Showing papers on "Microstructure published in 1970"

On the nature of freckles in nickel base superalloys

Abstract: Freckles are chains of equiaxed grains that have previously been observed in macroetched ingots of both iron and nickel base alloys formed by the consumable arc process. These defects have been observed in nickel base superalloys in both directionally solidified polycrystalline and single crystal ingots. Such ingots exhibit a relatively periodic distribution of vertical freckle trails around their circumference which are particularly suitable for study. This paper investigates the nature of “Freckles,” examines the conditions under which they occur and specifies a set of consistent observations which any successful model for freckling must explain. Visual, chemical, electron probe, metallographic, and X-ray diffraction data are presented which establish the appearance, composition and orientation differences between freckled and normal regions in single crystals of the nickel base superalloys Udimet 700 and Mar-M200. It is shown that the freckle lines are linear assemblies of small random equiaxed grains which are enriched in all but the inversely segregated solute species. Excessive interdendritic porosity and feeding shrinkage are observed in the vicinity of a freckle line. Some correlations of the tendency toward freckling and the geometric distribution of freckle lines are made with growth rate, thermal gradient, and interface shape. The dependence of freckling tendency on composition is examined in the Ni-Al system.

The microstructure of PAN-base carbon fibres

Abstract: The microstructure of polyacrylonitrile-base (PAN-base) type I and type II carbon fibres has been studied by X-ray small-angle and wide-angle scattering. Recent work in this field is critically reviewed and it is shown that the structural model proposed by Johnson & Tyson for these types of fibre is inadequate. The microstructure of PAN-base carbon fibres is essentially the same as that of rayon-base carbon fibres. Differences appear only in the volume fraction occupied by the microvoids.

The strength and oxidation of reaction-sintered silicon nitride

Abstract: The structure of reaction-sintered silicon nitride is studied using scanning electron and optical microscopy at various stages during nitriding, for a range of nitriding and compacting conditions. The strength is then evaluated and interpreted in terms of the microstructure. It is found that fracture always occurs in a brittle manner by the extension of the largest pores. The effects of prolonged annealing in air above 1000† C on both the structure and strength are investigated. At 1400† C, cristobalite is formed. If the temperature is then maintained above 250† C, the strength is enhanced, but below this temperature the oxide layer cracks and reduces the strength.

Deflagration rate, surface structure, and subsurface profile of self-deflagrating single crystals of ammonium perchlorate

Abstract: Pure ammonium perchlorate single crystal self deflagration, determining energy transfer mechanisms from pressure effects, combustion characteristics and subsurface profile

The Deformation of Single Crystals of Copper and Copper-Zinc Alloys Containing Alumina Particles. I. Macroscopic Properties and Workhardening Theory

Abstract: The stress-strain curves of Cu and Cu + Zn alloys containing alumina particles have been measured as a function of temperature, strain rate, composition, volume fraction and radius of the particles. Around room temperature the stress-strain curves of the Cu alloys are strongly temperature dependent and there is a marked recovery effect. A theory of work­ hardening is developed based on a model deduced from the electron microscope observations in which the glide dislocations generate rows of loops at the particles which act as parallel linear obstacles, leading to ‘self-hardening’ of a slip line. In addition, the interaction of other dislocations on parallel glide planes is considered, and the slip line spacing and the number of dislocations per slip line are adjusted to minimize the flow stress for a given strain. The theory predicts a parabolic stress-strain curve following an initial region of relatively low hardening rate, in good quantitative agreement with the stress-strain curves for the Cu alloys at 77 K. The predicted slip line spacing agrees with electron microscope observations. The behaviour of the Cu + Zn alloys, the temperature and strain rate dependence of the stress-strain curve, and the recovery effect are discussed qualitatively and are correlated with observed changes in the microstructure.

Oriented eutectic microstructures in the system Al2O3/ZrO2

Abstract: Oriented eutectic microstructures have been produced in the system Al2O3/ZrO2 using a Bridgman-type crystal-growing furnace. Ingots consisted of elongated columnar grains or colonies. Inside the colonies a rod-type eutectic microstructure consisting of rods of ZrO2 surrounded by an Al2O3 matrix was observed. The eutectic point was re-established at 63 mol % Al2O3/37.0 mol % ZrO2 and 1870±5° C. Al2O3 is the first phase to nucleate when eutectic growth occurs.

Microstructural changes in soft quick clay at failure

Abstract: A series of unconfined compression tests has been made on a marine, quick clay and small specimens were extracted for microstructural investigation. The natural microstructural pattern was characte...

