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At least 307 records · Page 17

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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.↗

NASA NTRS1976

Microstructural changes caused by thermal treatment and their effects on mechanical properties of a gamma/gamma prime - delta eutectic alloy

Microstructural changes due to thermal treatments of a directionally solidified gamma/gamma'-delta eutectic alloy were investigated. Aging treatments of 8 to 48 hours and ranging from 750 to 1120 C were given to the alloy in both its as directionally solidified condition and after gamma' solutioning. Aging resulted in gamma' coarsening gamma precipitates in delta, and delta and gamma'' precipitates in delta. The tensile strength was increased about 12 percent at temperatures up to 900 C by a heat treatment. Times to rupture were essentially the same or greater than for as directionally solidified material. Tensile and rupture ductility in the growth direction of the alloy were reduced by the heat treatment.

Tewari, S. N.↗

NASA NTRS1977

Microstructural and wear properties of sputtered carbides and silicides

Sputtered Cr3C2, Cr3Si2, and MoSi2 wear-resistant films (0.05 to 3.5 microns thick) were deposited on metal and glass surfaces. Electron transmission, electron diffraction, and scanning electron microscopy were used to determine the microstructural appearance. Strong adherence was obtained with these sputtered films. Internal stresses and defect crystallographic growth structures of various configurations within the film have progressively more undesirable effects for film thicknesses greater than 1.5 microns. Sliding contact and rolling element bearing tests were performed with these sputtered films. Bearings sputtered with a duplex coating (0.1-micron-thick undercoating of Cr3Si2 and subsequently 0.6-micron coating of MoS2) produced marked improvement over straight MoS2 films.

Spalvins, T.↗

NASA NTRS1977

Microstructure of a magnetotail fireball

We use high time resolution data from the magnetometer, LEPEDEA and plasma wave analyzer on IMP-7 to examine the microstructure of a long-duration magnetotail fireball event observed during an extended quiet period on November 9, 1972. We demonstrate that the magnetic field is turbulent with fluctuation time scales down to several seconds. These findings suggest that the magnetic merging process may be a highly turbulent one, possibly associated with a form of the tearing mode instability.

Coroniti, F. V.↗

NASA NTRS1977

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

Fractography and metallographic sectioning were used to investigate the influence of microstructure and strength on the fracture toughness (KIc) and fracture mechanism of an 18 Ni, 300 grade maraging steel. Increased yield strength from 1442 to 2070 MN/m squared through precipitation hardening results in a KIc loss from 143 to 55 MN/m superscript 3/2. Ti (C,N) Ti2S, and TiC inclusions in sizes from 1 to 8, 1 to 15, and 0.1 to 2 microns respectively serve as sites for void nucleation and lead to fracture by the dimpled rupture process in all strength levels considered. TiC nucleated dimples occupy more than half the fracture in all conditions. Void nucleation rate and resultant number of dimples per unit area of fracture increase with increasing yield strength. Average dimple size decreases with increasing strength and/or overaging which follows from the decreasing amount of stable void growth measured by sectioning tensile specimens. Void growth is assisted by crack branching along a path of TiC inclusions. Coalescence occurs in the highest strength materials by a combination of TiC void nucleation and premature separation at strengthening precipitates.

Psioda, J. A.↗

NASA NTRS1978

The effect of microstructure on hydrogen embrittlement of the nickel-base superalloy, Udimet 700

Material from a single heat of cast and wrought Udimet 700 was processed and/or heat treated to produce five material conditions with identical chemical compositions but with distinct microstructural variations, and then evaluated for susceptibility to hydrogen embrittlement. Two prealloyed powder conditions exhibited significantly improved resistance to hydrogen embrittlement, as compared to wrought material. No degradation in notch or smooth tensile strengths occurred, and average ductilities of 25 percent reduction of area were determined for 2 hydrogen evaluation procedures. For the most severe hydrogenation procedure, ductility levels were reduced to 15 percent. These improvements were attributed to cleaner grain boundaries and decreases grain size.

Gray, H. R.↗

NASA NTRS1978

Microstructure and mechanical properties of quenched and tempered 300M steel

Type 300M steel, which is being used for the landing gear on the space shuttle orbiter, was subjected to a wide range of quenched and tempered heat treatments. The plane-strain fracture toughness and the tensile ultimate and yield strengths were evaluated. Cryogenic mechanical properties were obtained for conventionally heat-treated steel. The microstructure of all heat-treated test coupons was studied both optically and by transmission electron microscopy. Fracture surfaces were studied by means of scanning electron microscopy. Results indicate that substantial improvement in toughness with no loss in strength can be accomplished in quenched and tempered steel by austenitizing at 1255 K or higher. Low fracture toughness in conventionally austenitized 300M steel (1144 K) appears to be caused by undissolved precipitates, seen both in the submicrostructure and on the fracture surface, which promote failure by quasi-cleavage. The precipitates appeared to dissolve in the range 1200 to 1255 K.

