Macrosegregation and nucleation in undercooled Pb-Sn alloys

A technique resulting in large undercoolings in bulk samples (23g) of lead-tin alloys was developed. Samples of Pb-12.5 wt percent Sn, Pb-61 wt percent Sn, and Pb-77 wt percent Sn were processed with undercoolings ranging from 4 to 34 K and with cooling rates varying between 0.04 and 4 K/sec. The nucleation behavior of the Pb-Sn system was found to be nonreciprocal. The solid Sn phase effectively nucleated the Pb phase of the eutectic; however, large undercoolings developed in Sn-rich eutectic liquid in the presence of the solid Pb phase. This phenomenon is believed to be mainly the result of differences in interfacial energies between solid Sn-eutectic liquid, and solid Pb-eutectic liquid rather than lattice misfit between Pb and Sn. Large amounts of segregation developed in the highly undercooled eutectic ingots. This macrosegregation was found to increase as undercooling increases. Macrosegregation in these undercooled eutectic alloys was found to be primarily due to a sink/float mechanism and the nucleation behavior of the alloy. Lead-rich dendrites are the primary phase in the undercooled eutectic system. These dendrites grow rapidly into the undercooled bath and soon break apart due to recalescence and Sn enrichment of the liquid. These fragmented Pb dendrites are then free to settle to the bottom portion of the ingot causing the macrosegregation observed in this study. A eutectic Pb-Sn alloy undercooled 20 K and cooled at 4 K/sec had a composition of about Pb-72 wt percent Sn at the top and 55 percent Sn at the bottom

Effects of crucible wetting during solidification of immiscible Pb-Zn

Many industrial uses for liquid phase miscibility gap alloys are proposed. However, the commercial production of these alloys into useful ingots with a reasonable amount of homogeneity is arduous because of their immiscibility in the liquid state. In the low-g environment of space gravitational settling forces are abated, thus solidification of an immiscible alloys with a uniform distribution of phases becomes feasible. Elimination of gravitational settling and coalescence processes in low-g also makes possible the study of other separation and coarsening mechanisms. Even with gravitational separation forces reduced, many low-g experiments have resulted in severely segregated structures. The segregation in many cases was due to preferential wetting of the crucible by one of the immiscible liquids. The objective was to analyze the wetting behavior of Pb-Zn alloys on various crucible materials in an effort to identify a crucible in which the fluid flow induced by preferential wetting is minimized. It is proposed that by choosing the crucible for a particular alloy so that the difference in surface energy between the solid and two liqud phases is minimized, the effects of preferential wetting can be diminished and possibly avoided. Qualitative experiments were conducted and have shown the competitive wetting behavior of the immiscible Pb-Zn system and 13 different crucible materials

Geo-Taxonomy Service Development Summer Internship at the United Nations Office for the Coordination of Humanitarian Affairs, NYC

This report provides a detailed account of my internship experience with the United Nations Office for the Coordination of Humanitarian Affairs (UN-OCHA) in New York, New York during the summer, autumn, and winter of 2016. The internship was completed at Two United Nations Plaza and remotely from Clark University under the supervision of UN-OCHA Information Management Programme Officer, Andrej Verity. While temporarily serving as an international civil servant, I had the unique opportunity to contribute my knowledge and skills to an intergovernmental organization that continuously aims to improve the nations of our world. Adhering to the requirements set forth by the M.S. GISDE program at Clark University, this report thoroughly describes my responsibilities as an intern and relates the opportunity to my career goals and academic history

Minimum control airspeed testing of the F/A-18 E/F airplane

The minimum control airspeed for an airplane has been classically defined based upon theory and methodology applicable to multi-engine, propeller-driven and low thrust-to-weight engine airplanes. This testing has traditionally been performed assuming aerodynamic characteristics remain constant throughout the test angle of attack (AOA) range, and controllability was primarily a function of dynamic pressure. For these airplanes, thrust level, thrust degradation and the interdependencies with the single engine minimum control airspeed were simple to flight test and analyze, as the results could be linearly extrapolated to a reference, sea level, standard day, condition. These extrapolations to reference conditions are critical to shipboard operations as these airspeeds are used as a basis to establish minimum catapult takeoff airspeeds during shipboard operations. Once established, safety margins over-and-above these airspeeds are applied to ensure controllability of the airplane is maintained in the event of a catastrophic engine failure during the critical catapult takeoff flight phase. For the modern high thrust-to-weight fighter airplane, VmcA is largely dependent on atmospheric conditions and the classical test techniques are no longer valid and are unsafe. During the F/A-18 E/F Engineering and Manufacturing Development (EMD) program, flight test results revealed additional VmcA dependencies on AOA, and lateral weight asymmetry. As a result, the test techniques and analysis of the results were significantly more complex to analyze. This thesis discusses the methodology used to establish and normalize the single engine minimum control airspeed flight test data for the F/A-18 E/F airplane, carrier environment, shipboard launching process, and the flight test demonstration requirements for airplanes which are catapult launched from ships. These discussions also include operational considerations, which must be made relative to operating in the shipboard environment

Meteoroid and Orbital Debris Threats to NASA's Docking Seals: Initial Assessment and Methodology

The Crew Exploration Vehicle (CEV) will be exposed to the Micrometeoroid Orbital Debris (MMOD) environment in Low Earth Orbit (LEO) during missions to the International Space Station (ISS) and to the micrometeoroid environment during lunar missions. The CEV will be equipped with a docking system which enables it to connect to ISS and the lunar module known as Altair; this docking system includes a hatch that opens so crew and supplies can pass between the spacecrafts. This docking system is known as the Low Impact Docking System (LIDS) and uses a silicone rubber seal to seal in cabin air. The rubber seal on LIDS presses against a metal flange on ISS (or Altair). All of these mating surfaces are exposed to the space environment prior to docking. The effects of atomic oxygen, ultraviolet and ionizing radiation, and MMOD have been estimated using ground based facilities. This work presents an initial methodology to predict meteoroid and orbital debris threats to candidate docking seals being considered for LIDS. The methodology integrates the results of ground based hypervelocity impacts on silicone rubber seals and aluminum sheets, risk assessments of the MMOD environment for a variety of mission scenarios, and candidate failure criteria. The experimental effort that addressed the effects of projectile incidence angle, speed, mass, and density, relations between projectile size and resulting crater size, and relations between crater size and the leak rate of candidate seals has culminated in a definition of the seal/flange failure criteria. The risk assessment performed with the BUMPER code used the failure criteria to determine the probability of failure of the seal/flange system and compared the risk to the allotted risk dictated by NASA's program requirements

Distributed control using linear momentum exchange devices

MSFC has successfully employed the use of the Vibrational Control of Space Structures (VCOSS) Linear Momentum Exchange Devices (LMEDs), which was an outgrowth of the Air Force Wright Aeronautical Laboratory (AFWAL) program, in a distributed control experiment. The control experiment was conducted in MSFC's Ground Facility for Large Space Structures Control Verification (GF/LSSCV). The GF/LSSCV's test article was well suited for this experiment in that the LMED could be judiciously placed on the ASTROMAST. The LMED placements were such that vibrational mode information could be extracted from the accelerometers on the LMED. The LMED accelerometer information was processed by the control algorithms so that the LMED masses could be accelerated to produce forces which would dampen the vibrational modes of interest. Experimental results are presented showing the LMED's capabilities