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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.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2026

Controlled Parametric Forcing During Directional Solidification of a Bulk Organic Alloy Under Microgravity

The response of dendritic microstructures to step-like pulling velocity conditions is investigated using microgravity directional solidification experiments conducted on DECLIC-DSI combined with phase-field simulations. Under a constant pulling velocity of 1.5 µm/s, the evolution toward steady-state growth is characterized in terms of primary spacing, dendrite drift, and tip dynamics. For the first time, side-view observations enabled direct measurement of tip radius and sidebranching frequency. When step-like oscillations of the pulling velocity are imposed, the dendritic array exhibits a strongly period-dependent response: short periods lead to rapid tip adaptation, whereas longer periods induce a phase lag between tip position and morphology, resulting in progressive tip flattening and, above a critical period, interface destabilization and dendrite splitting. Quantitative phase-field simulations, including a realistic thermal field and stochastic noise, reproduce the experimental observations and provide insight into the governing mechanisms, highlighting the role of characteristic relaxation times, sequence-dependent effects, and the irreversible reorganization of the microstructure following splitting.

Microgravity↗
NASA NTRS1981

Infrared experiments for spaceborne planetary atmospheres research. Full report

The role of infrared sensing in atmospheric science is discussed and existing infrared measurement techniques are reviewed. Proposed techniques for measuring planetary atmospheres are criticized and recommended instrument developments for spaceborne investigations are summarized for the following phenomena: global and local radiative budget; radiative flux profiles; winds; temperature; pressure; transient and marginal atmospheres; planetary rotation and global atmospheric activity; abundances of stable constituents; vertical, lateral, and temporal distribution of abundances; composition of clouds and aerosols; radiative properties of clouds and aerosols; cloud microstructure; cloud macrostructure; and non-LTE phenomena.

Source record↗
NASA NTRS1994

Review of modelling small-crack behavior and fatigue-life predictions for aluminum alloys

The small-crack effect, where small fatigue cracks grow faster and at lower stress-intensity factors than large cracks, has been found to be significant for many materials and loading conditions. In this paper, plasticity effects and crack-closure modelling of small fatigue cracks are reviewed. A crack-closure model with a cyclic-plastic-zone-corrected effective stress-intensity factor range (related to the cyclic J-integral) and microstructural data on crack-initiation sites were used to calculate small-crack growth rates and fatigue lives for unnotched and notched specimens made of two aluminum alloys. The crack-closure transient from the plastic wake was shown to be the dominant cause of the small-crack effect and plasticity effects on the cyclic-plastic-zone-corrected stress-intensity factor range were negligible except at extremely high stress levels. Small-crack growth rates and fatigue lives under both constant-amplitude and spectrum loading from tests and analyses agreed well.

CRACK CLOSURE MODELLING↗
NASA NTRS1994

A Review Of Modelling Small-Crack Behavior And Fatigue-Life Predictions For Aluminum Alloys

The small-crack effect, where small fatigue cracks grow faster and at lower stress-intensity factors than large cracks, has been found to be significant for many materials and loading conditions. In this paper, plasticity effects and crack-closure modelling of small fatigue cracks are reviewed. A crack-closure model with a cyclic-plastic zone-corrected effective stress-intensity factor range (related to the cyclic J-integral) and microstructural data on crack-initiation sites were used to calculate small-crack growth rates and fatigue lives for unnotched and notched specimens made of two aluminum alloys. The crack-closure transient from the plastic wake was shown to be the dominant cause of the small-crack effect and plasticity effects on the cyclic-plastic zone-corrected stress-intensity factor range were negligible except at extremely high stress levels. Small-crack growth rates and fatigue lives under both constant-amplitude and spectrum loading from tests and analyses agreed well.

