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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

On the electron affinities of the Ca, Sc, Ti and Y atoms

For the Ca, Sc, Ti and Y atoms calculations are performed for the ground states of the neutrals and the ground and several low-lying excited states of the negative ions. Overall the computed electron affinities are in good accord with experiment. The calculations show the rapid stabilization of the 3d orbital relative to the 4p as the nuclear charge increases. The 3F(0) and 3D(0) terms are found to be close in energy in Sc(-) and in Y(-). This confirms earlier speculation that some of the peaks in the photodetachment spectra of Y(-) originate from the bound excited 3F(0) term of Y(-).

Bauschlicher, Charles W., Jr.↗
NASA NTRS1987

Superplastic forming/weld-brazing of Ti-6Al-4V panels for enhanced structural efficiency

An account is given of the results obtained by NASA-sponsored studies which exploit the processing advantages of superplastic forming (SPF) and joining by weld-brazing (WB) for the fabrication of Ti-alloy skin-stiffened structural components. The buckling strengths of panels with stiffeners having the geometry of a conventional hat shape are compared with those of panels having advanced-geometry stiffereners. Both single- and multiple-stiffened panels have been evaluated at room temperature; the SPF/WB panels with advanced-geometry stiffeners developed up to 60-percent higher buckling strength than panels with conventional-geometry stiffeners. Selective stiffener reinforcement with a boron-reinforced Al-matrix MMC was also evaluated.

Royster, Dick M.↗
NASA NTRS1989

On the electron affinities of the Ca, Sc, Ti and Y atoms

For the Ca, Sc, Ti, and Y atoms calculations are performed for the ground states of the neutrals and the ground and several low-lying excited states of the negative ions. Overall, the computed electron affinities are in good accord with experiment. The calculations show the rapid stabilization of the 3d orbital relative to the 4p as the nuclear charge increases. The 3F0 and 3D0 terms are found to be close in energy in Sc(-) and in Y(-). This confirms earlier speculation that some of the peaks in the photodetachment spectra of Y(-) originate from the bound excited 3F0 term of Y(-).

Bauschlicher, Charles W., Jr.↗
NASA NTRS1989

Static and dynamic oxidation of Ti-14Al-21Nb and coatings

The oxidation of Ti-14Al-21Nb (wt pct) was studied under static conditions at 649 to 1093 C for as long as 120 hr, and under simulated hypersonic flight (dynamic oxidation) conditions at 982 C for as many as 16 half-hour cycles. Under simulated hypersonic flight conditions heavy oxidation and spalling of the oxide was observed. It was concluded that titanium aluminides used in hypersonic applications must have oxidation-protective coatings. In this preliminary study coatings about 1 micron thick were applied by sputter deposition, from solutions, and from sol-gels. It was found that, because of cracks and porosity, the sputter-deposited coatings did not have sufficient film integrity to shield the alloy. Some of the coatings applied from sol-gels demonstrated film integrity in 1 hr exposures at 982 C.

Wiedemann, K. E.↗
NASA NTRS1990

The significance of fractional crystallization in the petrogenesis of Apollo 17 Type A and B high-Ti basalts

Whole-rock and mineral analyses of 26 'new' type A and B Apollo 17 basalts are reported. The petrography and mineral chemistry of these basalts are similar to previously reported Apollo 17 basalts. However, these 'new' whole-rock data extend the compositional ranges of previously reported type A and B basalts and require the division of the type B basalts into type B1 and B2 varieties. These three types display similar trends when both major and trace elements are plotted against a fractionation index of Cr/La ratio. Major element compositions of basalts from all three types fall on olivine + Ti oxide control lines. This study demonstrates that Apollo 17 type A, B1, and B2 basalts have a relatively simple petrogenesis, with the only postmagma-generation process being fractional crystallization.

Neal, Clive R.↗
NASA NTRS1990

Oxidation characteristics of Ti-25Al-10Nb-3V-1Mo intermetallic alloy

Static oxidation kinetics of the super-alpha 2 titanium-aluminide alloy Ti-25Al-10Nb-3V-1Mo (at. percent) were investigated in air over the temperature range of 650 to 1000 C using thermogravimetric analysis. The oxidation kinetics were complex at all exposure temperatures and displayed up to three distinct oxidation rates. Breakaway oxidation occurred after long exposure times at high temperatures. Oxidation products were determined using x ray diffraction techniques, electron microprobe analysis, and energy dispersive x ray analysis. Oxide scale morphology was examined by scanning electron microscopy of the surfaces and cross sections of oxidized specimens. The oxides during the parabolic stages were compact and multilayered, consisting primarily of TiO2 doped with Nb, a top layer of Al2O3, and a thin bottom layer of TiN. The transition between the second and third parabolic stage was found to be linked to the formation of a TiAl layer at the oxide-metal interface. Porosity was formed during the third stage, causing degradation of the oxide and the beginning of breakaway oxidation.

