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At least 1,297 records · Page 72
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Environmental fatigue of an Al-Li-Cu alloy. Part 2: Microscopic hydrogen cracking processes

Based on a fractographic analysis of fatigue crack propagation (FCP) in Al-Li-Cu alloy 2090 stressed in a variety of inert and embrittling environments, microscopic crack paths are identified and correlated with intrinsic da/dN-delta K kinetics. FCP rates in 2090 are accelerated by hydrogen producing environments (pure water vapor, moist air, and aqueous NaCl), as defined in Part 1. For these cases, subgrain boundary fatigue cracking (SGC) dominates for delta K values where the crack tip process zone, a significant fraction of the cyclic plastic zone, is sufficiently large to envelop 5 micron subgrains in the unrecrystallized microstructure. SGC may be due to strong hydrogen trapping at T1 precipitates concentrated at sub-boundaries. At low delta K, the plastic zone diameter is smaller than the subgrain size and FCP progresses along (100) planes due to either local lattice decohesion or aluminum-lithium hydride cracking. For inert environments (vacuum, helium, and oxygen), or at high delta K where the hydrogen effect on da/dN is small, FCP is along (111) slip planes; this mode does not transition with increasing delta K and plastic zone size. The SGC and (100) crystallographic cracking modes, and the governing influence of the crack tip process zone volume (delta K), support hydrogen embrittlement rather than a surface film rupture and anodic dissolution mechanism for environmental FCP. Multi-sloped log da/dN-log delta K behavior is produced by changes in process zone hydrogen-microstructure interactions, and not by purely micromechanical-microstructure interactions, in contradiction to microstructural distance-based fatigue models.

Robert S Piascik↗
NASA NTRS1992

Environmental fatigue of an Al-Li-Cu alloy. Part 3: Modeling of crack tip hydrogen damage

Environmental fatigue crack propagation rates and microscopic damage modes in Al-Li-Cu alloy 2090 (Parts 1 and 2) are described by a crack tip process zone model based on hydrogen embrittlement. Da/dN sub ENV equates to discontinuous crack advance over a distance, delta a, determined by dislocation transport of dissolved hydrogen at plastic strains above a critical value; and to the number of load cycles, delta N, required to hydrogenate process zone trap sites that fracture according to a local hydrogen concentration-tensile stress criterion. Transgranular (100) cracking occurs for process zones smaller than the subgrain size, and due to lattice decohesion or hydride formation. Intersubgranular cracking dominates when the process zone encompasses one or more subgrains so that dislocation transport provides hydrogen to strong boundary trapping sites. Multi-sloped log da/dN-log delta K behavior is produced by process zone plastic strain-hydrogen-microstructure interactions, and is determined by the DK dependent rates and proportions of each parallel cracking mode. Absolute values of the exponents and the preexponential coefficients are not predictable; however, fractographic measurements theta sub i coupled with fatigue crack propagation data for alloy 2090 established that the process zone model correctly describes fatigue crack propagation kinetics. Crack surface films hinder hydrogen uptake and reduce da/dN and alter the proportions of each fatigue crack propagation mode.

Piascik, Robert S.↗
NASA NTRS1992

Reduced mobility and PPC in In(.20)Ga(.80)As / Al(.23)Ga(.77)As HEMT structure

Transport properties of a pseudomorphic In(.20)Ga(.80)As/Al(.23)Ga(.77)As High Electron Mobility Transistor (HEMT) structure were measured by Hall and SdH techniques. Two samples of identical structures but with different doping levels were compared. Low temperature mobility measurements as a function of concentration coincides with the onset of second subband occupancy, indicating that the decrease in mobility is due to intersubband scattering. In spite of the low Al content (23 percent), large persistent photoconductivity (PPC) was observed in the highly doped sample only, showing a direct correlation between the PPC and doping concentration of the barrier layer.

Schacham, S. E.↗
NASA NTRS1991

The effect of copper, chromium, and zirconium on the microstructure and mechanical properties of Al-Zn-Mg-Cu alloys

The present study evaluates the effect of the systematic variation of copper, chromium, and zirconium contents on the microstructure and mechanical properties of a 7000-type aluminum alloy. Fracture toughness and tensile properties are evaluated for each alloy in both the peak aging, T8, and the overaging, T73, conditions. Results show that dimpled rupture essentially characterize the fracture process in these alloys. In the T8 condition, a significant loss of toughness is observed for alloys containing 2.5 pct Cu due to the increase in the quantity of Al-Cu-Mg-rich S-phase particles. An examination of T8 alloys at constant Cu levels shows that Zr-bearing alloys exhibit higher strength and toughness than the Cr-bearing alloys. In the T73 condition, Cr-bearing alloys are inherently tougher than Zr-bearing alloys. A void nucleation and growth mechanism accounts for the loss of toughness in these alloys with increasing copper content.