Work strengthening by a deformation-induced phase transformation in 'MP Alloys'

Abstract: Work strengthening and microstructure were investigated for a class of alloys, designated “MP Alloys”, containing 20 pct Cr, 10 pct Mo, and the remainder cobalt and nickel in proportions ranging from 60Co∶10Ni to 30Co∶40Ni. These alloys, in the fully annealed, homogenized condition, have a fcc structure with yield strengths ranging from about 45 to 60 ksi. Deformation at room temperature rapidly increases the yield strength of the alloys to about 250 ksi. Structural analyses by X-ray and electron diffraction techniques indicate that this marked increase in strength is associated with a deformation-induced martensitic transformation forming a network of extremely thin hcp platelets within the fcc grains. The nature of this martensitic transformation was studied as a function of alloy composition, deformation temperature, and structural variables, such as the platelet size, thec/a ratio of the hcp phase, and twinning.

Crystal nucleation in lithium silicate glasses

Abstract: The development of the microstructure of simple glass-ceramics, of molar compositions 70SiO2·30Li2O and 69SiO2·30Li2O·1P2O5, has been studied by the techniques of transmission electron microscopy, optical microscopy, X-ray powder diffraction and electrical loss measurements. Surface nucleation of lithium disilicate crystals occurred in all samples, and internal nucleation occurred in samples of the first glass treated for 1 h at 450, 475 or 500°C before crystallization at 750°C. Internal nucleation always occurred in samples of the glass containing P2O5 which were crystallized at 750°C, but the grain size of the crystalline product depended strongly on the temperature of the previous lower temperature heat treatment (the “nucleation stage”). The optimum nucleation range was close to 500°C for 1 h heat treatments. The results suggest that glass-in-glass phase separation, which was observed in both glasses, did not promote crystal nucleation directly, but effects due to an influence of phase separation on crystal growth rates are suggested. Possible reasons for the action of P2O5 as a nucleation catalyst are discussed, but firm conclusions cannot be reached on the present evidence.

Structure and properties of tantalum carbide crystals

Abstract: Single crystals of tantalum carbide, up to 2 mm in size have been grown from solution in a bath of molten iron. The slip plane was found to be {111} using a two-surface analysis on etch-pitted crystals deformed by microindentation at room temperature. Observations of etch-pit patterns around inclusions suggest that slip occurs on other planes at elevated temperatures. Maximum microhardness values between 3800 and 5200 Knoop (100 gm load) were found at a composition TaC0.83±0.01. In regions of crystals with a carbon content less than TaC0.83 a phase transformation was seen close to microhardness indentations in samples decarburised below 2200† C. The mechanical behaviour of tantalum carbide is discussed with reference to a general model for the electronic structure of carbides.

Two-Phase Microstructures of α-Fe–Al Alloys in the K-State

Abstract: Electron diffraction and microscopy have been utilized to investigate the Fe–Al system up to 20 at.-% A1. It is found that alloys at and above 10 at.-% Al aged at low temperatures consist of two phases, as indicated by coherent-particle strain-contrast images and by electron diffraction. At 15·5 at.-% A1 and above the particles are identified as the α1 phase, exhibiting DO3-type structure. Thus, the so-called K-state and other abnormal properties attributed previously to short-range order in these alloys are associated with very small ordered particles (≤50 A in dia.). Outside the conventional two-phase field, growth of these particles is limited.

The nucleation of aluminum grains in alloys of aluminum with titanium and boron

Abstract: The microstructure of a ternary alloy, Al-5 wt pct Ti, 1 wt pct B, has been examined by optical and electron transmission microscopy, by selected area diffraction, and electron probe microscopy, by selected area diffraction, and electron probe microanalysis. Particles of Al3Ti are found at the center of grains and there exist preferred epitaxial orientations between this compound and the surrounding aluminum. Particles containing titanium and boron occur at aluminum grain boundaries and have no preferred configurations with respect to the aluminum or to one another. It is concluded that the active heterogeneous nuclei are therefore Al3Ti and that particles of TiB2, AlB2, or a ternary compound are not active in this alloy. Grain size measurements in binary Al-Ti alloys suggest that particles of a nucleating phase must be present at concentrations as low as 0.01 wt pct Ti, and it is suggested that these could be Al3Ti if the existing binary phase diagram Al-Ti is in error.