Wallace, Terryl A.↗
NASA NTRS1990

Behavior of surface and corner cracks subjected to tensile and bending loads in Ti-6Al-4V alloy

The behavior of part-through flaws with regard to failure under monotonic loading and their growth under fatigue loading was studied experimentally and analytically. Comparisons are made of experimental values of toughness obtained using surface and corner cracked specimens with those obtained using standard test specimens, and also experimental growth cycles were compared with numerical predictions using the NASA/FLAGRO computer program. Tests were conducted on various types of surface and corner cracks under tensile and bending loads. Room temperature lab air provided the test environment. The material used in this study was the Ti-6Al-4V alloy in the solution treated and aged (STA) and stress relieved condition. Detailed tabulation of the fracture toughness data and results of life prediction using the NASA/FLAGRO program are presented. Fatigue crack growth rates for the part-through cracked specimens are compared with a base curve fitted from the data obtained using standard specimens. The fatigue loading used in the crack growth testing was constant-amplitude sinusoidal type.

Forman, Royce G.↗
NASA NTRS1990

Shape-memory properties in Ni-Ti sputter-deposited film

A Ni-Ti alloy, generically called nitinol, was prepared from sputtering targets of two different compositions on glass substrates using a dc magnetron source. The as-deposited films were amorphous in structure and did not exhibit a shape memory. The amorphous films were crystallized with a suitable annealing process, and the transformation properties were measured using differential scanning calorimetry. The annealed films demonstrated a strong shape-memory effect. Stress/strain measurements and physical manipulation were used to evaluate the shape recovery. These tests demonstrated sustained tensile stresses of up to 480 MPa in the high-temperature phase, and a characteristic plastic deformation in the low-temperature phase.

Busch, J. D.↗
NASA NTRS1991

Co-57 and Ti-44 production in SN 1987A

A survey of parameterized explosive silicon burning is carried out in order to limit the range of allowed masses of Co-57 produced by SN 1987A. The most likely value is between 0.5 and 2.5 times that implied by a solar ratio for Fe-57/Fe-56. Values more than four and less than one-third times the solar ratio would pose very severe problems for the theory of stellar nucleosynthesis, implying, for example, that Fe-56 was not the dominant iron group isotope or that no electron capture occurred during oxygen burning. Though dependent somewhat on the neutron excess, the Fe-57 yield is most sensitive to the occurrence of an alpha-rich freeze-out. An accurate measurement of the Fe-57 yield, as reflected observationally by the current abundance of radioactive Co-57, is thus an important constraint upon the supernova explosion mechanism. The abundance of Ti-44, another nucleus produced exclusively in the alpha-rich freeze-out, cannot be much greater than that implied by the solar ratio of Ca-44/Fe-56.

Woosley, S. E.↗
NASA NTRS1991

A strontium and neodymium isotopic study of Apollo 17 high-Ti mare basalts - Resolution of ages, evolution of magmas, and origins of source heterogeneities

The geochronological and compositional differences between previously identified magma types (A, B1, B2, and C) were investigated using high-precision Rb-Sr and Sm-Nd isotopic data for a set of Apollo 17 high-Ti mare basalt samples chosen to span the range of each of the magma types. These data, combined with previously reported geochemical ages, suggest that Apollo 17 volcanism was initially dominated by an eruption of Type B basalts. Data obtained from new whole-rock Sr and Nd isotopic analyses exhibited distinct differences in initial Sr and Nd isotopic compositions between Types A, B1, B2, and C basalts and were found to be consistent with existing petrogenetic models.

Paces, James B.↗
NASA NTRS1992

The effect of weld porosity on the cryogenic fatigue strength of ELI grade Ti-5Al-2.5Sn

The effect of weld porosity on the fatigue strength of ELI grade Ti-5Al-2.5Sn at cryogenic temperature was determined. A series of high cycle fatigue (HCF) and tensile tests were performed at -320 F on specimens made from welded sheets of the material. All specimens were tested with weld beads intact and some amount of weld offset. Specimens containing porosity and control specimens containing no porosity were tested. Results indicate that for the weld configuration tested, the fatigue life of the material is not affected by the presence of spherical embedded pores.

