Showing papers on "Microstructure published in 1974"

The effect of austenitizing temperature on the microstructure and mechanical properties of as-quenched 4340 steel

Abstract: The effect of austenitizing temperature on both the plane strain fracture toughness,KIC, and the microstructure of AISI 4340 was studied. Austenitizing temperatures of 870 and 1200°C were employed. All specimens austenitized at 1200°C were furnace cooled from the higher austenitizing temperature and then oil quenched from 870°C. Transmission electron microscopy revealed an apparent large increase in the amount of retained austen-ite present in the specimens austenitized at the higher temperature. Austenitizing at 870°C resulted in virtually no retained austenite; only minor amounts were found sparsely scat-tered in those areas examined. A considerably altered microstructure was observed in specimens austenitized at 1200°C. Fairly continuous 100 to 200A thick films of retained austenite were observed between the martensite laths throughout most of the area exam-ined. Additionally, specimens austenitized at 870°C contained twinned martensite plates while those austenitized at 1200°C showed no twinning. Plane strain fracture toughness measurements exhibited an approximate 80 pct increase in toughness for specimens austen-itized at 1200°C compared to those austenitized at 870°C. The yield strength was unaffected by austenitizing temperature. The possible role of retained austenite and the elimination of twinned martensite in the enhancement of the fracture toughness of those specimens austen-itized at the higher temperature will be discussed.

Microstructure and phase stability of INCONEL alloy 617

Abstract: INCONEL alloy 617 (54 Ni, 22 Cr, 12.5 Co, 9 Mo, 1 Al, 0.07 C) is a solid-solution alloy with good corrosion resistance and an exceptional combination of high-temperature strength and oxidation resistance. A laboratory study was performed to determine the effects of long-time (215 to over 10,000 h) exposure to temperatures up to 2000°F (1093°C) on the microstructure and phase stability of the alloy. To investigate the strengthening response exhibited by the alloy during high-temperature exposure, microstructures were correlated with mechanical properties. The major phase present in the alloy after exposure to all temperatures from 1200 to 2000°F (649 to 1093°C) was found to be M23C6. The phase precipitated as discrete particles and remained stable at au temperatures. No MC or M6C carbides were found. A small amount of gamma prime was found in samples exposed at 1200°F (649°C) and 1400°F (760°C). A PHACOMP analysis indicated 0.63 pct gamma prime could form. No topological close-packed phases such as sigma, mu, and chi were found. Strengthening of the alloy during exposure to temperature was found to result primarily from the precipitation of M23C6. The phase provides effective strengthening because it precipitates in discrete particles and remains stable at temperatures to 2000°F (1093°C). The amount of gamma prime formed is not sufficient to cause appreciable hardening, but it does provide some strengthening at 1200 to 1400°F (649 to 760°C).

The effect of yttrium and thorium on the oxidation behavior of Ni-Cr-Al alloys

Abstract: The effect of quaternary additions of 0.5% Y, 0.5 and 1.0% Th to a base alloy of Ni-10CR-5Al on the oxidation behavior and mechanism was studied during oxidation in air over the range of 1000 to 1200 C. The presence of yttrium decreased the oxidation kinetics slightly, whereas, the addition of thorium caused a slight increase. Oxide scale adherence was markedly improved by the addition of the quaternary elements. Although a number of oxides formed on yttrium containing alloys, quantitative X-ray diffraction clearly showed that the rate-controlling step was the diffusion of aluminum through short circuit paths in a thin layer of alumina that formed parabolically with time. Although the scale adherence of the yttrium containing alloy was considerably better than the base alloys, spalling did occur that was attributed to the formation of the voluminous YAG particles which grew in a mushroom-like manner, lifting the protective scale off the subrate locally. The YAG particles formed primarily at grain boundaries in the substrate in which the yttrium originally existed as YNi9.

The microstructures of silicon nitride ceramics during hot-pressing transformations

Abstract: The microstructure of silicon nitride hot-pressed with a magnesium oxide additive has been studied by transmission electron microscopy. This includes material at various stages in a hot-pressing process: the initial (∼ 90%α) silicon nitride powder; specimens partially densified and partially transformed from α-silicon “nitride” (Si11.5N15O0.5) to β-silicon nitride (Si3N4); and almost fully dense and fully transformed β-Si3N4. The observations substantiate a solid/liquid/solid transformation mechanism, whereby Si and N are transported from α grains through a silicate liquid phase to nucleation sites for β at α/liquid interfaces or to β grains nucleated homogeneously in the liquid phase.

The effect of ingot processing treatments on the grain size and properties of Al alloy 7075

Abstract: An investigation was carried out to determine the effect of various ingot thermal mechanical processing treatments on the grain size and mechanical properties of high purity homogeneous 7075 aluminum alloy sheet and plate. The results indicate that the recrystallization of 7075 alloy into a fine grained structure can be controlled by the distribution of the Cr in the microstructure, as well as by the distribution of the major alloying elements, Zn, Mg and Cu. A new ingot processing technique, FA-ITMT, was developed for producing fine grained 7075 sheet and plate. Data are presented which show that fine grained 7075 sheet and plate have equivalent strength and significantly better ductility than conventionally processed material.

