SEARCH · Search NASA

Results for “Rapid Solidification”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 253 records · Page 14

All sources NASA NTRS DOE OSTI

NASA NTRS1992

The three stages of magma ocean cooling

Models of magma ocean (MO) cooling and crystallization can provide important constraints on MO plausibility for a given planet, on the origin of long term, stable crusts, and even on the origin of the solar system. Assuming the MO is initially extensive enough to have a mostly molten surface, its first stage of cooling is an era of radiative heat loss from the surface, with extremely rapid convection below, and no conductive layer in between. The development of the chill crust starts the second stage of MO cooling. Heat loss is now limited by conduction through the crust. The third stage of cooling starts when the near surface MO evolves compositionally to the point of saturation with feldspar. At this point, the cooling rate again precipitously diminishes, the rate of crustal thickness growth as a function of temperature suddenly increases. More work on incorporating chemical constraints into the evolving physical models of MO solidification would be worthwhile.

Warren, Paul H.↗

NASA NTRS2001

Quench Module Insert Capabilities and Development Test Results

The Quench Module Insert is a directional solidification furnace that will operate in the Materials Science Research Rack (MSRR) on the International Space Station. It will provide high thermal gradient and quench capabilities for processing metals and alloys in microgravity. Numerical analyses and breadboard testing conducted to date show that the QMI can produce an axial temperature gradient approaching 150 C per centimeter in a 1 centimeter diameter aluminum sample with a maximum molten sample temperature of 1100 C. Breadboard testing and analysis have also shown that the quench capabilities of the furnace are sufficient to rapidly solidify at least a 5mm axial portion of a 1 centimeter diameter molten aluminum sample in significantly less than the required 2 seconds and prevent significant backmelt. This paper presents the furnace requirements and capabilities and a status of the associated development testing and analyses.

Carswell, William E.↗

NASA NTRS2000

The Life Cycle Application of Intelligent Software Modeling for the First Materials Science Research Rack

Marshall Space Flight Center (MSFC) has been funding development of intelligent software models to benefit payload ground operations for nearly a decade. Experience gained from simulator development and real-time monitoring and control is being applied to engineering design, testing, and operation of the First Material Science Research Rack (MSRR-1). MSRR-1 is the first rack in a suite of three racks comprising the Materials Science Research Facility (MSRF) which will operate on the International Space Station (ISS). The MSRF will accommodate advanced microgravity investigations in areas such as the fields of solidification of metals and alloys, thermo-physical properties of polymers, crystal growth studies of semiconductor materials, and research in ceramics and glasses. The MSRR-1 is a joint venture between NASA and the European Space Agency (ESA) to study the behavior of different materials during high temperature processing in a low gravity environment. The planned MSRR-1 mission duration is five (5) years on-orbit and the total design life is ten (IO) years. The MSRR-1 launch is scheduled on the third Utilization Flight (UF-3) to ISS, currently in February of 2003). The objective of MSRR-1 is to provide an early capability on the ISS to conduct material science, materials technology, and space product research investigations in microgravity. It will provide a modular, multi-user facility for microgravity research in materials crystal growth and solidification. An intelligent software model of MSRR-1 is under development and will serve multiple purposes to support the engineering analysis, testing, training, and operational phases of the MSRR-1 life cycle development. The G2 real-time expert system software environment developed by Gensym Corporation was selected as the intelligent system shell for this development work based on past experience gained and the effectiveness of the programming environment. Our approach of multi- uses of the simulation model and its intuitive graphics capabilities is providing a concurrent engineering environment for rapid prototyping and development. Operational schematics of the MSRR-1 electrical, thermal control, vacuum access, and gas supply systems, and furnace inserts are represented graphically in the environment. Logic to represent first order engineering calculations is coded into the knowledge base to simulate the operational behavior of the MSRR-1 systems. An example of engineering data provided includes electrical currents, voltages, operational power, temperatures, thermal fluid flow rates. pressures, and component status indications. These type of data are calculated and displayed at appropriate instrumentation points, and the schematics are animated to reflect the simulated operational status of the MSRR-1. The software control functions are also simulated to represent appropriate operational behavior based on automated control and response to commands received by the crew or ground controllers. The first benefit of this simulation environment is being realized in the high fidelity engineering analysis results from the electrical power system G2 model. Secondly, the MSRR-1 simulation model will be embedded with a hardware mock-up of the MSRR-1 to provide crew training on MSRR-1 integrated payload operations. G2 gateway code will output the simulated instrumentation values, termed as telemetry, in a flight-like data stream so that the crew has realistic and accurate simulated MSRR-1 data on the flight displays which will be designed for crew use. The simulation will also respond appropriately to crew or ground initiated commands, which will be part of normal facility operations. A third use of the G2 model is being planned; the MSRR-1 simulation will be integrated with additional software code as part of the test configuration of the primary onboard computer, or Master Controller, for MSRR-1. We will take advantage of the G2 capability to simulate the flight like data stream to test flight software responses and behavior. A fourth use of the G2 model will be to train the Ground Support Personnel that will monitor the MSRR-1 systems and payloads while they are operating aboard the ISS. The intuitive, schematic based environment will provide an excellent foundation for personnel to understand the integrated configuration and operation of the MSRR-1, and the anticipated telemetry feedback based on operational modes of the equipment. Expert monitoring features will be enhanced to provide a smart monitoring environment for the operators. These features include: (1) Animated, intuitive schematic-based displays which reflect telemetry values, (1) Real-time plotting of simulated or incoming sensor values, (3) High/Low exception monitoring for analog data, (4) Expected state monitoring for discrete data, (5) Data trending, (6) Automated malfunction procedure execution to diagnose problems, (7) Look ahead capability to planned MSRR-1 activities in the onboard timeline. And finally, the logic to calculate telemetry values will be deactivated, and the same environment will interface to the incoming data for the real-time telemetry stream to schematically represent the onboard hardware configuration. G2 will be the foundation for the real-time monitoring and control environment. In summary, our MSRR-1 simulation model spans many elements of the life cycle development of this project: Engineering Analysis, Test and Checkout, Training of Crew and Ground Personnel, and Real-time monitoring and control. By utilizing the unique features afforded by an expert system development environment, we have been able to synergize a powerful tool capable of addressing our project needs at every phase of project development.

