Showing papers on "Microstructure published in 1976"

Sintering of Covalent Solids

Abstract: The sintering behavior of primarily covalently bonded β-SiC, Si, and Si3N4 was studied using surface area and densification measurements as well as observations of microstructures developed during firing. The existence of highly dense, microscopic regions and large (≥100°) dihedral angles in fired compacts of β-SiC and Si which experience little macroscopic densification suggests that macroscopic densification is not intrinsically limited by the effects of surface energy. The mechanism proposed to explain the microstructure that develops in unsinterable covalent solids which do not undergo a phase change is based on the existence of a high ratio of surface and/or vapor-phase matter transport-to-volume and/or grain-boundary transport. The addition of boron to both β-SiC- and Si-containing carbon retards surface and/or vapor-phase transport and grain growth at lower temperatures, which results in enhanced densification at high temperatures. Macroscopic densification of β-SiC and α-Si3N4 can also be retarded by the formation of a continuous network of high-aspect-ratio grains of the polymorphic form that rigidities the sintering body. Finally, the sintering of pure Si depends sensitively on particle size in the submicron range. Nearly theoretical density is achieved in Si powder of ∼0.06-μm size. This result suggests that other pure covalently bonded solids can also be sintered to high density without applied pressure.

Dependence of Electrical Conductivity of ZnO on Degree of Sintering

Abstract: The electrical resistivity of ZnO doped with Al2O3 was measured in air and under reduced pressure (∼0.5 mm Hg) in the range from 30° to 680°C as a function of the degree of sintering. The data obtained were explained in terms of the effects of the microstructure of the sintered body and the chemisorbed oxygen. There are two mechanisms of electrical conduction involved, only one of which is affected by the microstructure.

Crack Healing and Strengthening of Thermally Shocked Alumina

Abstract: Crack healing and strengthening of thermally shocked alumina are related to pore evolution in the microstructure during annealing Grain growth is not necessary for either an increase or a decrease in strength following heat treatment

High temperature creep in a semi-coherent NiAl-Ni 2 AlTi alloy

Abstract: Creep experiments have been made on a Ni-Ti-Al alloy, which has a microstructure consisting of a distribution of semi-coherent NiAl(β) precipitates with a Ni2AlTi(β′) Heusler phase matrix. The creep strength of this bcc type structure alloy is at least comparable with that of the nickel-base superailoy MARM-200 for values ofT/T m in the range 0.68 to 0.82. Quantitative electron microscope experiments show that both undissociated α0〈110〉 dislocations, and paired α0〈100〉 dislocations coupled by a sublattice A.P.B. exist within the β′ phase;α 0 is the lattice parameter of a bcc cell of which the large Ni2AlTi unit-cell is composed. The sublattice A.P.B. is a crystallographic fault created by wrong bonds between atoms on the Al-Ti sublattice. Theoretically the energy γ of a sublattice A.P.B. is shown to be minimum on {100}, and the experimental value for γ on {100} is ~40 mJ/m2.

Microstructure and mechanical properties relationships in the Ti-11 alloy at room and elevated temperatures

Abstract: To establish correlations between microstructure and mechanical properties for the Till alloy, twelve different combinations of hot die forging and heat treatment, in the α+β and β phase regions, were investigated. The resulting heat treated forgings were classified into four distinct categories based on their microstructural appearance. The room temperature tensile, post-creep tensile, fracture toughness and fatigue crack propagation properties were measured along with creep and low cycle fatigue at 566°C. The creep, tensile, fatigue crack propagation and fracture toughness properties, grouped in a manner similar to the microstructural categories. The fracture appearance and behavior of the cracks during propagation in fatigue and in fracture toughness tests were examined, and correlations with the microstructure discussed. In the case of the fully transformed acicular microstructure, it was found that the size and the orientation of colonies of similarly aligned α needles are dominant factors in the crack behavior.