Structure property relationships in deposits produced by plasma spray and detonation gun techniques

Abstract: The microstructures of materials produced by plasma spraying or detonation gun techniques consists of thin lenticular particles with a very fine grained (perhaps even amorphous) structure. They are rapidly quenched from the molten, or near molten state, and may have a high degree of residual stress. This review is concerned with the relationship between these unique microstructures and the properties of the material. The areas of bond strength, transient and residual stresses, density, corrosion characteristics, and wear resistance, as well as mechanical, thermal, and electrical properties are at least briefly surveyed. The methods of deposition, i.e., plasma torch operating conditions, heat transfer to the particles, etc., are not covered. It is evident that, although a sufficient amount of information is available for most engineeering applications of these materials, a great deal remains to be learned about the fundamental relationships between their structure and properties.

Improvements in the fatigue strength of Ti-6Al-4V through microstructure control

Abstract: Fatigue deformation and stage I (shear mode) crack initiation in Ti-6Al-4V alloy test pieces have been studied using optical microscopy. Two types of stage I fatigue crack initiation were observed, (a) alongα/β interfaces and (b) transcrystalline initiation acrossα grains in partly transformed microstructures and acrossαβ interfaces in fully transformed microstructures. The α/β interface cracking occurred predominantly in the low stress regions of the test pieces. These observations suggested that a microstructure with a smallα grain size, to minimize the mean free slip path, and with minimum lengths ofα/β interface, would have a high fatigue strength. Such a microstructure, with anα grain size of < 10 μm, and spheroidal or near spheroidalβ particles, was produced by thermo-mechanical processing. The rotating cantilever fatigue strength of this microstructure, ± 670 MN m−2 at 10−7 cycles, compares with fatigue strengths in the range ± 480 to ± 590 MN m−2 for commercial Ti-6Al-4V bars.

The microstructure of Al-8 wt% Fe-based alloys prepared by rapid quenching from the liquid state

Abstract: The results of a transmission electron microscope study of the microstructure of splat-quenched Al-8% Fe are described in detail. Two distinct structures, zone A and zone B, are examined in as-quenched samples, and each characterized in terms of the dispersions and types of phases present. The decomposition behaviours of zone A and zone B during annealing at temperatures between 573 and 823 K (300 and 550°C) are investigated and the associated phase transformations determined. The effect on the as-quenched, and annealed, microstructures of adding either 3% Mn or 1% Zr to the alloy is described. The observed microstructures and phase transformations are correlated with micro-hardness measurements.

Anionic polymerization initiators containing protected functional groups and functionally terminated diene polymers

Abstract: Organolithium reagents substituted with hydroxyl-carrying mixed acetals (i.e., tetra-hydropyranyl and α-ethoxyethyl ethers) have been prepared in high yields and used to polymerize 1,3-butadiene to various acetal-terminated polybutadiene polymers. A method is described for converting acetal-containing polymers into hydroxyl-containing polymers. The polybutadienes have been characterized with regard to endgroup types, quantitative functionalities, molecular weights, molecular weight distributions, and microstructures. Dihydroxyl terminated polymers are prepared anionically in the absence of gel. Such materials are more suitable from the standpoint of f(OH) and Mw/Mn for chain extension studies than are prepolymers prepared by radical methods.

Structure/property relationships in evaporated thick films and bulk coatings

Abstract: This paper reviews the known data on structure/property relationships in thick films or bulk coatings of metals, alloys, and compounds produced by high-rate evaporation/deposition technology. The effects of process variables (depostion temperature and/or kinetic energy of depositing atoms) on the microstructure, texture, and mechanical properties is given. The influence of coatings on corrosion resistance and thermal fatigue is also stated.

Mechanical properties of al-8 wt% fe-based alloys prepared by rapid quenching from the liquid state

Abstract: Aluminium-iron-based alloys have been prepared in bulk form from flakes that were splat-cast from the liquid. A detailed study has been made of the binary Al-8 wt % Fe alloy and the influence of several ternary additions is discussed. Mechanical properties of these materials are described and it is shown that the refinement in microstructure resulting from the extremely rapid cooling rates employed gives rise to improved mechanical behaviour, particularly in the temperature range 300 to 620 K.

