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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Noncontact temperature measurement: Requirements and applications for metals and alloys research

Temperature measurement is an essential capability for almost all areas of metals and alloys research. In the microgravity environment many of the science priorities that have been identified for metals and alloys also require noncontact temperature measurement capability. For example, in order to exploit the full potential of containerless processing, it is critical to have available a suitable noncontact temperature measurement system. This system is needed to track continuously the thermal history, including melt undercooling and rapid recalescence, of relatively small metal spheres during free-fall motion in drop tube systems. During containerless processing with levitation-based equipment, accurate noncontact temperature measurement is required to monitor one or more quasi-static samples with sufficient spatial and thermal resolution to follow the progress of solidification fronts originating in undercooled melts. In crystal growth, thermal migration, coarsening and other experiments high resolution thermal maps would be a valuable asset in the understanding and modeling of solidification processes, fluid flows and microstructure development. The science and applications requirements place several constraints on the spatial resolution, response time and accuracy of suitable instrumentation.

Perepezko, J. H.↗
NASA NTRS1987

Crystallization behavior of a melt-spun Fe-Ni based steel

Whether Fe-Ni-based alloys solidify with a bcc or fcc structure has been observed by many investigators to be a stronger function of kinetics and undercooling than strictly free-energy minimization. Such behavior has been observed in an Fe(52.8)Ni(28.7)Al(3.4)Ti(6.1)B(9.0) alloy. The alloy was cast as ribbons about 45 microns thick using a dual free-jet variation of chillbock melt spinning against a Cu wheel. Optical, X-ray, and electron analyses of the as-cast and annealed ribbons were performed. A microstructure of at least four layers containing combinations of ecc, bcc, and amorphous phases in differing proportions was observed in the as-cast ribbon. The midthickness layer had the most unusual features, containing fcc grains about 75 nm in size encompassing spherulitic regions as large as 15 microns comprised of fcc grains about 25 nm in size. The crystallization sequence responsible for the as-cast microstructure is discussed in terms of the competition between the formation of bcc and fcc phases as influenced by undercooling, recalescence, and variations in cooling rate experienced by the as-cast ribbon.

Michal, G. M.↗
NASA NTRS1985

Fundamentals of rapid solidification processing

An attempt is made to illustrate the continuous change that occurs in the solidification behavior of undercooled melts, as cooling rates increase from 0.0001 K/sec to about 1000 K/sec. At the higher cooling rates, more significant changes occur as the dendrite tip temperature begins to drop from the equilibrium liquidus. Discontinuous solidification behavior changes will occur if absolute stability is reached, or a metastable phase forms, or solidification proceeds to a glass rather than to a crystalline solid, or if there is significant undercooling prior to nucleation.

Flemings, Merton C.↗
NASA NTRS1989

Morphological transitions in the rapid solidification regime - A re-examination of the fundamental validity of the absolute stability concept of Mullins and Sekerka

The validity of the notion of absolute stability (AS) of a planar solid-liquid interface, proposed by Mullins and Sekerka (M-S, 1964), was tested using experimental data obtained for a number of dilute Ag-Cu and Sn-In alloys and for ultrahigh-purity succinonitrile. It is shown that the restriction applied by the M-S theory is satisfied during the steady-state planar solidification in an undercooled melt, provided the undercooling is quite small. The findings obtained on the Ag-Cu and Sn-In alloys are compared with early measurements by Davis and Fryzuk (1971) in dilute Sn-In alloys with 25 to 400 ppm additions of In.

Laxmanan, V.↗
NASA NTRS1989

Non-ideality near the monotectic composition of a miscibility-gap type system - Succinonitrile-water

Differential scanning calorimetry (DSC) of near monotectic succinonitrile-water solutions, fast-quenched in hydrophilic and hydrophobic DSC pans, indicate, by degree of undercooling, thst there may be significant dependence of final ingot microstructure on the pre-quench equilibration temperature. Partial molal-volume determinations from density data, along with DSC data, suggest the nature of temperature dependent component associations from 20 to 55 C in homogeneous solutions. The undercooling profile in a hydrophilic container is explained in terms of solution-composition shifts arising from the Gibbs surface excess. The evidence shows that temperature-dependent preferred component aggregates may modulate surface-composition gradients. Similar effects may be present through intermetallic compound formation in metallic monotectic alloys.