The relationship of microstructure to monotonic and cyclic straining of two age hardening aluminum alloys

Abstract: The effect of microstructure on the monotonic and low cycle fatigue properties of a high purity, large grain, ternary aluminum-zinc, magnesium (Al-Zn-Mg) alloy and a high strength 7050 aluminum alloy was investigated. The best combination of fatigue life, strength, and ductility for the ternary alloy resulted when aged to produce a microstructure containing predominately η′ having a Guinier radius of approximately 65a and a small amount of incoherent η (MgZn2). Superior fatigue life, strength, and ductility were found when the 7050 alloy was aged to produce the maximum number of partially coherent η′ precipitates having a Guinier radius of approximately 35a. Aging the 7050 alloy to produce particles larger than 50a gave a microstructure that had lower fatigue properties at the low plastic strain amplitudes, δep/2 <1.0 pct. The empirical CoffinManson relationship was found to hold for a given deformation process, however changes in deformation character resulted in changes in the Coffin-Manson parameters.

Effect of microstructure on notch fatigue properties of Ti-6Al-4V

Abstract: The effect of microstructure on the notch fatigue properties of Ti-6A1-4V was investigated. Specimens with five distinctly different microstructures were tested and subsequently examined in detail. It was found that the notch fatigue performance of the alloy varied significantly as the microstructure was altered by heat treatment. The best high cycle fatigue strength was found in specimens heat treated above the beta transus temperature, containing an almost totally transformed acicular alpha structure. The fatigue performance of specimens with this microstructure appeared to be controlled by the size of the nucleated crack. It is suggested that at low stress levels the nucleated crack is limited in size to the width of a single alpha needle, while at high stresses the nucleated crack may be as large as an entire colony of similarly aligned alpha needles.

Creep of titanium-silicon alloys

Abstract: Operative creep mechanisms in laboratory melts of Ti-5Zr-0.5Si and Ti-5Al-5Zr-0.5Si have been investigated as a function of microstructure, creep stress, and temperature. From creep rate data and transmission electron microscopy results, it has been shown that an important creep strengthening mechanism at 811 K in Si bearing Ti alloys is clustering of solute atoms on dislocations. All of the alloys investigated showed anomalously high apparent activation energies and areas for creep, and a high exponent (n) in the Dorn equation. In addition, the effect of heat treatment was investigated and it is shown that the highest creep strength was obtained by using a heat treatment which retained the maximum amount of silicon in solution. This is consistent with the proposed creep strengthening mechanism. An investigation of the creep behavior of several other Si containing alloys including two commercial alloys, Ti-11 and IMI-685 indicated similar results.

The effect of thermal exposure on the mechanical properties of aluminum-graphite composites

Abstract: The mechanical properties of aluminum-graphite composites were measured at room temperature in the as-received condition, after elevated temperature exposure and after thermal cycling. The composites were fabricated by solid-state diffusion bonding of liquid-phase Al-infiltrated Thornel 50 fibers. The results showed that the maximum longitudinal tensile strength of the as-received material was 80,000 psi, which corresponds well with the rule of mixture value. The composite strength was observed to vary widely, depending on the extent of wetting of the fibers by the aluminum. The strength of the composites in the transverse direction was generally very low, due to poor interfacial bonding. Aluminum carbide (Al4C3) formed at the surface of the fibers at temperatures greater than 500 C. Development of the carbide was shown to be diffusion-controlled and was dependent on the time and temperature used. It was shown that the tensile strength was virtually unaffected by heat-treatment up to 500 C; beyond that temperature a drastic degradation of tensile strength occurred. Thermal cycling of the composites below 500 C resulted in an observable degradation of the composite strength.

Microstructure of glass fibre-reinforced cement composites

Abstract: The microstructure and fracture surfaces of glass fibre-reinforced cement (grc) composites have been examined by scanning electron microscopy. The toughness of grc stored in dry environments is attributed to the formation of a complex structure of subsidiary cracks, resulting in a large failure surface area, and the pull-out of the glass fibres. The reduction of the toughness of grc stored in wet conditions over a period of time reflects the reduction in the amount of subsidiary cracking and fibre pull-out. Observations indicate that this is due to changes in the microstructure of the matrix in and around the fibre bundles.

