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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS2019

The Effect Stacking Fault Segregation and Phase Transformations have on Creep Strength in Ni-based Superalloys

In this study, two commercially similar polycrystalline Ni-based disk superalloys (LSHR and ME3) were creep tested at 760C and 552MPa to approximately 0.3% plastic strain. LSHR consistently displayed superior creep properties at this stress/temperature regime even though the microstructural characteristics between the two alloys were comparable. High resolution structural and chemical analysis, however, revealed significant differences between the two alloys among active gamma prime shearing modes involving superlattice intrinsic and extrinsic stacking faults. In ME3, Co and Cr segregation and Ni and Al depletion were observed along the intrinsic faults - revealing a gamma prime -to- gamma phase transformation. Conversely in LSHR, an alloy with a higher W content, Co and W segregation was observed along the intrinsic faults. This observation combined with scanning transmission electron microscopy (STEM) simulations confirm a gamma prime-to-D019 phase transformation along the intrinsic faults in LSHR. Using experimental observations and density functional theory calculations, a novel local phase transformation strengthening mechanism is proposed that could be further utilized to improve the high temperature creep capabilities of Ni-base disk alloys.

Superalloy↗
NASA NTRS1968

Microprobe investigation of brittle segregates in aluminum MIG and TIG welds

Quantitative microprobe analysis of segregated particles in aluminum MIG /Metal Inert Gas/ and TIG /Tungsten Inert Gas/ welds indicated that there were about ten different kinds of particles, corresponding to ten different intermetallic compounds. Differences between MIG and TIG welds related to the individual cooling rates of these welds.

Larssen, P. A.↗
NASA NTRS1972

Evaporative segregation in 80 percent Ni-20 percent Cr and 60 percent Fe-40 percent Ni alloys

The phenomenon of evaporative segregation in binary alloys has been investigated through a study of some experimental evaporation data relating to the Ni-Cr and Ni-Fr systems. In normal evaporation it is assumed that (1) the evaporating alloy is always homogeneous, (2) the vapor is instantly removed, and (3) the alloy follows Raoult's law. The solutions of the evaporation equations for the two most important cases are presented and experimental data are analyzed with these equations. The difference between observed and calculated values of evaporation constants lies within one order of magnitude. This is surprising because of the major assumptions stated above. Experimental results have shown that the evaporation time and final solute concentration are logarithmically related, further supporting our evaporation equations. It is further shown that neglecting the nonlogarithmic term in these evaporation equations may introduce considerable errors in the analysis.

Mukherjee, J. L.↗
NASA NTRS1974

Evaporative segregation in 80% Ni-20% Cr and 60% Fe-40% Ni alloys

An analytical approach is outlined to calculate the evaporative segregation behavior in metallic alloys. The theoretical predictions are based on a 'normal' evaporation model and have been examined for Fe-Ni and Ni-Cr alloys. A fairly good agreement has been found between the predicted values and the experimental results found in the literature.

Gupta, K. P.↗
NASA NTRS1982

Segregation effects during growth of pseudo-binary system with large liquidus-solidus separation

Results are reported for an experimental study of the seeded growth, of PbSnTe single crystals in a vertical Bridgman-Stockbarger (B-S) furnace. Profiles of axial and radial segregation of Sn are presented for crystal growth from melts with aspect ratios of 15 and 6.8; the profiles indicate intense convective mixing. A one-dimensional model of heat and mass transfer in an idealized B-S furnace with diffusion control is used to examine the importance of liquidus-solidus separation in determining growth-rate and composition transients in a pseudobinary system. It is shown that the coupling of heat and mass transfer in a pseudobinary system with a large liquidus-solidus separation causes the transient behavior in a nondilute system to differ markedly from such behavior in a dilute system and that PbSnTe crystals with a reasonable diameter can be grown under nearly convectionless conditions in a microgravity environment.