Frazier, D. O.↗
NASA NTRS1990

TEMPUS: First results

The electromagnetic levitation facility, TEMPUS, developed by Dornier is designed to operate under microgravity conditions. Compared to terrestrial levitation, microgravity offers the possibility to melt and undercool in ultra high vacuum, thereby, providing an ultra clean environment. The technical concept of the TEMPUS facility was tested on two KC 135 flights and in the Texus 22 mission. Preparative investigations concerning the coil system and the heating and positioning efficiencies were carried out in the TEMPUS laboratory version. Furthermore, temperature-time profiles were determined under various boundary conditions. As a consequence of processing liquid metals under UHV, correct temperature measurement arises as the most critical problem. Experiences with experiments in the TEMPUS laboratory module show that due to the evaporation losses of the sample, the transmission of the CaF2 shielding windows changes drastically during the processing time. The investigation of the effect of contamination on pyrometry and the development of alternative evaporation shielding methods were initiated. During the second KC 135 flight, it was possible to heat up and melt an FeNi sample under He atmosphere. Oscillations of the molten sample, which were excited by switching out the magnetic heating field, could be detected and afterwards analyzed. From the frequency of these oscillations the surface tension of the sample material could be derived. The measurement of the surface tension and viscosity of an undercooled metal is proposed for TEMPUS on IML-2. This document is presented in view graph form.

Neuhaus, P.↗
NASA NTRS1990

Influence of convection on rod spacing of eutectics

This paper describes a three-dimensional numerical model to study the influence of convection on the rod-like microstructure of an eutectic system. This model is based on a central finite difference approach. By applying it, the average concentration near the solid/liquid interface of a growing rod-like eutectic was determined for eutectic compositions C(e) of 0.03, 0.05, and 0.10. Following Jackson and Hunt (1966), the average interfacial composition was converted to a change of undercooling at the interface and, finally, to spacing between the rods. The change in rod spacing with increasing intensity of convection was calculated assuming the eutectic grows at minimum interfacial undercooling. It was confirmed that an increase in convection should coarsen the microstructure (i.e., the rod spacing increases with increasing intensity of stirring).

Caram, R.↗
NASA NTRS1991

Rapid solidification of polymorphic transition metals induced by nanosecond laser pulses

An Nd-YAG laser giving 5 ns, 266 nm pulses was used to melt and subsequently quench thin, pure metallic films on a liquid Al/Al2O3 substrate at 10 to the 10th - 10 to the 12th K/s. Transmission electron microscopy together with electron diffraction was used to study the competitive nucleation and growth behavior of the crystals from the undercooled melt. In the case of Fe and Co, the high-temperature bcc and fcc structures were retained after laser quenching. Ti and Zr in spite of the structural similarities exhibit different preferences for nucleation from the undercooled melt. In all the metals the solid-state transformations were completely suppressed and the crystal growth was found to be limited by the collisional frequency of the atoms onto the growing interface.

Vitta, Satish↗
NASA NTRS1991

Containerless processing and rapid solidification of Nb-Si alloys in the niobium-rich eutectic range

Containerless processing and rapid solidification techniques were used to process Nb-Si alloys in the Nb-rich eutectic range. Electromagnetically levitated drops were melted and subsequently splat-quenched from different temperatures. A variety of eutectic morphologies was obtained as a function of the degree of superheating or undercooling of the drops prior to splatting. Metallic glass was observed only in drops quenched from above the melting temperature. Microstructures of splats deeply undercooled prior to quenching were very fine and uniform. These results are discussed in terms of classic nucleation theory concepts and the expected heat evolution at different regions of the splat during the rapid quenching process. The locations of the coupled-zone boundaries for the alpha-Nb + Nb3Si eutectic are also suggested.

Hofmeister, W. H.↗
NASA NTRS1991

Buoyancy effects of a growing, isolated dendrite

The buoyancy effect of a growing isolated dendrite on the solidification process in the undercooling liquid material was investigated by developing an analytic solution to the growth/convection problem in powers of a buoyancy parameter G. The solution depends on the Prandtl number P and the Stefan number S (undercooling) for the local velocity and thermal fields and also the buoyant alteration of the interface shape. Results suggest that buoyancy effect for metals (low P) may be qualitatively different from that for organics (high P).

