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At least 1,261 records · Page 70
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Microstructure of the Interplanetary Medium

High time resolution measurements of the interplanetary magnetic field and plasma reveal a complex microstructure which includes hydromagnetic wave and discontinuities. The identification of hydromagnetic waves and discontinuities, their statistical properties, their relation to large-scale structure, and their relative contribution to power spectra are discussed.

Burlaga, L. F.↗
NASA NTRS1973

An exploratory study of the microstructure of mullite fibers

Mullite fibers of three compositions, ranging from SiO2-rich to Al2O3-rich, were investigated by a number of transmission electron microscopy (TEM) techniques. The fibers were examined in the as-received condition and after subjecting them to thermal exposures as high as 142 C. The investigative techniques used included direct TEM of microtomed sections of mounted fibers and TEM of replicas of polished, chemically etched, and cathodically etched fibers. In addition, X-ray diffraction line broadening analyses was used for determining average crystallite size. A preliminary description of the microstructure of mullite fibers is given.

Santoro, G. J.↗
NASA NTRS1973

Correlation of creep rate with microstructural changes during high temperature creep

The techniques of electron microscopy were used to examine the microstructural changes which occur during primary creep for two important types of engineering alloys: (1) alloys strengthened by solid-solution additions, and (2) dispersion-strengthened alloys. The metals chosen for study are unalloyed titanium, Ti-6Al-4V, and the cobalt-base alloy, Haynes 188. Results to date on NGR 47-004-108 show that development of prior dislocation substructure in Haynes 188 by 10% prestrain and annealing for one hour at 1800 F increases the time to reach 0.5% creep strain at 1600 F by more than an order of magnitude for creep stresses from 3 to 20 ksi. For creep at 1800 F, similar results were obtaind for stresses above 7 ksi, but the prior substructure decreases creep resistance below 7 ksi. This effect appears to be related to instability of grain structure at 1800 F in prestrained material.

Young, C. T.↗
NASA NTRS1973

An Exploratory Study of the Microstructure of Mullite Fibers

The microstructures of mullite composition fibers were examined by transmission electron microscopic techniques and by X-ray diffraction analyses. Both spun and monofilament fiber structures contained little porosity, nor did the degree of porosity change after heating the monofilament to 1205 C for one half hour. Thermal exposure produced crystallite growth and nucleation from the amorphous phase in all three of the spun fibers. After 116 hours at 1426 C, the crystallites of the standard spun fibers became contiguous. In addition, the surface crystallites of these fibers were found to be more than twice as large as the interior crystallites.

Santoro, G.↗
NASA NTRS1973

The effect of microstructure on the fracture toughness of titanium alloys

The high-strength titanium alloys are widely used in aircraft and aerospace structures due to their high strength to density ratios. In such applications, the fracture toughness rather than the strength is often the factor which requires larger size sections and lower useful payloads. The response of the strength and toughness of titanium alloys was analyzed generally without regard to the fracture mode or the effect of microstructure on the fracture mechanisms. Research on the fracture mechanisms in aluminum alloys and steels showed that the toughness may be improved by decreasing the sizes of inclusions and sub-micron precipitates. An investigation was conducted to study the fracture mechanisms in titanium alloys which may lead to suggestions for the improvement of the fracture toughness without a corresponding loss in strength.

Vanstone, R. H.↗
NASA NTRS1974

The effect of microstructure and strength on the fracture toughness of an 18 ni, 300 grade maraging steel

Methods for increasing the strength of maraging steels are discussed. An investigation was conducted to systematically vary the strength of 18 weight percent nickel, 300 grade maraging steel, to isolate any attending microstructural changes, and to study the effects of these changes on the fracture toughness of the alloy. A study aimed at determining the aging behavior of the program alloy was carried out to provide data by which to estimate yield strength. The effects of various alloying materials on the strength of the maraging steel are examined. The mechanical properties of the 300 grade maraging steel were determined by tension tests, fatigue precracked Charpy impact tests, and plane strain fracture toughness tests.

Psioda, J. A.↗
NASA NTRS1975

The effect of microstructure and strength on the fracture toughness of an 18 Ni, 300 grade maraging steel

A 300 grade maraging steel was chosen as a vehicle by which to understand the inverse relationship between strength and toughness in high strength alloys such as the 18 Ni maraging steels. The 18 Ni, 300 grade maraging material was a commercial grade consumable-electrode, vacuum arc remelted heat obtained in the form of forged and annealed plate. The matrix contained a population of second-phase impurity inclusions which was a product of the casting and hot working processes. These inclusions did not change with subsequent precipitation hardening. Changes in microstructure resulting in strength increases were brought about by variations in aging temperature and time. Maximum strength was attained in the 300 grade maraging steel by aging at 427 C (800 F) for 100 hours. Tensile, fatigue precracked Charpy impact, and plane-strain fracture toughness tests were performed at room temperature, 20 C (68 F). With increasing strength the fracture toughness decreases as smaller and smaller inclusions act as sites for void initiation.

Psioda, J. A.↗
NASA NTRS1975

Microstructure of the solar wind

In this review microstructure is understood to summarize the dynamical properties of the solar wind that enter the fluid equations, usually employed to describe the overall flow, only in one or the other averaged form. Thus, first a description of the kinetic picture of the solar wind is given; it mainly involves the structure of the particle distributions in velocity space and associated instabilities. Waves and discontinuities are discussed next. Two selected examples are mentioned where the effects of waves and discontinuities play a role. Specific physical properties of the interplanetary magnetic irregularity spectrum and its relevance for cosmic ray propagation are discussed at the end.

Voelk, H. J.↗