Newman, J. C., Jr.↗
NASA NTRS1990

Elevated temperature crack growth in aluminum alloys: Tensile deformation of 2618 and FVS0812 aluminum alloys

Understanding the damage tolerance of aluminum alloys at elevated temperatures is essential for safe applications of advanced materials. The objective of this project is to investigate the time dependent subcritical cracking behavior of powder metallurgy FVS0812 and ingot metallurgy 2618 aluminum alloys at elevated temperatures. The fracture mechanics approach was applied. Sidegrooved compact tension specimens were tested at 175, 250, and 316 C under constant load. Subcritical crack growth occurred in each alloy at applied stress intensity levels (K) of between about 14 and 25 MPa/m, well below K (sub IC). Measured load, crack opening displacement and displacement rate, and crack length and growth rate (da/dt) were analyzed with several continuum fracture parameters including, the C-integral, C (sub t), and K. Elevated temperature growth rate data suggest that K is a controlling parameter during time dependent cracking. For FVS0812, da/dt is highest at 175 C when rates are expressed as a function of K. While crack growth rate is not controlled by C (sub t) at 175 C, da/dt appears to better correlate with C (sub t) at higher temperatures. Creep brittle cracking at intermediate temperatures, and perhaps related to strain aging, is augmented by time dependent transient creep plasticity at higher temperatures. The C (sub t) analysis is, however, complicated by the necessity to measure small differences in the elastic crack growth and creep contributions to the crack opening displacement rate. A microstructural study indicates that 2618 and FVS0812 are likely to be creep brittle materials, consistent with the results obtained from the fracture mechanics study. Time dependent crack growth of 2618 at 175 C is characterized by mixed transgranular and intergranular fracture. Delamination along the ribbon powder particle boundaries occurs in FVS0812 at all temperatures. The fracture mode of FVS0812 changes with temperature. At 175 C, it is characterized as dimpled rupture, and at 316 C as mixed matrix superplastic rupture and matrix-dipersoid debonding.

Leng, Yang↗
NASA NTRS1994

Strength characteristics and crack growth behavior of a composite with well aligned fibers

Continuous fiber composites have shown tremendous promise in industrial applications. Their microstructures, however, are very complex and in many instances difficult to characterize. In this project, the fracture characteristics of a specially made fiber reinforced composite with different fiber spacing are investigated. The experimental results so far have shown that after an initial transient phase the crack speed reaches a steady phase, i.e., independent of the crack length. Within the steady crack growth phase debonding along the fibers in the bridging zone grows in a self-similar manner. During the steady phase the energy dissipation per cycle is constant. Afterwards, an increase of the energy dissipation is observed that is accompanied by a decrease in crack speed. This latter trend is presumed to be the result of relatively large amounts of energy dissipated in the bulk of the specimen. Using appropriate Green's function and computer simulations, the stress intensity factor at the crack tip is evaluated for various cases of bridging stresses. In this way the effects of specimen size and fiber spacing on the overall fracture behavior of the composite system are analyzed. The steady crack speed and the steady rate of debonding have a similar power dependence on stress level. Dimensional analysis demonstrates that the particular fracture process is not governed by dimensional invariance but on the detailed micromechanisms in the bridging zone.

Botsis, John↗
NASA NTRS1999

Thermal Barrier Coatings for Advanced Gas Turbine and Diesel Engines

Ceramic thermal barrier coatings (TBCS) have been developed for advanced gas turbine and diesel engine applications to improve engine reliability and fuel efficiency. However, durability issues of these thermal barrier coatings under high temperature cyclic conditions are still of major concern. The coating failure depends not only on the coating, but also on the ceramic sintering/creep and bond coat oxidation under the operating conditions. Novel test approaches have been established to obtain critical thermomechanical and thermophysical properties of the coating systems under near-realistic transient and steady state temperature and stress gradients encountered in advanced engine systems. This paper presents detailed experimental and modeling results describing processes occurring in the ZrO2-Y2O3 thermal barrier coating systems, thus providing a framework for developing strategies to manage ceramic coating architecture, microstructure and properties.

Zhu, Dongming↗
NASA NTRS2016

Primary Dendrite Arm Spacing and Trunk Diameter in Al-7-Weight-Percentage Si Alloy Directionally Solidified Aboard the International Space Station