Canright, D.↗
NASA NTRS1992

An analytical approach for the prediction of gamma-to-alpha phase transformation of aluminum oxide (Al2O3) particles in the Space Shuttle ASRM and RSRM exhausts

The analytical approach developed here utilizes the flow-field output from industry standard nozzle and plume codes as input into a particle phase conversion code which predicts the amount of gamma-to-alpha conversion in SRM exhausts. Sixty different cases were considered which varied such parameters as particle size, degree of undercooling, motor type, and altitude. On-centerline calculations were made for both the ASRM and RSRM at an altitude of 100,000 feet with particle sizes varying from 3.5 to 9.1 micron radius and undercooling varying from 0 to 20 percent. Additional calculations were made for the ASRM at 100,000 feet off centerline and at an altitude of 60,000 feet on centerline. The results indicate that significant amounts of metastable alumina will be present in ASRM and RSRM exhausts. Though not significant to motor performance, this may be important in such issues as environmental effects of rocket exhausts, plume radiative heating predictions, and particle size determination by laser scattering.

Oliver, S. M.↗
NASA NTRS1992

Solidification kinetics and metastable phase formation in binary Ti-Al

Electromagnetic levitation was used to solidify near-equiatomic alloys of Ti-Al at various bulk undercoolings. Detailed thermal histories were acquired during experiments using optical pyrometry with sampling rates as fast as 500 KHz. Solidification and other high-temperature transformation pathways were deduced from the thermal data and microstructural analysis. Recalescence rise times were employed to determine semiquantitative primary solidification kinetics for the different phases. Primary beta solidification was observed at compositions well into the equilibrium alpha regime; this is presented as part of a near-equiatomic nucleation domain diagram that shows the primary solidification phase that results for each combination of nucleation temperature and composition. Solidification kinetics are faster for primary beta than they are for primary alpha. For undercoolings less than about 150 K, the primary solidification kinetics are about an order of magnitude slower for gamma than for alpha.

Anderson, C. D.↗
NASA NTRS1993

Formation of a metastable ferromagnetic tau phase during containerless melt processing in Mn-Al-C alloys

This paper reports the production of a metastable ferromagnetic tau phase directly from the melt in Mn(0.55)Al(0.433)C(0.017), using a containerless processing method involving levitation melting followed by quenching. Using the results from differential thermal analysis measurements and an analysis of the phase equilibria, it was found that the minimum amount of undercooling level required for ferromagnetic metastable tau phase formation in this alloy was Delta T = 87 K. The attainment of this undercooling may be facilitated by the application of containerless melt processing.

Kim, Y. J.↗
NASA NTRS1994

Magnetic properties and magnetic hardening mechansim of Pt-Co-B alloys

The intrinsic coercivity is found to be maximized in the Pt42Co45B13 ternary alloy which is undercooled and rapidly solidified (quenched using a 70 m/s wheel speed after undercooling), and then annealed (800 C for 2400 min). The same alloy, processed at slower cooling rates and annealed in the same way, has a much larger scale microstructure and a much lower resulting magnetic coercivity. The microstructure which would optimize the coercitvity of this coercivity of this ternary alloy is a completely ordered L1(sub zero) Pt-Co matrix with a submicron magnetic single-domion Co-boride precipitate. The L1(sub zero) phase is highly anistropic magnetically while the Co-boride precipate is somewhat less so. Annealing treatments designed to produced single-domain Co-boride precipitates enhance the coercivity. This suggests that the refined microstructures is responsible for the high coercivities found in the rapidly solidified and annealed alloy. The magnetic domain wall thickness for a Co-boride precipitate is determined from both experimental observation and theoretical calculation in order to evaluate its influence on the coercivity of the alloy. The effects of the pinning of domain walls and the barrier to the nucleation of reverse domains on the coercivity are discussed. Both microstrucutral analysis and theoretical calculation indicate that the high coercivities in the Pt42Co45B13 alloy are due to the difficult nucleation of reverse magnetic domains.

Qiu, Ning↗
NASA NTRS1993

Growth of binary organic NLO crystals: m.NA-p.NA and m.NA-CNA system

Experiments were carried out to grow 3.Nitroaniline (m.NA) crystals doped with 4.Nitroaniline (p.NA) and 2.chloro 4.Nitroaniline (CNA). The measured undercooling for m.NA, p.NA, and CNA were 0.21 tm K, 0.23 tm K, and 0.35 tm K respectively, where tm represents the melting temperature of the pure component. Because of the crystals' large heat of fusion and large undercooling, it was not possible to grow good quality crystals with low thermal gradients. In the conventional two-zone Bridgman furnace we had to raise the temperature of the hot zone above the decomposition temperature of CNA, p.NA, and m.NA to achieve the desired thermal gradient. To avoid decomposition, we used an unconventional Bridgman furnace. Two immiscible liquids, silicone oil and ethylene glycol, were used to build a special two-zone Bridgman furnace. A temperature gradient of 18 K/cm was achieved without exceeding the decomposition temperature of the crystal. The binary crystals, m.NA-p.NA and m.NA-CNA, were grown in centimeter size in this furnace. X-ray and optical characterization showed good optical quality.

