Engineering topics

Whittenberger, J. D.

Publications and source records attributed to Whittenberger, J. D..

At least 55 records · Page 3

NASA NTRS1991

Compression studies on particulate composites of ternary Al-Ti-Fe, and quaternary Al-Ti-Fe-Nb and Al-Ti-Fe-Mn L1(2) compounds

Compression studies were conducted on monolithic and TiB2 particulate-reinforced composites of Al22Fe3Ti8, both with and without minor quaternary alloying additions (2 at. pct Nb and 2 at. pct Mn) as a function of temperature and as a function of strain rate at high temperature. The volume fraction of reinforcement was varied between 0 and 20 percent. The particulate reinforcements were found to be effective in increasing ambient- and warm-temperature strength; at high temperatures, the monolithic material is stronger than the composites, although the composites are superior at slow strain rates. The microstructures of the monolithic and composite specimens were examined before and after deformation to explain these observations.

Kumar, K. S.↗

NASA NTRS1991

Comments on the paper 'Does the subgrain size influence the rate of creep'?

A reply is given to a recent paper which questioned the validity of experimental observations and conclusions on the importance of cells in determining the creep behavior of materials. The reply clarifies experimental procedures and observations on NaCl single crystals and examines the importance of cells in determining creep behavior. It is concluded that cells are not spurious microstructural features and that they should be included along with subgrains and 'free' dislocations in developing creep models.

Raj, S. V.↗

NASA NTRS1991

Mechanical properties of monolithic and particulate composites of L12 forms of Al3Ti

Preliminary studies of simple compressive properties of particulate composites with L1(2)-modified Al3Ti's as the matrices were performed as functions of matrix composition, amount of TiB2, processing technique, temperature, and strain rate. At and below about 900 K the introduction of TiB2 particles overwhelms minor chemical effects, and extremely high-yield strengths (about 2000 MPa at the 20 vol pct TiB2 level) were obtained. As L1(2)-modified Al3Ti has about half the density of the Ni-base superalloys, such values translate to strength/density ratios approximately four times those for superalloys. This advantage is not maintained by the Al3Ti-based composites above about 1000 K, particularly under slow strain rate plastic flow conditions, and it is proposed that this behavior could be due to the dissolution of intermetallic second phases as well as grain boundary creep mechanisms.

Whittenberger, J. D.↗

NASA NTRS1991

1200 and 1300 K slow plastic compression properties of Ni-50Al composites

XD synthesis, powder blending, and hot pressing techniques have been utilized to produce NiAl composites containing 4, 7.5, 15, and 25 vol pct alumina whiskers and hybrid composite materials with 15 vol pct Al2O3 + 10 or 20 vol pct, nominally 1 micron TiB2 particles. The resistance to slow plastic flow was determined at 1200 and 1300 K via compression testing in air under constant velocity conditions. The stress-strain behavior of the intermetallic composites depended on the fraction of second phases where the 4 and 7.5 percent Al2O3 materials flowed at a nominally constant stress after about 2 percent deformation, while all the other composites exhibited diffuse yielding followed by strain softening. The flow stress-strain rate properties increased with volume fraction of Al2O3 whiskers except for the 4 and 7.5 percent materials, which had similar strengths. The hybrid composite NiAl + 15Al2O3 + 10TiB2 was substantially stronger than the materials simply containing alumina. Deformation in these composites can be described by the Kelly and Street model of creep in perfectly bonded, rigid, discontinuous fiber materials.

Whittenberger, J. D.↗

NASA NTRS1991

Properties of pure nickel after long term exposures to LiOH and vacuum at 775 K

The solid to liquid phase transformation of LiOH at 744.3 K is considered to be an ideal candidate thermal energy storage (TES) mechanism for a Rankine heat engine based solar dynamic system operating at approximately 682 K. While pure nickel is thought to be a suitable containment material for LiOH, long term containment is of concern because molten hydroxides are usually corrosive. Two commercially pure nickel alloys, Ni-200 and Ni-201, were exposed to molten LiOH, its vapor, and vacuum at 775 K for periods ranging from 50 to 5000 h, and simple mechanical property measurements (77 to 900 K tensile and 750 K creep rupture) of exposed alloys were undertaken. The mechanical property test procedures are described and tabular lists of the test data are presented.