Under a NASA-ESA collaborative research project, three Al-7-weight-percentage Si samples (MICAST-6, MICAST-7 and MICAST 2-12) were directionally solidified aboard the International Space Station to determine the effect of mitigating convection on the primary dendrite array. The samples were approximately 25 centimeters in length with a diameter of 7.8 millimeter-diameter cylinders that were machined from [100] oriented terrestrially grown dendritic Al-7Si samples and inserted into alumina ampoules within the Sample Cartridge Assembly (SCA) inserts of the Low Gradient Furnace (LGF). The feed rods were partially remelted in space and directionally solidified to effect the [100] dendrite-orientation. MICAST-6 was grown at 5 microns per second for 3.75 centimeters and then at 50 microns per second for its remaining 11.2 centimeters of its length. MICAST-7 was grown at 20 microns per second for 8.5 centimeters and then at 10 microns per second for 9 centimeters of its remaining length. MICAST2-12 was grown at 40 microns per second for 11 centimeters. The thermal gradient at the liquidus temperature varied from 22 to 14 degrees Kelvin per centimeter during growth of MICAST-6, from 26 to 24 degrees Kelvin per centimeter for MICAST-7 and from 33 to 31 degrees Kelvin per centimeter for MICAST2-12. Microstructures on the transverse sections along the sample length were analyzed to determine nearest-neighbor spacing of the primary dendrite arms and trunk diameters of the primary dendrite-arrays. This was done along the lengths where steady-state growth prevailed and also during the transients associated with the speed-changes. The observed nearest-neighbor spacings during steady-state growth of the MICAST samples show a very good agreement with predictions from the Hunt-Lu primary spacing model for diffusion controlled growth. The observed primary dendrite trunk diameters during steady-state growth of these samples also agree with predictions from a coarsening-based model. The radial macrosegregation and "steepling" caused by thermosolutal convection during terrestrial growth of the Al-7Si was not observed in the space-grown MICAST samples.

Ghods, M.↗
NASA NTRS1999

A Phenomenological Description of Primary Creep in Class M Materials

Observations of creep microstructures in the primary creep region in class M materials show a remarkable similarity with those formed in the exponential creep regime. As a result, it is proposed that the constitutive creep law for normal primary creep is similar to that for the exponential creep regime. A phenomenological description is discussed to rationalize these microstructural observations in terms of a normalized strain rate vs. stress plot. The implications of this plot in describing different testing procedures, steady-state flow, and on the observed deviations from the universal creep law are discussed. The plot is also extended to explain the observed similarities in the transient creep behavior in pre-strained materials and in stress change experiments.

Raj, S. V.↗
NASA NTRS1984

Microstructural development of protective Al2O3 scales

Microstructural characteristics of Al2O3 scales grown as protective coatings on NiCrAl alloys used in jet engines are described. The alloys were pure or doped with 0.3 percent Zr or Y and oxidized in 1 atm air at 1100 C for 0.1, 1 or 20.0 hr. The scales were then examined under a microscope. Transient epitaxial scales, formed during the 0.1 hr treatment and containing Ni, Cr and Al, consisted of a mosaic of subgrains and precipitates of different phases. The Y and Zr dopants had no effect on the nucleation site locations. The appearance of intergranular porosity at 0.1 hr was exacerbated after the 1 hr treatment. A bimodal void distribution appeared after 20 hr, when no porosity was evident. The detection of local areas of preferred orientation is taken as a spur to further studies of scale growth to gain control of the grain size or even to produce single crystal scales.

Smialek, J. L.↗
NASA NTRS1981

Energy efficient engine high-pressure turbine single crystal vane and blade fabrication technology report

The objective of the High-Pressure Turbine Fabrication Program was to demonstrate the application and feasibility of Pratt & Whitney Aircraft-developed two-piece, single crystal casting and bonding technology on the turbine blade and vane configurations required for the high-pressure turbine in the Energy Efficient Engine. During the first phase of the program, casting feasibility was demonstrated. Several blade and vane halves were made for the bonding trials, plus solid blades and vanes were successfully cast for materials evaluation tests. Specimens exhibited the required microstructure and chemical composition. Bonding feasibility was demonstrated in the second phase of the effort. Bonding yields of 75 percent for the vane and 30 percent for the blade were achieved, and methods for improving these yield percentages were identified. A bond process was established for PWA 1480 single crystal material which incorporated a transient liquid phase interlayer. Bond properties were substantiated and sensitivities determined. Tooling die materials were identified, and an advanced differential thermal expansion tooling concept was incorporated into the bond process.