Rice, Amanda↗

NASA NTRS2000

A Technique for Rapidly Deploying a Concentration Gradient with Applications to Microgravity

The latter half of the last century has seen rapid advancements in semiconductor crystal growth powered by the demand for high performance electronics in myriad applications. The reduced gravity environment of space has also been used for crystal growth tests, especially in instances where terrestrial growth has largely been unsuccessful. While reduced gravity crystal growth affords some control of the gravity parameter, many crystals grown in space, to date, have structural flaws believed to result from convective motions during the growth phase. The character of these instabilities is not well understood but is associated with thermal and solutal density variations near the solidification interface in the presence of residual gravity and g-jitter. In order to study these instabilities in a separate, controlled space experiment, a concentration gradient would first have to be artificially established in a timely manner as an initial condition. This is generally difficult to accomplish in a microgravity environment because the momentum of the fluid injected into a test cell tends to swirl around and mix in the absence of a restoring force. The use of magnetic fields to control the motion and position of liquids has received growing interest in recent times. The possibility of using the force exerted by a non-uniform magnetic field on a ferrofluid to not only achieve fluid manipulation but also to actively control fluid motion makes it an attractive candidate for space applications. This paper describes a technique for quickly establishing a linear or exponential fluid concentration gradient using a magnetic field in place of gravity to stabilize the deployment. Also discussed is a photometric technique for measuring the concentration profile using light attenuation. Results of the ground-based experiments indicate that the concentration distribution is within 3% of the predicted value. Although any range of concentations can be realized, photometric constraints are discussed which impose some limitations on measurements.

Leslie, Fred↗

NASA NTRS1986

Mechanical property anisotropy in superalloy El-929 directionally solidified by an exothermic technique

Directional solidification (DS) of the nickel-based superalloy El-929 was carried out by employing the exothermic technique for preparing several 150 mm long x 55 mm diameter rods. Specimens machined from blanks cut at 0, 45, 75 and 90 deg to the chill surface were tensile and stress-rupture tested at different temperatures. The air-melted DS alloy, when loaded parallel to the growth direction, shows considerable improvement in stress-rupture life and tensile ductility as compared with the vacuum induction melted, forged and heat-treated alloy. However, these property advantages rapidly degrade with the increasing deviation of the load axis from the growth direction.

