. The microgravity environment eliminates the melt flow, which develops on Earth due to gravity. Four ground-based (GB) experiments were performed under Earth-like conditions with the same (similar) solidification parameters in a vertical Bridgman-type furnace having four heating zones. The detailed analysis of the grain structure, amount of eutectic, and secondary dendrite arm spacing (SDAS) for different process conditions is reported and compared with the results of the microgravity experiments. GB experiments showed that the microstructure was columnar in the samples that do not contain GF material or in case the solid/liquid (vSL front velocity was slow (0.02 mm/s)). In contrast, in the sample which contained GF material, progressive columnar/equiaxed transition (PCET) was observed at vSL = 0.077 mm/s and GSL = 3.9 K/mm. The secondary (SDAS) dendrite arm spacing follows the well-known power law, SDAS=K[t0]13, where K is a constant, and t0 is the local solidification time for both GB and µg experiments.

In situ investigation of the Columnar-to-Equiaxed Transition during directional solidification of Al–20 wt.%Cu alloys on Earth and in microgravity

Solidification experiments on Al–20 wt.%Cu alloys with grain refiner were performed in a Bridgman-type furnace to investigate the impact of gravity-driven phenomena on the Columnar-to-Equiaxed Transition (CET) during directional solidification (i) in microgravity, on board the sounding rocket MASER-14 and (ii) on Earth in three growth directions, namely horizontal, vertical upward (counter-gravity direction) and vertical downward (in-gravity direction). The CET was provoked by a step increase of the cooling rate and was visualised in situ and in real-time using X-radiography. This paper reports direct observations of dendritic columnar growth, CET and the subsequent equiaxed regime, and quantitative characterisation of the microstructures. The increase in constitutional undercooling that induces the nucleation of the first grains ahead of the columnar front can be related to the rapid thermal change following the increase in cooling rate. The nucleation distance of equiaxed grains ahead of the columnar front is larger in microgravity than on Earth and possible origins for this difference are discussed. Both mechanical and solutal blocking mechanisms were observed at CET. Mechanical blocking is attributed to the occurrence of solidification-induced shrinkage flow. The relative importance of each blocking mechanism is detailed for each growth configuration. In particular, mechanical blocking is shown to be the principal type of impingement in the microgravity experiment. The final equiaxed grain structure was characterised, and the results confirm that gravity effects become less significant at high growth rates.

Structure and Property Modulations of 13Cr Stainless Steel Through Microgravity Solidification and Minor Ce Addition

Solidification controlling and RE doping are effective ways to manipulate the microstructures and performances of 13Cr martensitic stainless steel. In this work by Ying Ruan and co-workers, article number 2100062, the influences of microgravity solidification and Ce addition on microstructure refinement, inclusion spheroidization and mechanical properties of 13Cr steels were explored comparatively.

Live record of vortex-shaped flow by microgravity solidification

A unique type of vortex-shaped microstructure reflecting the flow pattern during the microgravity solidification formed in Al70Ag20Ge10 alloy droplets using drop tube technique. A 2D axisymmetric model coupling heat transfer, fluid flow with phase change was established to simulate the fluid flow variation of alloy droplets during microgravity solidification. The Stokes motion was suppressed and the Marangoni motion was prominent in driving the vortex-shaped flow. The melt flow affected the phase selection, consequently the Ag2Al phase nucleated primarily instead of (Al) phase under near-equilibrium solidification, and the flow pattern was preserved during the rapid growth of the metastable Ag2Al dendrites. The Marangoni effect was weakened with the decrease of droplet size, and anomalous (Al) + Ag2Al and intergranular (Al) + (Ge) eutectics formed owing to the enhanced nucleation and growth competition among phases. In comparison, Al70Ag20Ge10 alloy was solidified under gravity condition by DSC and vacuum arc melting methods respectively. No vortex-shaped microstructure formed in DSC sample ascribed to the suppressed natural convection at small cooling rate. During arc melting, the electromagnetic force and enhanced natural convection caused strong turbulent flow, the metastable Ag2Al dendrites rapidly formed with increased vortex density.

Structure and Property Modulations of 13Cr Stainless Steel Through Microgravity Solidification and Minor Ce Addition

Both rare‐earth doping and microgravity solidification processing bring significant influences upon the structure formation and mechanical properties of various alloys. Herein, the microstructure and performance of 13Cr martensitic stainless steel are modulated by adding minor Ce content and microgravity solidification by means of drop tube technique. The lattice structure of constituent phase α(Fe) is deformed evidently by microgravity solidification, and the lattice parameter variation is 1.8 times of that caused by Ce addition. The microstructures of this steel processed by arc melting consist of lath‐shaped martensites and a few feather‐shaped bainites whose volume fraction increases with the Ce content. The bainite is replaced by ferrite in the rapidly solidified microstructure of 13Cr steel under microgravity condition. Although both microgravity solidification and Ce addition induce the microstructure refinement and inclusion spheroidization, the former effect plays the dominant role. The compressive properties and microhardness decrease moderately with Ce addition due to the slight increase in bainite, however, the microgravity solidification brought the hardening effect and thus the microhardness of the steel droplets increases with undercooling.