Bourret, E. D.↗
NASA NTRS1982

Investigation of compositional segregation during unidirectional solidification of solid solution semiconducting alloys

Compositional segregation of solid solution semiconducting alloys in the radial direction during unidirectional solidification was investigated by calculating the effect of a curved solid liquid interface on solute concentration at the interface on the solid. The formulation is similar to that given by Coriell, Boisvert, Rehm, and Sekerka except that a more realistic cylindrical coordinate system which is moving with the interface is used. Analytical results were obtained for very small and very large values of beta with beta = VR/D, where V is the velocity of solidification, R the radius of the specimen, and D the diffusivity of solute in the liquid. For both very small and very large beta, the solute concentration at the interface in the solid C(si) approaches C(o) (original solute concentration) i.e., the deviation is minimal. The maximum deviation of C(si) from C(o) occurs for some intermediate value of beta.

Wang, J. C.↗
NASA NTRS1984

The mechanisms of formation and prevention of channel segregation during alloy solidification

Using a base chilled configuration, conditions for the formation and prevention of segregation channels are studied in the ammonium chloride-water and lead-tin systems. Such channels develop when the rejected solute is less dense than the solvent and are therefore a result of density inversion, but slow (less than 5 rpm) rates of mold rotation, about axes inclined to the vertical by 20 to 30 deg, throughout the time of solidification, effectively prevent the formation or propagation of these channels. In experiments with artificially created or blocked channels, a simultaneous solute plume in the bulk liquid must be present in order for a channel to develop. The results indicate that channels originate not within the dendritic array, but immediately ahead of the growth front as a result of perturbation from the less dense boundary layer into the bulk liquid. The effect of mold movement is to translate laterally the bulk liquid relative to the growth front so that perturbations are sheared off. The nature of the liquid movement is considered and shown to be a function of the mold dimensions.

Hellawell, A.↗
NASA NTRS1988

Regelation and ice segregation

Macroscopic processes can have an important effect on the state of regolith water. The two primary mechanisms responsible for the formation of segregated ice on Earth, thermally induced regelation and hydraulic fracturing, are reviewed while their potential importance on Mars is examined. While regelation is the dominant terrestrial process, it requires a warmer and wetter environment than currently exists on Mars. In this respect, the conditions required for hydraulic fracturing are less demanding. In assessing its potential importance on Mars, it is noted that hydraulic fracturing can produce a localized zone of high pressure water that could readily disrupt an overburden of frozen ground. Such a process, it is concluded, may have triggered the release of groundwater that led to the formation of the major outflow channels.

Miller, Robert D.↗
NASA NTRS1992

Terrestrial magma ocean and core segregation in the earth

According to the recent theories of formation of the earth, the outer layer of the proto-earth was molten and the terrestrial magma ocean was formed when its radius exceeded 3000 km. Core formation should have started in this magma ocean stage, since segregation of metallic iron occurs effectively by melting of the proto-earth. Therefore, interactions between magma, mantle minerals, and metallic iron in the magma ocean stage controlled the geochemistry of the mantle and core. We have studied the partitioning behaviors of elements into the silicate melt, high pressure minerals, and metallic iron under the deep upper mantle and lower mantle conditions. We employed the multi-anvil apparatus for preparing the equilibrating samples in the ranges from 16 to 27 GPa and 1700-2400 C. Both the electron probe microanalyzer (EPMA) and the Secondary Ion Mass spectrometer (SIMS) were used for analyzing the run products. We obtained the partition coefficients of various trace elements between majorite, Mg-perovskite, and liquid, and magnesiowustite, Mg-perovskite, and metallic iron. The examples of the partition coefficients of some key elements are summarized in figures, together with the previous data. We may be able to assess the origin of the mantle abundances of the elements such as transition metals by using the partitioning data obtained above. The mantle abundances of some transition metals expected by the core-mantle equilibrium under the lower mantle conditions cannot explain the observed abundance of some elements such as Mn and Ge in the mantle. Estimations of the densities of the ultrabasic magma Mg-perovskite at high pressure suggest existence of a density crossover in the deep lower mantle; flotation of Mg-perovskite occurs in the deep magma ocean under the lower mantle conditions. The observed depletion of some transition metals such as V, Cr, Mn, Fe, Co, and Ni in the mantle may be explained by the two stage process, the core-mantle equilibrium under the lower mantle conditions in the first stage, and subsequent downwards separation of the ultrabasic liquid (and magnesiowustite) and flotation of Mg-perovskite in the lower mantle.