Pradhan, D. C.↗

DOE OSTI2024

MaterialsMap: A CALPHAD-based tool to design composition pathways through feasibility map for desired dissimilar materials, demonstrated with resistance spot welding joining of Ag-Al-Cu

Assembly of dissimilar metals can be achieved by different methods, for example, casting, welding, and additive manufacturing (AM). However, undesired phases formed in liquid-phase assembling processes due to solute segregation during solidification diminish mechanical and other properties of the processed parts. In the present work, an open-source software named MaterialsMap, has been developed based on the CALculation of Phase Diagrams (CALPHAD) approach. The primary objective of MaterialsMap is to facilitate the design of an optimal composition pathway for assembling dissimilar alloys with liquid-phases based on the formation of desired and undesired phases along the pathway. In MaterialsMap, equilibrium thermodynamic calculations are used to predict equilibrium phases formed at slow cooling rate, while Scheil-Gulliver simulations are employed to predict non-equilibrium phases formed during rapid cooling. Additionally, by combining these two simulations, MaterialsMap offers a thorough guide for understanding phase formation in various manufacturing processes, assisting users in making informed decisions during material selection and production. As a demonstration of this approach, a compositional pathway was designed from pure Al to pure Cu through Ag using MaterialsMap. The design was experimentally verified using resistance spot welding (RSW).

36 MATERIALS SCIENCE↗

NASA NTRS1986

Comparison of structures of gas atomized and of emulsified highly undercooled Ni-Sn alloy droplets

A comparison is made of microstructures of droplets of Ni-Sn alloys rapidly solidified by gas atomization and in a glass emulsifying medium. Cooling rate of the gas atomized particles ranged from 10 to the 3rd to 10 to the 6th K/s depending on droplet diameter (20-230 microns). In the hypoeutectic alloy studied, /Ni-(25 wt pct Sn)/, most particles showed a dendritic structure. These same particles, melted and resolidified in a glass medium using DTA (Differential Thermal Analysis), showed undercoolings up to 280 K: the structures were dendritic at low undercoolings and nondendritic at undercoolings above 220 K. It is concluded that the gas atomized particles exhibited little or no undercooling before nucleation; the solidification time of the undercooled emulsified droplets is substantially less than that of gas atomized droplets, and the undercooling required to achieve nondendritic structure depends on sample size.

Yamamoto, M.↗

NASA NTRS1992

Solidification and morphology of submarine lavas - A dependence on extrusion rate

The results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity. The experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter, the ratio of the time taken for the surface to solidify, and a time scale for lateral flow. For submarine basalts a solution of the cooling problem (which is dominated by conduction in the lava but convective heat transfer in the water) and estimates of lava viscosities place this parameter within the empirically determined 'pillowing' regime over a wide range of extrusion rates. This results is consistent with the observation that pillow basalts are the most common products of submarine eruptions. Smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusion of basaltic magma. Still higher eruption rates in regions of low topographic relief may produce submarine lava lakes. Minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type.

Griffiths, Ross W.↗

NASA NTRS1986

A study of the microstructure of a rapidly solidified nickel-base superalloy modified with boron

The microstructures of melt-spun superalloy ribbons with variable boron levels have been studied by transmission electron microscopy. The base alloy was of approximate composition Ni-11% Cr-5%Mo-5%Al-4%Ti with boron levels of 0.06, 0.12, and 0.60 percent (all by weight). Thirty micron thick ribbons display an equiaxed chill zone near the wheel contact side which develops into primary dendrite arms in the ribbon center. Secondary dendrite arms are observed near the ribbon free surface. In the higher boron bearing alloys, boride precipitates are observed along grain boundaries. A concerted effort has been made to elucidate true grain shapes by the use of bright field/dark field microscopy. In the low boron alloy, grain shapes are often convex, and grain faces are flat. Boundary faces frequently have large curvature, and grain shapes form concave polygons in the higher boron level alloys. It is proposed that just after solidification, in all of the alloys studied, grain shapes were initially concave and boundaries were wavy. Boundary straightening is presumed to occur on cooling in the low boron alloy. Boundary migration is precluded in the higher boron alloys by fast precipitation of borides at internal interfaces.