Wagner, John A.↗
NASA NTRS1992

Fracture resistance and fatigue crack growth characteristics of two Al-Cu-Mg-Zr alloys

The dependence of strength, fracture resistance, and fatigue crack growth rate on the aging conditions of two alloy compositions based on Al-3.7Cu-1.85Mg-0.2Mn is investigated. Mechanical properties were evaluated in two heat treatment conditions and in two orientations (longitudinal and transverse). Compact tension specimens were used to determine fatigue crack growth characteristics and fracture resistance. The aging response was monitored on coupons using hardness measurements determined with a standard Rockwell hardness tester. Fracture resistance is found to increase with increasing yield strength during artificial aging of age-hardenable 2124-Zr alloys processed by powder metallurgy techniques. Fatigue crack growth rate increases with increasing strength. It is argued that these changes are related to deformation modes of the alloys; a homogeneous deformation mode tends to increase fracture resistance and to decrease the resistance to the fatigue crack propagation rate.

Sarkar, Bhaskar↗
NASA NTRS1992

Al II emission-line strengths in low-density astrophysical plasmas

Theoretical values of the emission-line ratio R are derived for the transitions 3s2 1S-3s3p 3P2 and 3s2 1S-3s3p 3P1 in Al II, at 2660 and 2669 A, respectively. These ratios are compared with IUE observations of the planetary nebula NGC 7027 and the symbiotic star RR Tel, to illustrate the usefulness of the R as an electron density diagnostics. A value of R = 0.72 was deduced for NGC 7027, which implies log Ne = 4.2 for Te = 14,000 K. This is consistent with densities deduced earlier for this planetary nebula.

Keenan, F. P.↗
NASA NTRS1990

Continuous recrystallization during thermomechanical processing of a superplastic Al-10Mg-0.1Zr alloy

Microstructural evolution via static continuous recrystallization during thermomechanical processing of an Al-Mg-Zr alloy is addressed. Mechanical property data demonstrated that as-rolled material was capable of superplastic response without further treatment. Further, superplastic ductility at 300 C was enhanced by a factor of five by increasing the reheating time between rolling passes during processing also at 300 C. This enhanced ductility was associated with a Cu-texture and a microstructure consisting of predominantly high-angle boundaries. Processing to minimize recovery resulted in a strong Brass-texture component, a predominantly low-angle boundary microstructure and poorer ductility.

Hales, S. J.↗
NASA NTRS1992

Solidification processing of intermetallic Nb-Al alloys

Several Nb-Al alloys, including single-phase NbAl3 and the eutectic of Nb2Al and NbAl3, were prepared either by nonconsumable arc melting in Ar or by zone processing in He following initial induction melting and rod casting, and the effect of the solidification route on the microstructure and room-temperature mechanical properties of these alloys was investigated. Automated control procedures and melt conditions for directional solidification of NbAl3 and the Nb2Al/Nb3Al eutectic were developed; high purity and stoichiometry were obtained. The effects of ternary additions of Ti and Ni are described.

Smith, Preston P.↗
NASA NTRS1992

Fracture toughness of an Al-Li-Cu-In alloy

The crack initiation and growth fracture toughness of select AL-Li-Cu alloy variants are characterized and elucidated. Conventionally processed plates form large DC cast ingots are investigated to eliminate the variation in microstructure associated with laboratory scale and SPF-processed material. Fracture resistance is characterized using the J-integral method to establish crack initiation and growth behavior at 25 and -185 C. It is shown that state-of-the-art 2090-T81 has superior toughness compared to 2090 + In-T6 at both test temperatures, with the low toughness of 2090 + In-T6 associated with intersubgranular fracture attributed to a high density of subboundary precipitates.

Wagner, John A.↗
NASA NTRS1991

Shifts of the Al-26 line due to Galactic rotation

The shape of the Al-26 line is evaluated using a Galactic disk rotation curve and assuming various galactic distributions of radioactive aluminum. The line shape depends on galactic longitude, detector opening angle, and the assumed aluminum distribution. For a detector of 20 deg aperture and 2.5 keV resolution, the shift of the line peak can be as large as 0.4 keV. Such a shift could be detected with the proposed nuclear astrophysics explorer.

Skibo, Jeff↗
NASA NTRS1992

dc and microwave performance of a 0.1-micron gate InAs/In(0.52)Al(0.48)As MODFET

The performance characteristics of 0.1-micron gate InAs/In(0.52)Al(0.48)As MODFETs grown by molecular beam epitaxy are measured and analyzed. The transistors are characterized by measured g sub m (max) = 840 mS/mm, f sub T = 128 GHz, and a very high current carrying capability, e.g., I sub dss = 934 mA/mm at V sub gs = 0.4 V and V sub ds = 2.7 V. The value of f sub T is estimated from extrapolation of the current gain (H21) at a -6-dB/octave rolloff. This is the first report on the microwave characteristics of an InAs-channel MODFET and establishes the superiority of this heterostructure system.