Structural Evolution and Micromechanical Properties of Ternary Ni-Fe-Ti Alloy Solidified Under Microgravity Condition

Both the microgravity solidification mechanism and resultant micromechanical properties of ternary Ni41Fe40Ti19 alloy were investigated by means of drop tube, nanoindentation, and nano-dynamic mechanical analysis (Nano-DMA) techniques. The microstructure of the Ni41Fe40Ti19 alloy droplets consisted of γ-(Fe,Ni) dendrites and interdendritic pseudobinary eutectic phases. The average cooling rate and undercooling increased significantly as the droplet diameter decreased during free fall. Owing to the refinement of the rapidly solidified microstructure, and the Ti solute hardening of the primary γ-(Fe,Ni) dendrites, the microhardness of this alloy was remarkably increased with the decrease of droplet size. Moreover, the nanohardness of the γ-(Fe,Ni) dendrite increased as the indentation displacement decreased within the depth range of 40 to 244 nm, indicating a conspicuous indentation size effect (ISE). However, the ISE increased as the undercooling increased, because additional geometrically necessary dislocations (GNDs) were required, while intragranular dislocation motion was further hindered as the Ti content increased. The size effect factor increased linearly with the reduced droplet diameter.

Intern Designed Modules for Conducting Potential Microgravity Solidification Experiments aboard the International Space Station

MMaJIC (Microgravity Materials Joining Investigation Chamber) is a modular experiment chamber for performing materials science investigations. MMaJIC provides a controlled and sealed test environment for a wide variety of soldering and brazing experiment to examine porosity in microgravity. MMaJIC’s modular cartridge design includes a tray for housing the solder experiments and demonstrates simplicity to astronauts. SoLIDD (Solid Liquid Interface Directional Device) is a device for conducting directional solidification experiments which enables control of the microstructural development. SoLIDD has capability for varying temperature gradient and growth velocity. These quantities utilize heaters, coolers, and a directional drive unit. ICED-T (Interface Control Experiment with Directional Translation) is an apparatus that conducts directional solidification experiments of transparent materials which allows direct observation of the solid/liquid interface. Currently, it is in a breadboard phase that consists of the components mounted to a surface that allows the sample to be placed in an upright position. Orientation of the experiment (solid on the bottom, liquid on top) is important during testing ground samples to maintain stability. Ultimately, the goal of ICED-T is to conduct successful ground tests and then be configured to fit into an enclosure similar to MMaJIC and SoLIDD that could be utilized aboard the ISS.

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.

Effect of solidification conditions and surface pores on the microstructure and columnar-to-equiaxed transition in solidification under microgravity

Microgravity solidification experiments were carried out in the Material Science Laboratory on board the International Space Station. The influence of grain refinement, rotating magnetic field (RMF) and surface pores on the microstructure and columnar-to-equiaxed transition (CET) were investigated in two selected Al-based samples solidified under microgravity conditions. The increase of the furnace pulling velocity leads to a finer dendrite structure, a smaller eutectic percentage and a more uniform eutectic distribution in the interdendritic regions. On the one hand, grain refinement ensures the occurrence of CET, which is progressive in the studied experiment because of the high temperature gradient. On the other hand, in the non-refined alloy a RMF applied during solidification fails to trigger the CET, because the forced liquid flow is too weak compared to the solidification front velocity to transport fragments from the mushy zone above the solidification front. The presence of the pores at the sample surface leads to a peculiarity in the eutectic percentage and weakens the decrease of the dendrite arm spacing for both samples. These effects are ascribed to a forced extra liquid flow into the mushy zone due to the pore that promotes the growth of the dendrites along the liquid flow direction, resulting in elongated grains and postponing the CET in the refined alloy.