Whittenberger, J. D.↗

NASA NTRS1990

Compression behavior of the forged Ll2 compounds Al67Ti25Cr8 and Al66Ti25Mn9

The paper presents the findings of a compression study as a function of temperature from cryogenic temperatures to 1373 K at a strain rate of about 0.0001/s on forged Al67Ti25Cr8 and Al66Ti25Mn9 compounds. The creep response of these alloys is analyzed at strain rates varying from 2 x 10 to the -4th to 2 x 10 to the -7th /s at 1000 K and 1100 K. It is shown that as-cast microstructures for both compounds are dendritic, while both intermetallics forged very well with no edge cracking, with the microstructures appearing fully recrystallized. True comprehensive stress-strain diagrams determined at 1100 K as a function of nominal strain rate reveal a range of behaviors in both materials.

Brown, S. A.↗

NASA NTRS1990

High-temperature slow-strain-rate compression studies on CoAl-TiB2 composites

Results are presented of compressive deformation tests performed on particulate-reinforced CoAl-TiB2 composites in the temperature range 1100-1300 K. Hot-pressed and postdeformation microstructures were characterized by TEM and by optical microscopy. It was found that the addition of TiB2 particles improves the deformation resistance of the matrix, due to dislocation-particle interactions.

Mannan, S. K.↗

NASA NTRS1990

Slow plastic deformation of extruded NiAl-10TiB2 particulate composites at 1200 and 1300 K

A dispersion of 1-micron TiB2 particles in the B2 crystal structure NiAl intermetallic can effectively increase its elevated temperature strength, in association with increasing deformation resistance with TiB2 volume fraction. Attention is presently given to alternative densification methods, which may increase the initial as-fabricated dislocation density and lead to enhanced elevated-temperature strength. The 'XD' extrusion method was used to produce NiAl with 10 vol pct TiB2. Although apparent extrusion defects were occasionally found, neither grain-boundary cracking nor particle-matrix separation occurred.

Whittenberger, J. D.↗

NASA NTRS1990

Deformation of as-cast LiF-22 mol pct CaF2 hypereutectic salt between 500 and 1015 K

Results are presented on compression tests conducted on as-cast LiF-22 mol pct CaF2 hypereutectic specimens at nominal strain rates between 1.8 x 10 to the -6th/sec and 0.25/sec over the temperature range 500-1015 K. In all instances, the stress-strain curves showed broad maxima, with negative strain-hardening rates after the peak stress sigma(max). It was found that, at low temperatures and high stresses, the CaF2 lamellae are rigid while the LiF matrix exhibits extensive transgranular cavitation, while at high temperatures and low stresses the CaF2 lamellae break down and spheroidize while the LiF matrix does not cavitate. It was concluded that the mechanical properties of the as-cast hypereutectic LiF-22 mol pct CaF2 are governed by the rate of deformation of the CaF2 phase. It is suggested that, for thermal energy storage applications, a spheroidal microstructure is more desirable than a lamellar structure.

Raj, S. V.↗

NASA NTRS1990

Cyclic oxidation resistance of a reaction milled NiAl-AlN composite

Based upon recent mechanical property tests a NiAl-AlN composite produced by cryomilling has very attractive high temperature strength. This paper focuses on the oxidation resistance of the NiAl-AlN composite at 1473 and 1573 K as compared to that of Ni-47Al-0.15Zr, one of the most oxidation resistant intermetallics. The results of cyclic oxidation tests show that the NiAl-AlN composite has excellent properties although not quite as good as those of Ni-47Al-0.15Zr. The onset of failure of the NiAl-AlN was unique in that it was not accompanied by a change in scale composition from alumina to less protective oxides. Failure in the composite appears to be related to the entrapment of AlN particles within the alumina scale.