Rogers, P. R.↗
NASA NTRS1990

The effect of nitric acid exposure on Galileo spacecraft titanium alloy Ti-6Al-4V propellant tanks

The Ti-6Al-4V-constructed retropropulsion-module tanks of the Galileo spacecraft were purged with nitrogen tetroxide in order to wait out a major launch rescheduling; nitric acid is among the residual products of such an operation. A test program was conducted on representative samples to ascertain the fracture toughness and stress corrosion threshold of the tanks' material, in view of Space Shuttle safety and mission-reliability requirements. It was found that the tanks' structural integrity was not degraded by nitric acid exposure.

Hsieh, Cheng↗
NASA NTRS1992

Gibbs free energy difference between the undercooled liquid and the beta phase of a Ti-Cr alloy

The heat of fusion and the specific heats of the solid and liquid have been experimentally determined for a Ti60Cr40 alloy. The data are used to evaluate the Gibbs free energy difference, delta-G, between the liquid and the beta phase as a function of temperature to verify a reported spontaneous vitrification (SV) of the beta phase in Ti-Cr alloys. The results show that SV of an undistorted beta phase in the Ti60Cr40 alloy at 873 K is not feasible because delta-G is positive at the temperature. However, delta-G may become negative with additional excess free energy to the beta phase in the form of defects.

Ohsaka, K.↗
NASA NTRS1991

Phase stability in binary Ti-Al

Binary Ti-Al samples containing from 46 to 54 at. pct Al were solidified while undercooled by various amounts using electromagnetic levitation techniques. A detailed thermal history of these samples was obtained with sampling rates as high as 500 KHz during recalescence. This very high sampling rate was essential to resolve the thermal events. Primary alpha solidification was observed in samples containing from 51 to 54 at. pct Al that were undercooled less than about 100 K at solidification. Primary beta solidification was found for all undercoolings tested in samples containing less than 51 at. pct Al and for undercoolings greater than about 100 K in samples containing 51 to 54 at. pct Al.

Anderson, C. D.↗
NASA NTRS1992

Structural relationships between the T and O phases in Ti-24Al-11Nb

TEM and electron diffraction are presently used to characterize the structural relationships between the ordered tetragonal (T) and ordered orthorhombic (O) phases of a rapidly solidified Ti-24Al-11Nb alloy. It is established that the transition from the T to the O phase is explainable in terms of a shape-deformation mechanism, and that the T/O interface (habit) plane approximates -2 2 3 sub T. Attention is given to the orientation relationships between the two phases.

Hsiung, L. M.↗
NASA NTRS1992

Oxidation characteristics of Ti-25Al-10Nb-3V-1Mo

Static oxidation kinetics of the super-alpha 2 titanium-aluminide alloy Ti-25Al-10Nb-3V-1Mo (at. percent) were investigated in air over the temperature range of 650 to 1000 C using thermogravimetric analysis. The oxidation kinetics were complex at all exposure temperatures and displayed up to three distinct oxidation rates. Breakaway oxidation occurred after long exposure times at high temperatures. Oxidation products were determined using X-ray diffraction techniques, electron microprobe analysis, and energy dispersive X-ray analysis. Oxide scale morphology was examined by scanning electron microscopy of the surfaces and cross sections of oxidized specimens. The oxides during the parabolic stages were compact and multilayered, consisting primarily of TiO2 doped with Nb, a top layer of Al2O3, and a thin bottom layer of TiN. The transition between the second and third parabolic stage was found to be linked to the formation of a TiAl layer at the oxide-metal interface. Porosity was formed during the third stage, causing degradation of the oxide and the beginning of breakaway oxidation.

Wallace, T. A.↗
NASA NTRS1992

Stability of the ordered orthorhombic phase in Ti-24Al-11Nb

The ordered orthorhombic 'O' phase in the Ti-Al-Nb ordered alloy system emerges either through slow cooling from above 1200 C or through isothermal aging after quenching from that temperature regime. An experimental investigation is presently conducted of the structural relationship between the O and alpha(2) phases, in order to clarify the mechanism of transition between the two. The transition occurs with the formation of a platelike alpha(2) phase within the O matrix, and may be characterized as due to a shape-deformation mechanism associated with diffusion of excess Nb away from the O/alpha(2) interface.

Hsiung, L. M.↗