Speck, J. S.↗

NASA NTRS1998

Acquisition and Early Losses of Rare Gases from the Deep Earth

Direct observations show that the deep Earth contains rare gases of solar composition distinct from those in the atmosphere. We examine the implications of mantle rare gas characteristics on acquisition of rare gases from the solar nebula and subsequent losses due to a large impact. Deep mantle rare gas concentrations and isotopic compositions can be obtained from a model of transport and distribution of mantle rare gases. This model assumes the lower mantle closed early, while the upper mantle is open to subduction from the atmosphere and mass transfer from the lower mantle. Constraints are derived that can be incorporated into models for terrestrial volatile acquisition: (1) Calculated lower-mantle Xe-isotopic ratios indicate that the fraction of radiogenic Xe produced by I-129 and Pu-244 during the first about 10(exp 8) yr was lost, a conclusion also drawn for atmospheric Xe. Thus, either the Earth was made from materials that had lost >99% of rare gases about (0.7-2) x 10(exp 8) yr after the solar system formed, or gases were then lost from the fully formed Earth. (2) Concentrations of 3He and 20Ne in the lower mantle were established after these losses. (3) Neon-isotopic data indicates that mantle Ne has solar composition. The model allows for solar Ar/Ne and Xe/Ne in the lower mantle if a dominant fraction of upper mantle Ar and Xe are subduction-derived. If Earth formed in the presence of the solar nebula, it could have been melted by accretional energy and the blanketing effect of a massive, nebula-derived atmosphere. Gases from this atmosphere would have been sequestered within the molten Earth by dissolution at the surface and downward mixing. It was found that too much Ne would be dissolved in the Earth unless the atmosphere began to escape when the Earth was only partially assembled. Here we consider conditions required to initially dissolve sufficient rare gases to account for the present lower mantle concentrations after subsequent losses at 10(exp 8) yr. It is assumed that equilibration of the atmosphere with a thoroughly molten mantle was rapid, so that initial abundances of gases retained in any mantle layer reflected surface conditions when the layer solidified. For subsequent gas loss of 99.5% and typical solubility coefficients, a total pressure of 100 atm was required for an atmosphere of solar composition. Calculations of the pressure at the base of a primordial atmosphere indicate that this value might be exceeded by an order of magnitude or more for an atmosphere supported by accretional energy. Surface temperatures of about 4000 K would have been produced, probably high enough to melt the deep mantle. Initial distributions of retained rare gases would then be determined by the history of surface pressure and temperature during mantle cooling and solidification, i.e., the coupled cooling of Earth and atmosphere. The Earth's thermal state was determined by its surface temperature and the efficiency of convection in the molten mantle, estimated to be sufficient to maintain an adiabatic gradient. Because the melting curve is steeper than the adiabat, solidification of the mantle proceeded outward from the interior. Incorporation of atmospheric gases in the mantle therefore occurred over a range in surface temperature of a few thousand degrees Kelvin. The thermal state of the atmosphere was controlled by total luminosity of the Earth (energy) released by accreting planetesimals and the cooling Earth), nebular temperature and pressure, and atmospheric opacity. The energy released by accretion declined with time as did nebular pressure. Analytical solutions for an idealized (constant opacity radiative atmosphere show that declining energy sources under constant nebular conditions result in slowly diminishing surface temperature but dramatically increasing surface pressure. For such an atmosphere with declining nebular pressure but constant total luminosity, surface pressure decreases gradually with decreasing temperaure. A decline in accretion luminosity might be compensated by energy released as the mantle cools for about 10(exp 5) year, after which luminosity must decline. The total complement of dissolved rare gases will depend on the particular evolutionary path determined by the declining accretional luminosity, the Earth thermal history, removal of the nebula, and opacity variations of the atmosphere. Models for these coupled evolutionary histories for Earth's acquisition of nebular-derived noble gases are in progress. The later losses required at about 10(exp 8) yr (depleting the interior concentrations of the sequestered solar gases by a factor of > 100) were presumably related to the major impact in which the Moon formed.

Porcelli, D.↗

NASA NTRS1974

Study of ceramic products and processing techniques in space

An analysis of the solidification kinetics of beta alumina in a zero-gravity environment was carried out, using computer-simulation techniques, in order to assess the feasibility of producing high-quality single crystals of this material in space. The two coupled transport processes included were movement of the solid-liquid interface and diffusion of sodium atoms in the melt. Results of the simulation indicate that appreciable crystal-growth rates can be attained in space. Considerations were also made of the advantages offered by high-quality single crystals of beta alumina for use as a solid electrolyte; these clearly indicate that space-grown materials are superior in many respects to analogous terrestrially-grown crystals. Likewise, economic considerations, based on the rapidly expanding technological applications for beta alumina and related fast ionic conductors, reveal that the many superior qualities of space-grown material justify the added expense and experimental detail associated with space processing.