Youngblood, J. L.↗

NASA NTRS1977

Microstructure of hot-pressed Al2O3-Si3N4 mixtures as a function of holding temperature

Powder mixtures of 40 m/o Si3N4-60 m/o Al2O3 were hot-pressed at 4000 psi at various holding temperatures from 1100 C to 1700 C. Scanning and transmission electron microscopy results were correlated to X-ray phase analysis and density measurements. The progressively developed microstructure was used to interpret the densification behavior of SiAlON.

Yeh, H. C.↗

NASA NTRS1977

Effect of microstructure on Na/+/ diffusion in beta alumina

The effect of microstructural features, such as grain size and orientation, on sodium ion diffusion in polycrystalline beta alumina is investigated; the ionic solid is suitable for use in high-energy and power density batteries. Two types of hot-pressed polycrystalline samples are studied, one with grains highly oriented in the direction normal to the pressing direction, the other with significantly less preferred orientation normal to the pressing direction. Results of the experimental assessments indicate that grain size and diffusion path tortuosity have little effect on diffusion kinetics, while grain boundary type and grain aspect ratio are important factors in determining the diffusion rate.

Sarian, S.↗

NASA NTRS1977

Magnetic and microstructural properties of some lodestones

A lodestone is an iron ore that behaves as a permanent magnet. A protolodestone is an iron ore capable of being charged sufficiently strongly to behave as a permanent magnet. In the present paper, important criteria are established which distinguish lodestones from other iron ores. Magnetic hysteresis-loop data provide evidence that the massive pieces of lodestone behave magnetically as fine intergrowths rather than coarse multidomain material. This means that the iron ores have been magnetically hardened. The nature and scale of the hardening microstructure is illustrated by photo micrographs. The mechanism of charging the protolodestone appears to be either transient magnetic fields with lightning-discharge currents, or presently obscure aspects of magnetization intensity enhancement associated with maghemitization of massive iron ores.

Wasilewski, P. J.↗

NASA NTRS1977

Microstructural and wear properties of sputtered carbides and silicides

Sputtered Cr3C2, Cr3Si2, and MoSi2 wear-resistant films (0.05 to 3.5 microns thick) were deposited on metal and glass surfaces. Electron transmission, electron diffraction, and scanning electron microscopy were used to determine the microstructural appearance. Strong adherence was obtained with these sputtered films. Internal stresses and defect crystallographic growth structures of various configurations within the film have progressively more undesirable effects for film thicknesses greater than 1.5 microns. Sliding contact and rolling-element bearing tests were also performed with these sputtered films.

Spalvins, T.↗

NASA NTRS1978

The effect of microstructure on hydrogen embrittlement of the nickel base superalloy, Udimet 700

Gray, H. R.↗

NASA NTRS1978

A microstructural study of the Tishomingo meteorite

Metallography, electron microscopy, and X-ray diffraction techniques were employed to study a fragment of the Tishomingo iron meteorite. The results suggest the following thermal-mechanical history: The fragment was originally a large crystal of taenite (gamma). Cooling through the alpha + gamma phase boundary did not result in accompanying precipitation of kamacite (alpha). Transformation to a martensitic structure initiated between -25 and -65 C. Transformation continued as the temperature fell to -75 to -115 C, resulting in approx 80% martensite (alpha-prime). Subsequent shock deformation and thermal aging processes substantially modified the taenite and martensite microstructures. Twins in the retained taenite phase are attributed to shock deformation at a pressure estimated for a single event at about 170 kbar. The existing complex altered martensite structure containing both taenite and kamacite (3-15% Ni) particles was apparently the product of both shock deformation and thermal aging processes. The maximum temperature reached during thermal aging is estimated to be less than 400 C and perhaps below 310 C.

Ives, L. K.↗

NASA NTRS1978

Growth parameters for aligned microstructures in directionally solidified aluminum-bismuth monotectic

Microstructures are shown for directionally solidified Al-3.4 wt pct Bi alloys with 0.2 wt pct Fe and 0.6 wt pct Fe additions. The third element causes the LI/SI + LII growth interface to become cellular. The bismuth forms at the cell nodes, appearing either as uniformly spaced arrays of spheres in the case of 0.2 wt pct Fe, or as an irregular network in the case of 0.6 wt pct Fe. Changes in the growth conditions which are known to control cellular structure are seen to have a similar effect on the bismuth spacing, with the cross sectional spacing varying as the inverse of G (Temperature gradient) x R(Growth rate).

Parr, R. A.↗

NASA NTRS1979

The influence of microstructure on the strength of reaction bonded silicon nitride /RBSN/

The effect of the microstructure of reaction bonded silicon nitride on strength is examined. The morphologies of the various microconstituents (alpha-matte, beta-spikes, residual silicon and porosity) each play a role in determining the strength of the material. While density and hence percent porosity greatly affect strength, it is shown that the pore size and pore size distribution affect strength with equal importance. By minimizing the pore size and distribution and maximizing the alpha-beta ratio, a reaction bonded material (2.4 kg/cu m) has been produced with a room temperature modulus of rupture (three-point bending) of 297 MPa from a low green density starting material (1.5 kg/cu m).

Danforth, S. C.↗