Structure of Sputtered Molybdenum Disulfide Films at Various Substrate Temperatures

Abstract: MoS2 films (300 to 400 A) were radio-frequency sputtered on aluminum and nickel surfaces at elevated, ambient, cold water, and liquid nitrogen temperatures. Electron transmission micrographs and electron diffraction patterns were taken to determine the structural growth. These transmission micrographs revealed that sputtered MoS2 films at ambient and elevated temperatures (320 and 150 C) formed an irregular network of ridges. The electron diffraction patterns of these films showed relatively sharp diffraction rings, indicating crystallinity. At water temperatures (∼7 C), ridge formation was extensively reduced or completely eliminated. The transmission micrographs of sputtered films at liquid nitrogen temperatures revealed a continuous featureless film. The electron diffraction patterns showed broad, diffused rings indicating an amorphous film. The transmission micrographs of a post annealed (425 C) MoS2 film sputtered at liquid nitrogen temperature revealed the tendency for ridge formation. Electron diff...

Viscous Flow in Glass During Phase Separation

Abstract: The isothermal viscosities of two borosilicate glasses, one a commercial glass widely used for chemical glassware, increase by 4 to 5 orders of magnitude with heat-treatment time near the annealing point. The glasses are basically sodium borosilicates but differ greatly in phase-separation characteristics. Electron micrographs were used to analyze the development of microstructure during the suspected phase separation. In both glasses, structure development is primarily responsible for the viscosity increase. An analysis of the data and a theoretical interpretation of the effect are presented.

Mechanical Properties of Aluminum-Graphite Composites Prepared by Liquid Phase Hot Pressing:

Abstract: A continuous liquid metal infiltration process has recently been developed for making aluminum-graphite composite wire from commercially available multifiber graphite yarns. Composite specimens have been successfully fabricated by hot pressing the composite wire. The longitudinal properties of both the composite wire and hot pressed specimens approximated rule of mixtures behavior, but the transverse tensile and compressive strengths of the specimens were lower than expected. Examination of the microstructure indicates that better transverse and compressive strengths may be achieved through refinement of the matrix grain size, elimination of continuous networks of intermetallic compounds in the matrix, and homogenization of the matrix alloy constituents.

The microstructure of ti alloys as influenced by Thin-Foil artifacts

Abstract: : The occurrence of a variety of thin foil artifacts which form during preparation of electron microscope thin foils from Ti alloys have been described in the literature. These artifacts severely complicate the study of phase transformations and microstructure/property relations in Ti alloys. This paper illustrates several examples of such artifacts which occur in a wide range of alloys and show how their presence can be bothersome and, in some instances, grossly misleading. Examples include 'spontaneous relaxation' in beta-phase alloys, reversion of martensite and modification of omega-phase precipitates. An ion-thinning technique has been developed which suppresses the occurrence of thin foil artifacts, thereby permitting examination of alloys and microstructures which previously have not been analyzed. The structures of ion-thinned samples are illustrated and compared to the structures observed in conventionally thinned samples. (Author)

Use of the Notched‐Beam Test for Evaluation of Fracture Energies of Ceramics

Abstract: The mathematical analysis of the notched-beam specimen for fracture-energy determination considers a beam containing a zero-volume crack, i.e. a crack with zero width. Such a configuration is difficult to reproduce in practice with ceramics, and artificially cut notches with widths large relative to the scale of the ceramic microstructure are usually used. Evidence is presented which suggests that incorrect results can be obtained using such notches even when real cracks exist at the notch root. The effects of notch width and depth are examined for Al2O3, graphite, and SiC. A tentative specification of a root-crack-notch-radius relation is proposed for the application of this technique to ceramics.

Interrelationships between process parameters, structure, and properties of CVD tungsten and tungsten–rhenium alloys

Abstract: The development of CVD techniques for fabricating free-standing tungsten and tungsten–rhenium alloy structures is reviewed. Relationships between plating parameters, kinetics, morphology, microstructure, and properties of thick polycrystalline deposits are discussed. It is emphasized that porosity may be grown into the grain boundaries when the depostition rate is controlled by gas phase diffusion, and that fully dense deposits are generally obtained when the rate is limited by a surface process. The origin and control of many of the microstructural features peculiar to CVD are also discussed.

Sintering behaviour of ultrafine NbC and TaC powders

Abstract: Compositional and microstructural changes upon firing ultrafine (300 to 400 A) stoichiometric NbC and TaC powder lots have been studied up to a temperature of 1600° C. Substantial amounts of oxygen impurities, mostly oxide particles or layers are eliminated by reductions with hydrogen, free carbon or the carbides themselves. TGA showed these reactions to take place at 700 to 1400° C with maxima around 1000 to 1100° C. Low temperature sintering is inhibited by this impurity and its removal is thus essential. Other impurities (Ni, Cr, Fe) were also found in the starting powders in total concentration 0.5 to 1%. They give rise to a liquid phase located at grain edges at temperatures as low as 1100° C which then controls microstructure development. It dissolves to some extent in the carbide matrix at high temperature, and has a tendency to rise to the free surface of the samples. Compositional and structural heterogeneities are thus produced between bulk and surface at high temperatures. Owing to these impurity effects, it was not possible to clearly evaluate the influence of powder granulometry.