Ohtani, Eiji↗
NASA NTRS1992

Oxygen-induced Al surface segregation in Al(x)Ga(1-x)As and the effect of Y overlayers on the oxidation of the Y/Al(x)Ga(1-x)As interface

The oxidation of Al(x)Ga(1-x)As (x = 0.15, AlGaAs) was studied by AES and XPS at 350 C and different oxygen exposures (up to 5 x 10 exp 4 L). Also studied were the effects of yttrium overlayers (theta = 3 ML) on the oxidation of the AlGaAs surface. Substantial oxygen-induced Al surface segregation has been observed for both yttriated and nonyttriated AlGaAs surfaces which increased with increasing oxygen exposure. Also observed is a significant Y-enhanced oxidation of the AlGaAs surface. Oxidation of the yttriated AlGaAs surface was found to be a factor of 4 greater than that of the nonyttriated surface. Also, while oxidation of the nonyttriated AlGaAs yielded mainly Al2O(x) (x less than 3) and only little Ga2O3, the yttriated AlGaAs surface oxide layer was principally Ga2O3 and stoichiometric Al2O3. However, both the yttriated and nonyttriated surfaces were found to contain metallic As within the oxide layer.

Mesarwi, A.↗
NASA NTRS1993

Segregation of gas and stars in shell galaxies

Using a code which is capable of evolving composite systems of collisionless matter and gas, we explore mergers like those thought responsible for the shells seen around many elliptical galaxies. If a small companion containing both gas and stars is accreted by a more massive primary, the stellar and gaseous debris are rapidly segregated: while the stars are free to oscillate back and forth in the primary's potential, thereby forming shells, the oppositely directed flows near the center of the primary effectively dissipate the orbital kinetic energy of the gas. Consequently, the gas settles into compact disks or rings in the nucleus of the primary, depending on orbital parameters. We note implications of these findings for the production of polar rings and the onset of nuclear activity in galaxies.

Weil, Melinda L.↗
NASA NTRS1993

The ilmenite liquidus and depths of segregation for high-Ti picrite glasses

Lunar picrite glasses represent primitive and perhaps near primary liquids which have suffered only minor degrees of crystallization or near crustal modification. These glasses are multisaturated with olivine and orthopyroxene at pressures from 20-25 kb. I argue below that high TiO2 mare glasses were indeed equilibrated with orthopyroxene and were segregated from the lunar mantle at mean depths of 400-500 km. The glasses are typically modelled as products of relatively low degrees of melting of an hybridized source resulting from the overturn and mixing of the gravitationally unstable cumulate pile. But the models are neither unique nor, in some cases, correct.

Hess, P. C.↗
NASA NTRS1993

Convection effects on radial segregation and crystal melt interface in vertical Bridgman growth

We analytically study the influence of convection caused by horizontal heat transfer through the sides of a vertical Bridgman apparatus. We consider the case when the heat transfer across the side walls is small so that the resulting interfacial deformation and fluid velocities are also small. This allows us to linearize the Navier-Stokes equations and express the interfacial conditions about a planar interface through a Taylor expansion. Using a no tangential stress conditions on the side walls, asymptotic expressions for both the interfacial slope, and radial segregation at the crystal-melt interface are obtained in closed form in the limit of large thermal Rayleigh number. It is suggested that these can be reduced by appropriately controlling a specific heat transfer property at the edge of the insulation zone in the solid side.

Tanveer, S.↗