A Nucleation Progenitor Function approach to polycrystalline equiaxed solidification modelling with application to a microgravity transparent alloy experiment observed in-situ

A Nucleation Progenitor Function (NPF) approach that accounts for the interdependence between nucleation and growth during equiaxed solidification is proposed. An athermal nucleation density distribution, based on undercooling, is identified as a progenitor function. A Kolmogorov statistical approach is applied assuming continuous nucleation and growth conditions. The derived progeny functions describe the (supressed) distribution of actual nucleation events. The approach offers the significant advantage of generating progeny functions for volumetric (3D) data and projected image (2D) data. The main difference between 3D and 2D data in transparent alloy experiments is due to a stereological correction for over-projection. Progeny functions can be analysed to obtain statistical output information, e.g., nucleation counts, average nucleation undercooling and standard deviation. The statistical output data may be calculated in a formative (running) or a summative (final) mode. The NPF kinetics have been incorporated into a transient thermal model of equiaxed solidification. The model has been applied to characterise a microgravity solidification experiment with the transparent alloy system Neopentylgycol-30 wt%(d)Camphor. The model predicted thermal and observed nucleation and growth data with a good level of agreement.

Structural evolution and micromechanical properties of ternary Al[sbnd]Ag[sbnd]Ge alloy solidified under microgravity condition

Microgravity solidification provides a particular opportunity to produce an extraordinary microstructure and optimized mechanical properties. To shed further light on the influence of rapid solidification mechanism on mechanical properties, both the microgravity solidification mechanism and resultant micromechanical properties of ternary Al57Ag12Ge31 alloy were analyzed by means of drop tube, nanoindentation and frictional sliding techniques, which was compared with equilibrium solidification condition. The solidification pathways changed with the decrease of droplet size, owing to a larger cooling rate and a higher undercooling. Consequently, the microstructure transformed from dendrites plus two-phase eutectic to two-phase eutectics, eventually to anomalous ternary eutectic, while the thickness of surface (Ge) layer decreased. The micromechanical properties of rapidly solidified alloy droplets were evidently improved with the decrease of droplet size, which is mainly ascribed to the microstructure refinement and the homogenous distribution especially of hardening (Ge) phase. The measured microhardness, yield strength, strain hardening exponent, pile-up resistance and friction coefficient were analyzed as a function of droplet size.

Directional solidification of aqueous TiO2 suspensions under reduced gravity

Porous materials exhibiting aligned, elongated pore structures can be created by directional solidification of aqueous suspensions—where particles are rejected from a propagating ice front and form interdendritic, particle-packed walls—followed by sublimation of the ice, and sintering of the particle walls. Theoretical models that predict dendritic lamellae spacing—and thus wall and pore width in the final materials—are currently limited due to an inability to account for gravity-driven convective effects during solidification. Here, aqueous suspensions of 10–30 nm TiO2 nanoparticles are solidified on parabolic flights under micro-, lunar (∼0.17 g; gl = 1.62 m/s2), and Martian (∼0.38 g; gm = 3.71 m/s2) gravity and compared to terrestrially-solidified samples. After ice sublimation and sintering, all resulting TiO2 materials exhibit elongated lamellar pores replicating the ice dendrites. Increasing the TiO2 fraction in the suspensions leads to decreased lamellar spacing in all samples, regardless of gravitational acceleration. Consistent with previous studies of microgravity solidification of binary metallic alloys, lamellar spacing decreases with increasing gravitational acceleration. Mean lamellar spacing for 20 wt% TiO2 nanoparticles suspensions under micro-, lunar, Martian, and terrestrial gravity are, respectively: 50 ± 8, 34 ± 11, 30 ± 6, and 23 ± 9 μm, indicating that gravity-driven convection strongly affects lamellae spacing under terrestrial gravity conditions. Gravitational effects on lamellar spacing are highest at low TiO2 fractions in the suspension; for 5 wt% TiO2 suspensions, the microgravity lamellar spacing is more than twice that under terrestrial gravity (182 ± 21 vs. 81 ± 23 μm). Results of this study are in good agreement with previous studies of binary metallic alloy solidification where primary dendrite spacing increases under microgravity. Literature data from ice-templating systems are used to discuss a dependence on lamellae spacing of the density ratio of particles and fluid.

Numerical investigation of solidification and CET of the transparent alloy NPG-37.5 wt.% DC in microgravity “TRACE” experiment

A solidification experiment “TRACE” of the transparent alloy Neopentylglycol (NPG)-37.5wt.% D-Camphor (DC) was conducted on-board the sounding rocket TEXUS-47 in low-gravity environment to investigate the columnar growth and the columnar-to-equiaxed transition (CET). To improve the fundamental understanding of solidification and CET in microgravity, the current laboratory scale experiment was tried to be numerically reproduced by a recently developed 5-phase volume averaging model. The temperature gradient in the solidification cell is applied to the simulation. In absence of melt flow, the calculated cooling curves, columnar tip position, tip undercooling and velocity, and number density of equiaxed crystals were compared to the results of in-situ real-time observations of the experiment. The CET could be predicted at position close to that of experiment. Simulation reveals the competitive growth between the columnar and equiaxed crystals before CET. Modelling parameters of equiaxed nucleation and columnar tip growth are the key to regulate this competition and to locate the CET. Experimental verification of modelling parameters considering melt flow is intended in the future work.