Cryogenic fracture toughness of 9Ni steel enhanced through grain refinement

Abstract: A thermal cycling technique was used to refine the grain size of commercial “9Ni” cryogenic steel. The grain refined alloy was then tempered, a treatment which introduced a small admixture of retained austenite. The reprocessed alloy shows an excellent combination of strength and toughness to temperatures as low as 6 K, and shows little evidence of embrittlement in liquid helium. The microstructure and mechanical properties of the reprocessed alloy are described and compared to those obtained through conventional treatment.

Irradiation effects on the microstructure and properties of metals

Abstract: The conference included 29 papers. Separate abstracts were prepared for 23 papers. Other papers, having conference numbers 760515-4, 760515-6, 760515-1, and 760515-3, appeared previously in ERA. (JFP)

Microstructure and Toughness of High-Strength Aluminum Alloys

Abstract: The toughness of wrought, high-strength aluminum alloys is related to the amount, type, and morphology of coarse (larger than about 1 μm) constituent particles, intermediate size (about 0.02 to 0.5 μm) dispersoids, and fine (down to about 0.001 μm) precipitates. High toughness can be attained by minimizing the size and volume fraction of constituent particles, increasing the interdispersoid distance, refining the intragranular precipitate in 2XXX alloys, and controlling the intergranular precipitate in 7XXX alloys. For highest toughness in 7XXX alloy products where low residual stress is desired, rapid quenching followed by the minimum amount of cold work required for mechanical stress relief is recommended.

Morphological and electrical properties of rf sputtered Y2O3‐doped ZrO2 thin films

Abstract: The structural, compositional, and electrical properties of rf sputter‐deposited Y2O3‐doped ZrO2 films have been investigated as a function of sputtering conditions. The results show that the application of an rf substrate bias during deposition has a large effect on both the morphological and electrical properties of the films. Auger electron spectroscopy, electron diffraction, and scanning‐electron‐fractography results show that films deposited at P=20 mTorr (2.67 Pa), VT=−500 V, and VS=−40 V are nearly stoichiometric, have a cubic crystal structure, and a relatively equiaxed microstructure. The results of electrical measurements indicate that films grown under these sputtering conditions have an ionic transference number which is nearly zero below 100 °C and rises to approximately 0.4 at 200 °C.

Relationship between mechanical properties, microstructure, and fracture topography in α + β titanium alloys

Abstract: The influence of forging schedule and subsequent heat treatment on the microstructure and mechanical properties of three a + β titanium alloys has been investigated. The alloys studied were Ti-6Al-4V, Ti-6Al-6V-2Sn, and Ti-6Al-2Sn-4Zr-6Mo. The microstructural comparisons included (a +β) finished and β-processed material, variations in primary a volume fraction, primary a particle aspect ratio, and the nature of the transformation products. Careful selection of forging history and heat treatment permitted comparisons of fracture properties at essentially constant strength levels. This paper describes the influence of forging and heat treatment on the microstructure of these alloys as shown by light and thin foil electron microscopy. The microstructural information was then used to analyze the variations in topography of both fatigue and fast fracture regions as observed by scanning electron microscopy. In some cases, good correlation between fracture topography and microstructure has been established but such correlations depend on microstructure and on loading conditions. For example, fast fracture in the β-processed materials occurs largely along prior β-phase grain boundaries, whereas fatigue cracks propagate by a predominantly transgranular mode. Further, the occurrence of fatigue striations strongly depends on microstructure and on the cyclic stress intensity, with cyclic cleavage predominating at low growth rates in those alloy conditions containing primary a particles which have at least one dimension greater than ∼25 μm. Other correlations between fracture topography, microstructure, and properties were obtained and comments for microstructure selection to control properties are included.

Glass-ceramics with random and oriented microstructures

Abstract: The variation of a number of physical properties of a glass-ceramic composition as a function of the crystallization heat-treatment temperature are reported. The properties studied include mechanical strength, modulus of elasticity, electrical properties and linear coefficient of thermal expansion. It is shown that several of the properties are dependent on microstructural effects. This dependence is especially marked for modulus of rupture and is also apparent for electrical properties such as conductivity and loss tangent. For the latter, however, compositional changes in the residual glass phase also exert a significant influence. For other properties, such as indentation hardness, the microstructural dependence is less well defined.