Giamei, A. F.↗
NASA NTRS1999

Exploiting the Temperature/Concentration Dependence of Magnetic Susceptibility to Control Convection in Fundamental Studies of Solidification Phenomena

The objective of this new research project is to demonstrate by experiment, supplemented by mathematical modeling and physical property measurement, that the effects of buoyancy driven convection can be largely eliminated in ground-based experiments, and further reduced in flight, by applying a new technique. That technique exploits the dependence of magnetic susceptibility on composition or temperature. It is emphasized at the outset that the phenomenon to be exploited is fundamentally and practically different from the magnetic damping of convection in conducting liquids that has been the subject of much prior research. The concept suggesting this research is that all materials, even non-conductors, when placed in a magnetic field gradient, experience a force. Of particular interest here are paramagnetic and diamagnetic materials, classes which embrace the "model alloys", such as succinonitrile-acetone, that have been used by others investigating the fundamentals of solidification. Such alloys will exhibit a dependence of susceptibility on composition. The consequence is that, with a properly oriented field (gradient) a force will arise that can be made to be equal to, but opposite, the buoyancy force arising from concentration (or temperature) gradients. In this way convection can be stilled. The role of convection in determining the microstructure, and thereby properties, of materials is well known. Elimination of that convection has both scientific and technological consequences. Our knowledge of diffusive phenomena in solidification, phenomena normally hidden by the dominance of convection, is enhanced if we can study solidification of quiescent liquids. Furthermore, the microstructure, microchemistry and properties of materials (thereby practical value) are affected by the convection occurring during their solidification. Hitherto the method of choice for elimination of convection has been experimentation in microgravity. However, even in low Earth orbit, residual convection has effects. That residual convection arises from acceleration (drag on the spacecraft), displacement from the center of mass or transients in the gravitational field (g-jitter). There is therefore a need for both further reducing buoyancy driven flow in flight and allowing the simulation of microgravity during ground based experiments. Previous investigations, the research project description, theory behind the study and experimental methods as well as plots of magnetic fields and forces are presented.

Evans, J. W.↗
NASA NTRS2012

Temperature Induced Voltage Offset Drifts in Silicon Carbide Pressure Sensors

We report the reduction of transient drifts in the zero pressure offset voltage in silicon carbide (SiC) pressure sensors when operating at 600 C. The previously observed maximum drift of +/- 10 mV of the reference offset voltage at 600 C was reduced to within +/- 5 mV. The offset voltage drifts and bridge resistance changes over time at test temperature are explained in terms of the microstructure and phase changes occurring within the contact metallization, as analyzed by Auger electron spectroscopy and field emission scanning electron microscopy. The results have helped to identify the upper temperature reliable operational limit of this particular metallization scheme to be 605 C.

Okojie, Robert S.↗
NASA NTRS2002

Thermostructural Analysis of Carbon Cloth Phenolic Material Tested at the Laser Hardened Material Evaluation Laboratory

RSRM nozzle liner components have been analyzed and tested to explore the occurrence of anomalous material performance known as pocketing erosion. Primary physical factors that contribute to pocketing seem to include the geometric permeability, which governs pore pressure magnitudes and hence load, and carbon fiber high temperature tensile strength, which defines a material limiting capability. The study reports on the results of a coupled thermostructural finite element analysis of Carbon Cloth Phenolic (CCP) material tested at the Laser Hardened Material Evaluation Laboratory (the LHMEL facility). Modeled test configurations will be limited to the special case of where temperature gradients are oriented perpendicular to the composite material ply angle. Analyses were conducted using a transient, one-dimensional flow/thermal finite element code that models pore pressure and temperature distributions and in an explicitly coupled formulation, passes this information to a 2-dimensional finite element structural model for determination of the stress/deformation behavior of the orthotropic fiber/matrix CCP. Pore pressures are generated by thermal decomposition of the phenolic resin which evolve as a multi-component gas phase which is partially trapped in the porous microstructure of the composite. The nature of resultant pressures are described by using the Darcy relationships which have been modified to permit a multi-specie mass and momentum balance including water vapor condensation. Solution to the conjugate flow/thermal equations were performed using the SINDA code. Of particular importance to this problem was the implementation of a char and deformation state dependent (geometric) permeability as describing a first order interaction between the flow/thermal and structural models. Material property models are used to characterize the solid phase mechanical stiffness and failure. Structural calculations were performed using the ABAQUS code. Iterations were made between the two codes involving the dependent variables temperature, pressure and across-ply strain level. Model results comparisons are made for three different surface heat rates and dependent variable sensitivities discussed for the various cases.