Lowell, Carl E.↗

NASA NTRS1989

On the stability of the creep substructure in NaCl single crystals

Microstructural observations were conducted on NaCl single crystals after creep. The microstructure after a stress increase followed by a stress decrease consisted primarily of cells; no significant number of subgrains were observed although they were present in the microstructures produced during uninterrupted tests. Prestraining in the exponential creep regime produced an uniform distribution of dislocations and a few subboundaries which transformed to equiaxed subgrains when tested in the power law creep region. This substructure was similar to that observed in an as-received specimen deformed to an equivalent strain. Prior creep in the power law creep region produced equiaxed subgrains whose boundaries were found to be mechanically stable when the specimen was retested in the exponential creep region. The role of subboundary migration in the formation of new subgrains is discussed.

Raj, S. V.↗

NASA NTRS1989

The mechanical properties of fluoride salts at elevated temperatures

The deformation behavior of CaF2 and LiF single crystals compressed in the 111 and the 100 line directions, respectively, are compared with the mechanical properties of polycrystalline LiF-22 (mol pct) CaF2 eutectic mixture in the temperature range 300 to 1275 K for strain rates varying between 7 x 10 to the -7th and 0.2/s. The true stress-strain curves for the single crystals were found to exhibit three stages in an intermediate range of temperatures and strain rates, whereas those for the eutectic showed negative work-hardening rates after a maximum stress. The true stress-strain rate data for CaF2 and LiF-22 CaF2 could be represented by a power-law relation with the strain rate sensitivities lying between 0.05 and 0.2 for both materials. A similar relation was found to be unsatisfactory in the case of LiF.

Raj, S. V.↗

NASA NTRS1988

Use of pure nickel and LiOH for thermal energy storage

The solid to liquid phase transformation of LiOH has been proposed as an ideal candidate thermal energy storage media for a Rankine Cycle powered electrical generation unit envisioned in Space Station based solar dynamic systems. Due to the corrosive nature of molten hydroxides, long term containment of LiOH is of concern. Pure nickel is thought to be a suitably resistant material, and a program has been instituted to measure the effects of prolonged exposure of liquid and gaseous LiOH on the mechanical properties of pure nickel alloys. Results to date indicate that negligible weight and thickness changes occurred in Ni alloys exposed to LiOH for as long as 2500 hr at 775 K, and essentially no difference in 77-900 K tensile properties could be detected between LiOH exposed and vacuum annealed Ni specimens. Although there was little sign of outward damage, microstructural examination revealed that all hydroxide contaminated tensile test specimens had surface connected intergranular cracks along the gage lengths. Two other potential problems, which have strong implications with respect to a LiOH/Ni energy storage system, were also noted during the corrosion experiments. In particular stress corrosion cracking of weld joints in pressurized vessel and permeation of hydrogen through nickel were observed.

Whittenberger, J. D.↗

NASA NTRS1987

A preliminary report on the effects of long-term exposure of LiOH on pure nickel

A 'bread pan' capsule has been designed which allows large numbers of tensile specimens to be simultaneously exposed to molten LiOH, its vapor, and vacuum. Capsules and specimens fabricated from the pure nickel alloy Ni-200 were annealed for 401 hours and 2500 hours at 775 K. Examination of the exposed materials revealed that little outward damage in terms of visible attack, weight change, or loss of room temperature tensile properties occurred. In particular, the mechanical behavior of hydroxide-contaminated alloy was essentially identical to that receiving a simple thermal exposure in vacuum. Examination of the microstructures revealed that LiOH did produce some nonuniform, shallow intergranular corrosion in NI-200; however, the extent of the damage was insufficient to produce weakening or embrittlement.