Effect of Microstructure on Rate of Machining of Ceramics

Abstract: The effect of microstructure on the machinability of a variety of ceramics was studied. The machining difficulty, i.e. the inverse of sawing or grinding rate at constant force, increased with decreasing porosity and grain size. For materials with nonuniform distributions of grain size or porosity, machining rates were controlled by the smaller grains and the least porous regions. The grain-size dependence of the machining difficulty was generally greater for harder materials. The effect of porosity on machining difficulty generally followed the same trend as its effect on other mechanical properties. The results obtained were interpreted in terms of the Petch relation, which provided a rationale for the occurrence of flaw sizes independent of grain size. The results also indicate that theories of the machinability of ceramics must include the effects of compressive strength and, especially, hardness.

Effect of Composition and Structure on Crevice, Intergranular, and Stress Corrosion of Some Wrought Ni-Cr-Mo Alloys

Abstract: The relationship of composition and microstructure to the occurrence of localized corrosion in Hastelloy alloys C, C-276, and C-4 was investigated. One hour exposures of these alloys in th...

Annealing effect on the microstructure of poly(vinyl chloride)

Abstract: The effect of annealing on the microstructure of commercial grade poly(vinyl chloride) was investigated by calorimetric, X-ray and viscoelastic measurements. The degree of crystallinity increases with increasing annealing temperature from above the glass transition temperature up to 130°C, at which point the degree of crystallinity takes on a maximum value. Also, the crystal melting temperature increases with increasing annealing temperature. Thermal analysis and X-ray study suggest that the crystallite of poly (vinyl chloride) decomposes by thermal degradation when annealed, above 170°C. The isothermal crystallization process is analyzed using Avrami's equation employing the degree of crystallinity as a function of annealing time at various annealing temperatures. The crystallization rate has a maximum value at around 140°C. It is expected that the crystalline texture grows in the shape of a lineal-like habit, judging from the magnitude of Avrami's constant and from a study of the X-ray intensity distribution. The αf-transition was observed to occur at temperatures 5 to 10°C lower than the crystalline melting temperatures for annealed specimens of poly(vinyl chloride) using a dynamic spring analysis. The αf-transition may be attributed to thermal molecular motions with a long time scale, resulting from the cross-link points introduced by the small crystallites.

Effect of alloying elements on the microstructure and properties of a hot-rolled low-carbon low-alloy bainitic steel

Abstract: A two-level full factorial statistical experiment consisting of eight alloys was conducted to determine the effect of 2 pct cobalt, 1 pct nickel and 1 pct chromium on the hot-rolled microstructure and properties of a bainitic steel containing 0.2 pct C, 2 pct Mn, 1 pct Si, 0.75 pct Mo and 0.003 pct B. The results indicate that chromium induced the formation of the acicular bainitic structure while cobalt favored massive ferrite formation and resulted in islands of martensite and/or austenite. Nickel, when added singly, did not appear to influence the microstructure but in combination with chromium, enhanced the formation of the lower bainitic structure. The mechanical properties were statistically analyzed and statistical equations were obtained to predict optimized compositions. These equations indicate that chromium increased the toughness of these steels more than nickel. However, it was shown that with similar bainitic structures, nickel enhanced the toughness more than chromium. The results illustrate the short-coming of a pure statistical approach to the design of alloys.

Method of producing a high strength steel having uniform elongation

Abstract: A steel product having a combination of high strength and formability (as measured by percent tensile uniform elongation) is produced by austenitizing a steel consisting essentially of from 0.04 to 0.17% carbon, 0.8 to 2.0% manganese, up to 1.0% silicon, up to 0.12% vanadium, up to 0.1% columbium, up to an effective amount of titanium to form titanium carbonitrides, 0.001 to 0.025% nitrogen, balance essentially iron and then cooling at a rate of no more than about 70° F/sec. to about 850° F and at a rate of more than about 10° F/sec. to transform the freshly formed austenite to a microstructure of from 10 to 35% by volume of martensite and/or lower bainite (MLB), balance essentially proeutectoid ferrite. Slower cooling rates may be employed to obtain the desired microstructure if a restricted chemical composition is used. The heat-treated steel product is characterized by an ultimate tensile strength of 80,000 p.s.i. minimum and a uniform elongation of 16% minimum.