Yang, D.↗
NASA NTRS1990

Ductility enhancement from interface dislocation sources in a directionally solidified beta + gamma + gamma-prime Ni-Fe-Al composite alloy

A directionally solidified beta + gamma + gamma-prime Ni-Fe-Al in situ composite alloy of composition Ni50Fe30Al20 has been used to investigate the effect of a plastically soft second phase on the mechanical behavior of a B2 ordered intermetallic alloy. This material exhibits extensive plasticity during compressive deformation at room temperature and fails in shear with extensive gamma + gamma-prime lamellar or rod pullout. The material also exhibits about 10 percent tensile elongation to fracture at room temperature, with final fracture that includes substantial necking of the gamma + gamma-prime lamellae or rods. Observation of slip lines and dislocation substructures discloses that the normally brittle beta matrix undergoes extensive plasticity in order to deform compatibly with the more ductile gamma phase. The plasticity of the beta matrix is accomplished by the generation of glissile dislocations into the beta matrix from the beta/gamma interface region and is enhanced because of a favorable beta-gamma orientation relationship for slip transfer. Ductility enhancement from interface-generated mobile dislocations generated from beta-gamma interfaces is compared to that observed in film-coated beta-NiAl single crystals and FeAl polycrystals.

Larsen, M.↗
NASA NTRS1992

Bose-Einstein correlations in Si + Al and Si + Au collisions at 14.6A GeV/c

The E802 Spectrometer at the Brookhaven Alternating Gradient Synchrotron has been used to measure the correlation in relative momentum between like-sign pions emitted in central Si + Al and Si + Au collisions at 14.6A GeV/c. Data are presented in terms of the correlation function for both identified pi(-) and pi(+) pairs near the nucleon-nucleon center-of-mass rapidity. All parametrizations of the correlation function are consistent with a spherically symmetric source of rms radius 3.5 +/- 0.4 fm and lifetime fm/c.

Abbott, T.↗
NASA NTRS1992

Combustion synthesis of ceramic-metal composite materials - The TiC-Al2O3-Al system

Combustion synthesis was applied for producing ceramic-metal composites with reduced levels of porosity, by allowing an excess amount of liquid metal, generated by the exothermic reaction during synthesis, to infiltrate the pores. It is shown that this method, when applied to TiC-Al2O3 system, led to a decreased level of porosity in the resulting TiC-Al2O3-Al product, as compared with that of TiC-Al2O3 system. This in situ procedure is more efficient than the two-stage conventional processes (i.e., sintering followed by liquid metal infiltration), although there are limitations with respect to total penetration of the liquid metal and maintaining a stable propagation of the combustion reaction.

Feng, H. J.↗
NASA NTRS1992

Low cost, SPF aluminum cryogenic tank structure for ALS

Past production work has shown that cryogenic tank structure for the Shuttle Booster Rockets and the Titan system have very high life cycle costs for the fuel tank structure. The tanks are machined stiffener-skin combination that are subsequently formed into the required contour after machining. The material scrap rate for these configurations are usually high, and the loss of a tank panel due to forming or heat treatment problems is very costly. The idea of reducing the amount of scrap material and scrapped structural members has prompted the introduction of built-up structure for cryogenic tanks to be explored on the ALS program. A build-up structure approach that has shown improvements in life cycle cost over the conventional built-up approach is the use of superplastically formed (SPF) stiffened panels (reducing the overall part count and weight for the tank) resistance spot welded (RSW) to outer tank skin material. The stiffeners provide for general stability of the tank, while the skin material provides hoop direction continuity for the loads.

Anton, Claire E.↗
NASA NTRS1992

User's manual for the ALS base heating prediction code, volume 2

The Advanced Launch System (ALS) Base Heating Prediction Code is based on a generalization of first principles in the prediction of plume induced base convective heating and plume radiation. It should be considered to be an approximate method for evaluating trends as a function of configuration variables because the processes being modeled are too complex to allow an accurate generalization. The convective methodology is based upon generalizing trends from four nozzle configurations, so an extension to use the code with strap-on boosters, multiple nozzle sizes, and variations in the propellants and chamber pressure histories cannot be precisely treated. The plume radiation is more amenable to precise computer prediction, but simplified assumptions are required to model the various aspects of the candidate configurations. Perhaps the most difficult area to characterize is the variation of radiation with altitude. The theory in the radiation predictions is described in more detail. This report is intended to familiarize a user with the interface operation and options, to summarize the limitations and restrictions of the code, and to provide information to assist in installing the code.

Reardon, John E.↗
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.↗