Clayton, J. Louie↗
NASA NTRSDate not supplied

Convolution-Based Numerical Solutions of Transient Temperature Fields during Powder Bed Fusion Additive Manufacturing: Theory, Accuracy, and Computational Cost

Powder bed fusion (PBF) additive manufacturing has found numerous applications in the aerospace domain. However, components fabricated via PBF have a complex time-temperature history that significantly impacts subsequent mechanical performance. This study examines convolution-based numerical solutions of transient temperature fields that support simulations involving arbitrary beam shapes and paths during PBF. The convolutional approach is verified through comparisons with analytical solutions of the temperature field. The computational speed and accuracy of the method are assessed through comparisons with other explicit and implicit numerical techniques. In addition, the straightforward translation of the approach from a CPU to a GPU implementation and the resultant performance improvement are presented. The role of the technique in predicting microstructure evolution during PBF (for a greater process-structure-property-performance framework) is also demonstrated. This work supports the development of computational materials methods for understanding and controlling the time-temperature history during PBF.

powder bed fusion↗
NASA NTRS1979

Some TEM observations of Al2O3 scales formed on NiCrAl alloys

The microstructural development of Al2O3 scales on NiCrAl alloys has been examined by transmission electron microscopy. Voids were observed within grains in scales formed on a pure NiCrAl alloy. Both voids and oxide grains grew measurably with oxidation time at 1100 C. The size and amount of porosity decreased towards the oxide-metal growth interface. The voids resulted from an excess number of oxygen vacancies near the oxidemetal interface. Short-circuit diffusion paths were discussed in reference to current growth stress models for oxide scales. Transient oxidation of pure, Y-doped, and Zr-doped NiCrAl was also examined. Oriented alpha-(Al, Cr)2O3 and Ni(Al, Cr)2O4 scales often coexisted in layered structures on all three alloys. Close-packed oxygen planes and directions in the corundum and spinel layers were parallel. The close relationship between oxide layers provided a gradual transition from initial transient scales to steady state Al2O3 growth.

Smialek, J.↗
NASA NTRS1979

Some TEM observations of Al2O3 scales formed on NiCrAl alloys

The microstructural development of Al2O3 scales on NiCrAl alloys has been examined by transmission electron microscopy. Voids have been observed within grains in scales formed on a pure NiCrAl alloy. Both voids and oxide grains grew measurably with oxidation time at 1100 C. The size and amount of porosity decreased towards the oxide-metal growth interface. It was postulated that the voids resulted from an excess number of oxygen vacancies near the oxide-metal interface. Short-circuit diffusion paths were discussed in reference to current growth stress models for oxide scales. Transient oxidations of pure, Y-doped, and Zr-doped NiCrAl was also examined. Oriented alpha-(Al,Cr)2O3 and Ni(Al,Cr)2O4 scales often coexisted in layered structures on all three alloys. Close-packed oxygen planes and directions in the corundum and spinel layers were parallel. The close relationships between oxide layers provided a gradual transition from initial transient scales to steady state Al2O3 growth.

Smialek, J.↗
NASA NTRS1981

The properties of polycrystalline silicon solar cells with controlled titanium additions

By coupling the results of electrical measurements, such as spectral response, lighted and dark I-V determinations, and deep-level-transient spectroscopy with optical and laser scan photomicroscopy, the effects of grain boundaries and impurities on silicon solar cells were evaluated. Titanium, which produces two deep levels in silicon, degrades cell performance by reducing bulk lifetime and thus cell short-circuit current. Electrically active grain boundaries induce carrier recombination in the bulk and depletion regions of the solar cell. Experimental data imply a small but measurable segregation of titanium into some grain boundaries of the polycrystalline silicon containing high Ti concentration. However, for the titanium-contaminated polycrystalline material used in this study, solar cell performance is dominated by the electrically active titanium concentration in the grains. Microstructural impacts on the devices are of secondary importance

Rohatgi, A.↗