Markworth, A. J.↗

NASA NTRS2000

BUNDLE: A Novel Furnace for Performing Controlled Directional Solidification Experiments in a Microgravity Environment

NASA Marshall Space Flight Center has developed a novel directional solidification furnace prototype for processing metals and alloys experiments in a microgravity environment. The BUNDLE (Bridgman Unidirectional Dendrite in Liquid Experiment) furnace is intended to accommodate the science requirements of Flight Definition Principle Investigators studying cellular/dendritic growth in aluminum and lead alloys at processing temperatures up to 1200 C. The furnace implements a number of innovative features to achieve high thermal gradients and quench rates in a low-power, light-weight design. These include a pyrolytic boron nitride/graphite composite heating element surrounded by layers of self-supporting refractory metal shielding, and a graphite fiber enhanced cold zone allowing high levels of heat extraction from the sample crucible. Novel to the BUNDLE design is an in-situ helium gas quench capability that ensures rapid freezing of the solidifying region (mushy zone) of the metal sample within the furnace without sample disturbance prior to quenching; this is a stringent requirement for subsequent analysis and understanding of microstructural development. The experiment hardware concept features multiple furnaces that may be "bundled" together so many samples, currently up to eight, can be processed at one time. The design of BUNDLE is flexible enough to be implemented on the Shuttle and Space Station in a number of locations (SpaceHab, Express Rack, MPESS, ISPR, etc). BUNDLE prototype furnaces have directionally solidified and quenched 1cm diameter lead - 5.8 weight percent antimony and aluminum - 4 weight percent copper alloys. Quenching of the mushy zone, as recorded by in-situ thermocouples, occurred on the order of 0.5 seconds or less, a rate within the PI's requirements. Subsequent metallographic examination revealed the solidified microstructure to be, as expected, unidirectional. Both the dendrite tips and the eutectic reaction were planar in nature indicating uniform axial heat flow. Delineation between the growing dendrites and eutectic structure with the "quenched-in" liquid was sharp, attesting to the efficacy of the helium quench. BUNDLE's conception, development, capability, and adaptability are presented (in view of Flight PI's needs and science requirements) through viewgraphs depicting actual hardware, generated thermal analysis, and micrographs prepared from BUNDLE processed, flight-like samples.

Carrasquillo, Edgar J.↗

NASA NTRS2001

BUNDLE - A Novel Furnace for Performing Controlled Directional Solidification Experiments in a Microgravity Environment

Carrasquillo, Edgar J.↗

DOE OSTI2026

Solidification cracking of refractory alloys: a computational and machine learning study to investigate composition-dependence for improved weldability and additive manufacturability

Large-batch numerical, CALculation of PHAse Diagrams (CALPHAD)-based solidification cracking calculations are performed and then analyzed with machine learning methods to generate models that relate chemistry of refractory alloys to cracking susceptibility. Kou’s solidification cracking index is used to study the refractory alloys including O, N, C binary mixtures with Mo, Ta, Nb, and W, the molybdenum-based TZM, Niobium-based C103, and Tantalum-based T111 and Ta-10 W, as well as hypothetical refractory ternary alloys. Findings strongly validate Kou’s Crack Susceptibility Index (CSI) against Varestraint test data for Nb- and Ta-based alloys, establishing CSI thresholds where refractory alloys with CSI < 15,000 K are likely weldable, CSI > 15,000 K are prone to cracking, and CSI > 25,000 K are likely unweldable (or unprintable). Furthermore, interstitial elements C, N, and O significantly increase crack susceptibility, with some existing material specifications coinciding with peak cracking susceptibility concentrations. Finally, machine learning-derived elemental potency factors enable rapid prediction of CSI from alloy chemistry for C103, TZM, Ta-10 W, and T-111 alloys. These results provide practical guidance for feedstock selection, powder reuse limits, and alloy specification amendments for welding and additive manufacturing applications.

36 MATERIALS SCIENCE↗

NASA NTRS1986

Undercooled and rapidly quenched Ni-Mo alloys

Hypoeutectic, eutectic, and hypereutectic nickel-molybdenum alloys were rapidly solidified by both bulk undercooling and melt spinning techniques. Alloys were undercooled in both electromagnetic levitation and differential thermal analysis equipment. The rate of recalescence depended upon the degree of initial undercooling and the nature (faceted or nonfaceted) of the primary nucleating phase. Alloy melts were observed to undercool more in the presence of primary Beta (NiMo intermetallic) phase than in gamma (fcc solid solution) phase. Melt spinning resulted in an extension of molybdenum solid solubility in gamma nickel, from 28 to 37.5 at % Mo. Although the microstructures observed by undercooling and melt spinning were similar the microsegregation pattern across the gamma dendries was different. The range of microstructures evolved was analyzed in terms of the nature of the primary phase to nucleate, its subsequent dendritic growth, coarsening and fragmentation, and final solidification of interfenderitic liquid.