Singh, N. B.↗
NASA NTRS1998

Containerless Processing in Reduced Gravity Using the TEMPUS Facility during MSL-1 and MSL-1R

Containerless processing provides a high purity environment for the study of high-temperature, very reactive materials. It is an important method which provides access to the metastable state of an undercooled melt. In the absence of container walls, the nucleation rate is greatly reduced and undercooling up to (Tm-Tn)/Tm approx. equal to 0.2 can be obtained, where Tm and Tn are the melting and nucleation temperatures, respectively. Electromagnetic levitation represents a method particularly well-suited for the study of metallic melts. The TEMPUS (Tiegelfreies ElektroMagnetisches Prozessieren Unter Schwerelosgkeit) facility is a research instrument designed to perform electromagnetic levitation studies in reduced gravity. TEMPUS is a joint undertaking between DARA, the German Space Agency, and the Microgravity Science and Applications Division of NASA. The George C. Marshall Space Flight Center provides the leadership for scientific and management efforts which support the four US PI teams which performed experiments in the TEMPUS facility. The facility is sensitive to accelerations in the 1-10 Hz range. This became evident during the MSL-1 mission. Analysis of accelerometer and video data indicated that loss of sample control occurred during crew exercise periods which created disturbances in this frequency range. Prior to the MSL-1R flight the TEMPUS team, the accelerometer support groups and the mission operations team developed a strategy to provide for the operation of the facility without such disturbances. The successful implementation of this plan led to the highly successful operation of this facility during MSL-1R.

Rogers, Jan R.↗
NASA NTRS1999

Experiments on Nucleation in Different Flow Regimes

The vast majority of metallic engineering materials are solidified from the liquid phase. Understanding the solidification process is essential to control microstructure, which in turn, determines the properties of materials. The genesis of solidification is nucleation, where the first stable solid forms from the liquid phase. Nucleation kinetics determine the degree of undercooling and phase selection. As such, it is important to understand nucleation phenomena in order to control solidification or glass formation in metals and alloys. Early experiments in nucleation kinetics were accomplished by droplet dispersion methods. Dilatometry was used by Turnbull and others, and more recently differential thermal analysis and differential scanning calorimetry have been used for kinetic studies. These techniques have enjoyed success; however, there are difficulties with these experiments. Since materials are dispersed in a medium, the character of the emulsion/metal interface affects the nucleation behavior. Statistics are derived from the large number of particles observed in a single experiment, but dispersions have a finite size distribution which adds to the uncertainty of the kinetic determinations. Even though temperature can be controlled quite well before the onset of nucleation, the release of the latent heat of fusion during nucleation of particles complicates the assumption of isothermality during these experiments. Containerless processing has enabled another approach to the study of nucleation kinetics. With levitation techniques it is possible to undercool one sample to nucleation repeatedly in a controlled manner, such that the statistics of the nucleation process can be derived from multiple experiments on a single sample. The authors have fully developed the analysis of nucleation experiments on single samples following the suggestions of Skripov. The advantage of these experiments is that the samples are directly observable. The nucleation temperature can be measured by noncontact optical pyrometry, the mass of the sample is known, and post processing analysis can be conducted on the sample. The disadvantages are that temperature measurement must have exceptionally high precision, and it is not possible to isolate specific heterogeneous sites as in droplet dispersions.

Bayuzick, R. J.↗
NASA NTRS1999

Adaptive-Grid Methods for Phase Field Models of Microstructure Development

In this work the authors show how the phase field model can be solved in a computationally efficient manner that opens a new large-scale simulational window on solidification physics. Our method uses a finite element, adaptive-grid formulation, and exploits the fact that the phase and temperature fields vary significantly only near the interface. We illustrate how our method allows efficient simulation of phase-field models in very large systems, and verify the predictions of solvability theory at intermediate undercooling. We then present new results at low undercoolings that suggest that solvability theory may not give the correct tip speed in that regime. We model solidification using the phase-field model used by Karma and Rappel.

Provatas, Nikolas↗