Whittenberger, J. D.↗

NASA NTRS1987

Identification of salt-alloy combinations for thermal energy storage applications in advanced solar dynamic power systems

Thermodynamic calculations based on the available data for flouride salt systems reveal that a number of congruently melting compositions and eutectics exist which have the potential to meet the lightweight, high energy storage requirements imposed for advanced solar dynamic systems operating between about 1000 and 1400 K. Compatibility studies to determine suitable containment alloys to be used with NaF-22CaF2-13MgF2, NaF-32CaF2, and NaF-23MgF2 have been conducted at the eutectic temperature + 25 K for each system. For these three NaF-based eutectics, none of the common, commercially available high temperature alloys appear to offer adequate corrosion resistance for a long lifetime; however mild steel, pure nickel and Nb-1Zr could prove useful. These latter materials suggest the possibility that a strong, corrosion resistant, nonrefractory, elevated temperature alloy based on the Ni-Ni3Nb system could be developed.

Whittenberger, J. D.↗

NASA NTRS1986

Estimated heats of fusion of fluoride salt mixtures suitable for thermal energy storage applications

The heats of fusion of several fluoride salt mixtures with melting points greater than 973 K were estimated from a coupled analysis of the available thermodynamic data and phase diagrams. Simple binary eutectic systems with and without terminal solid solutions, binary eutectics with congruent melting intermediate phases, and ternary eutectic systems were considered. Several combinations of salts were identified, most notable the eutectics LiF-22CaF2 and NaF-60MgF2 which melt at 1039 and 1273 K respectively which posses relatively high heats of fusion/gm (greater than 0.7 kJ/g). Such systems would seemingly be ideal candidates for the light weight, high energy storage media required by the thermal energy storage unit in advanced solar dynamic power systems envisioned for the future space missions.

Misra, A. K.↗

NASA NTRS1986

Effect of multiple strain-anneal cycles on the 1000 C creep behaviour of gamma/gamma prime-alpha

Various multiple strain-anneal cycles (1000 C) were imposed on specimens of the directionally solidified eutectic (DSE) alloy gamma/gamma prime-alpha to identify thermomechanical processing methods (TMP) which would improve the creep behavior. Specimens of the Ni-32.3Mo-6.3Al wt pct alloy were grown with a modified Bridgeman technique. Some of the cylindrical specimens were alternately heat-treated at 900 C, then strained, or heat-treated only, while other specimens were annealed at 900 C after swaging and then worked at ambient temperature. The specimens were all examined microstructurally using transmission electron microscopy, some before and after being exposed to constant-load compression tests at 1000 C. The creep strain increased for all TMP specimens for strain rates of at least 2 millionths per sec. Strain rates of about 2 ten millionths per sec were only improved with strain annealing with 13 percent work at ambient temperature. A slight improvement, compared to as-grown materials, was observed in the 1000 C creep behavior of materials annealed at 900 C. Strain-annealing was found to introduce three-dimensional dislocation networks into the gamma-prime matrix.

Whittenberger, J. D.↗

NASA NTRS1986

The influence of grain size and composition on slow plastic flow in FeAl between 1100 and 1400 K

The slow plastic flow properties of a series of binary B2 FeAl intermetallics at elevated temperatures, and ranging in aluminum content from 39.8 to 48.7 at. pct, were investigated using constant-velocity compression tests at strain rates from 2 x 10 to the -3rd to 2 x 10 to the -7th. Two deformation mechanisms with the same activation energy are found. For the mode with a stress component of 6, the strength increases with decreasing grain size, probably as a result of Hall-Petch behavior, while for the mode with stress component of 3, the strength increases with increasing grain size, probably due to the action of diffusional creep. Creep in the high stress exponent mode can be described in terms of the initial grain size because the large-angle grain structure is replaced by a small-angle grain microstructure of similar grain diameter during deformation.

Whittenberger, J. D.↗