Tewari, S. N.↗

NASA NTRS1987

Undercooled and rapidly quenched Ni-Mo alloys

Hypoeutectic, eutectic, and hypereutectic nickel-molybdenum alloys were rapidly solidified by both bulk undercooling and melt spinning techniques. Alloys were undercooled in both electromagnetic levitation and differential thermal analysis equipment. The rate of recalescence depended upon the degree of initial undercooling and the nature (faceted or nonfaceted) of the primary nucleating phase. Alloy melts were observed to undercool more in the presence of primary Beta (NiMo intermetallic) phase than in gamma (fcc solid solution) phase. Melt spinning resulted in an extension of molybdenum solid solubility in gamma nickel, from 28 to 37.5 at. pct Mo. Although the microstructures observed by undercooling and melt spinning were similar, the microsegregation pattern across the gamma dendries was different. The range of microstructures evolved was analyzed in terms of the nature of the primary phase to nucleate, its subsequent dendritic growth, coarsening and fragmentation, and final solidification of interfenderitic liquid.

Tewari, S. N.↗

NASA NTRS2003

First Post-Flight Status Report for the Microgravity Science Glovebox

The Microgravity Science Glovebox (MSG) was launched to the International Space Station (ISS) this year on the second Utilization Flight (UF2). After successful on-orbit activation, the facility began supporting an active microgravity research program. The inaugural NASA experiments operated in the unit were the Solidification Using a Baffle in Sealed Ampoules (SUBSA, A. Ostrogorski, PI), and the Pore Formation and Mobility (PFMI, R. Grugel, PI) experiments. Both of these materials science investigations demonstrated the versatility of the facility through extensive use of telescience. The facility afforded the investigators with the capability of monitoring and operating the experiments in real-time and provided several instances in which the unique combination of scientists and flight crew were able to salvage situations which would have otherwise led to the loss of a science experiment in an unmanned, or automated, environment. The European Space Agency (ESA) also made use of the facility to perform a series of four experiments that were carried to the ISS via a Russian Soyuz and subsequently operated by a Belgium astronaut during a ten day Station visit. This imaginative approach demonstrated the ability of the MSG integration team to handle a rapid integration schedule (approximately seven months) and an intensive operations interval. Interestingly, and thanks to aggressive attention from the crew, the primary limitation to experiment thru-put in these early operational phases is proving to be the restrictions on the up-mass to the Station, rather than the availability of science operations.

Baugher, Charles R., III↗

NASA NTRS2003

Data and Modeling of Dendrites Subject to A Step Change in Pressure (TDSE)

There is considerable interest in dendritic solidification because of the influence dendrites have in the determination of microstructure, and thereby in the physical properties of cast metals and alloys. Current theories and models of dendritic growth generally couple diffusion effects in the melt with the physics of the interface. Data and subsequent analysis prior of the tip growth speed and radii of thermal succinonitrile dendrites in the near-convection free, on-orbit, free-fall environment demonstrate that these theories yield predictions that are reasonably in agreement with the results of experiment. However, data and analysis for assessing the interfacial physics component of theory are not sufficiently detailed or definitive. To study fundamental aspects of dendritic interface stability, we are measuring and modeling the kinetics and morphology of dendrites as they evolve from one well-defined steady state at a pre-set supercooling, through a transient stage, to a different well-defined steady state. More specifically, we subject succinonitrile dendrites, growing under steady-state conditions, to a rapid change in pressure. This leads to a rapid change in thermal driving force from the corresponding change in both the equilibrium melting temperature due to the Clapeyron effect, and a change in the far-field temperature due to adiabatic temperature changes in the bulk liquid and solid. Subsequently, we observe transformations from a well-characterized initial state into a new steady-state. Initial data reveal that the dendrite tip velocity changes almost as fast as the pressure charges, while the tip radius changes occur more slowly, taking from 10 60 seconds depending on the size of the step change and the final supercooling. Computer modeling of this process shows both agreements and disagreements with the experimental data. In making these observations and measurements, we are gaining new understandings of interfacial dynamics and state-selection physics.

Koss, Matthew B.↗