XRMON-GF: A novel facility for solidification of metallic alloys with in situ and time-resolved X-ray radiographic characterization in microgravity conditions

As most of the phenomena involved during the growth of metallic alloys from the melt are dynamic, in situ and time-resolved X-ray imaging should be retained as the method of choice for investigating the solidification front evolution. On Earth, the gravity force is the major source of various disturbing effects (natural convection, buoyancy/sedimentation, and hydrostatic pressure) which can significantly modify or mask certain physical mechanisms. Therefore solidification under microgravity is an efficient way to eliminate such perturbations to provide unique benchmark data for the validation of models and numerical simulations. Up to now, in situ observation during microgravity solidification experiments were limited to the investigations on transparent organic alloys, using optical methods. On the other hand, in situ observation on metallic alloys generally required synchrotron facilities. This paper reports on a novel facility we have designed and developed to investigate directional solidification on metallic alloys in microgravity conditions with in situ X-ray radiography observation. The facility consists of a Bridgman furnace and an X-ray radiography device specifically devoted to the study of Al-based alloys. An unprecedented experiment was recently performed on board a sounding rocket, with a 6min period of microgravity. Radiographs were successfully recorded during the entire experiment including the melting and solidification phases of the sample, with a Field-of-View of about 5mm×5mm, a spatial resolution of about 4µm and a frequency of 2 frames per second. Some preliminary results are presented on the solidification of the Al–20wt% Cu sample, which validate the apparatus and confirm the potential of in situ X-ray characterization for the investigation of dynamical phenomena in materials processing, and particularly for the studying of metallic alloys solidification.

Thermoelectric n-type silicon germanium synthesized by unidirectional solidification in microgravity

Thermoelectric n-type Si0.2Ge0.8 and Si0.7Ge0.3 added 1at%P for Si–Ge were synthesized by unidirectional solidification in microgravity. Microgravity with ±10−2g for 0.46s was obtained in free fall using a 2m-drop tower. The microstructure of the sample solidified in microgravity and 1g was dendrite on the surface that contacted a Cu chill block. The microstructure of the cross section along the cooling direction was dendrite and the columnar dendrite structure was mainly aligned along the solidification direction. The dendrite became larger with depth from the surface that contacted the Cu chill block, and the width of the primary dendrite arm solidified in microgravity exceeded that in 1g. Ge was segregated to the secondary arm of columnar dendrite. The Si/Ge atomic ratio in the primary and secondary arm of Si0.7Ge0.3-1at%P solidified in 1g and microgravity was slightly higher than that of solidified Si0.8Ge0.2-1at%P in 1g and microgravity. The secondary arm of solidified Si0.7Ge0.3-1at%P was wider than that of solidified Si0.8Ge0.2-1at%P. P was distributed uniformly in Si–Ge solidified in microgravity. The electrical conductivities of Si0.8Ge0.2-1at%P and Si0.7Ge0.3-1at%P were anisotropic in directions along and perpendicular to the solidification direction. The dimensionless Figure of Merit, ZT, of the direction along the solidification direction at 1000K was estimated from thermal conductivity, electrical conductivity adjusted by the anisotropy, and the Seebeck coefficient in 1g and microgravity. The ZT of the sample solidified in microgravity at 1000K along the solidification direction was 1.19.

Preparation of ZrNiSn half-Heusler compounds with crystalline alignment by unidirectional solidification in short-duration microgravity and their thermoelectric properties

A ZrNiSn half-Heusler compound was prepared by unidirectional solidification in short-duration microgravity. When a ZrNiSn half-Heusler pellet with Zr:Ni:Sn = 1:1:1(in mole) was solidified in microgravity (m-g) by two different cooling systems, the sample solidified in m-g had a different crystallographic alignment of ZrNiSn phase along the cooling direction. However, both sample solidified in normal gravity (1-g) had no crystalline alignment. The solidified sample in m-g had higher electrical conductivity along the cooling direction than perpendicular to the cooling direction, although these samples did not exhibit Seebeck coefficient anisotropy. This electrical conductivity anisotropy was found to be affected by crystalline alignment of the sample. The thermal conductivity was also affected by the structural difference of the sample.

Synthesis of Giant Magnetostrictive Iron-rich Sm-Fe Alloy by Unidirectional Solidification in Microgravity

Synthesis of Giant Magnetostrictive Iron-rich Sm-Fe Alloy by Unidirectional Solidification in Microgravity

微小重力環境を利用したZr-Ni-Sn合金の配向制御と熱電特性

微小重力環境を利用したZr-Ni-Sn合金の配向制御と熱電特性