Transport Properties, Microstructure, and Conduction Model of Cosputtered Au–SiO2 Cermet Films

Abstract: High‐resistivity Au–SiO2 cermet films have been cosputtered. Their transport and structural properties have been examined and a model for electrical conduction is proposed. The electrical resistivity was found to be thermally activated, and the activation energy increased linearly as the metal content decreased. Activation energies varied from 0.01 to 0.07 eV. At a fixed temperature, the resistivity increased exponentially as the weight percent gold decreased. The cermets obeyed Ohm's law. No Hall effect was observed. Transmission electron micrographs showed very small (30–100 A) discontinuous gold particles separated by very small SiO2 regions. Films were deposited at 20°, 500°, 520°, and 600°C. Gold particle size increased with increasing gold content or with increasing substrate temperature. Only polycrystalline gold was observed by electron diffraction. The typical infrared spectrum of SiO2 was recorded. 20°C cermets annealed at 600°C showed large particle growth and twinning. Electrical conduction in...

Mechanical properties and structure of age-hardened Ti-Cu alloys

Abstract: The room temperature tensile properties of age-hardened Ti-Cu alloys, 0.9 and 4 at. pct Cu, were investigated. The results were correlated to the microstructure and to the observed interaction mechanism between moving dislocations and precipitated particles. Two types of precipitates were observed upon aging between 400° and 500°C. One type was identified as a metastable, ordered precipitate coherent with the matrix. The other type was the stable Ti3Cu phase which was partially coherent with the matrix. Both types of particles were precipitated from a martensitic microstructure which resulted from the β → α transformation. Although the martensitic microstructure contributed to the high flow stress, the elongation to fracture was principally determined by the dislocation-particle interaction mechanism. The maximum elongation to fracture was obtained by inducing a dislocation by-pass mechanism in a structure containing homogeneously distributed, partially coherent Ti3Cu particles. The tendency of the Ti3Cu particles to precipitate preferentially on boundaries was minimized by low temperature aging.

Microstructure of Fine-Grain Ceramics

Abstract: This chapter describes the use of transmission electron-microscopy to characterize the microstructure of fine-grain ceramics. Observations have been made on a number of polycrystalline materials including alumina, magnesia, zirconia, metal-ceramic composites, and rock specimens.

Processing of nickel-base alloys for improved fatigue properties

Abstract: The fatigue properties of the precipitation-hardened, nickelbase alloys, such as Inconel 718, Incoloy 901 and Waspaloy, are significantly improved by a thermomechanical processing technique involving the generation of an intermetallic pinning phase, such as a spheriodal eta phase or an overaged gamma prime phase, with subsequent recrystallization to provide a uniform microstructure having a grain size of ASTM 10-13 or finer.

Semiconducting glass-ceramics

Abstract: Studies of the microstructure and thermal treatment of V2O5P2O5 based glasses have shown an initial ordering of the glass structure followed by detectable crystallization. DC conductivity increases markedly with microstructural changes. Unusual electrical behavior is associated with the partially crystallized glasses including ac absorption, elimination of high frequency conductivity dispersions and inversion of frequency effects at elevated temperatures. The effect of forming variables such as quenching temperatures and rates on electrical properties are documented. Neutron irradiation has also been shown to dramatically change certain characteristics of these materials, whereas other features are irradiation insensitive to flux levels of 1 × 1017 nvt. A summary of modified band theory calculations that are consistent with the above results is presented.

Penetration of aluminum alloys by projectiles

Abstract: Bands of intense shear may be formed during the penetration of aluminum alloys by projectiles. It is shown that the formation of these bands lowers the ability of the material to withstand further projectile penetration. The structure of these bands has been investigated by electron microscopy and the results obtained indicate that melting occurs within the bands. A simple model of a propagating shear band predicts that the material within the band will melt.

Crystallogenic Modification in the Formation Process at the Lead Electrode by the Addition of a Lignin Derivative

Abstract: Microscopic investigation of the effect of lignin and BaSO/sub 4/ on the negative lead electrode showed that lignin was able to modify the lead crystal structure produced during the forming operation, in addition to its effect on the structure of the PbSO/sub 4/ crystals formed on discharge the effect of the lignin was to reduce the size and complexity of the dendritic lead crystals in the formed plate. The presence of BaSO/sub 4/ had no appreciable effect on the microstructure of the formed plate.

The Effect of Potential and Time on Deposition Characteristics of Zinc on a Zinc Single Crystal in KOH

Abstract: Zn deposition on Zn single crystals in KOH solution, examining time and potential effects on deposit morphology

Phase separation in GeO–GeO2 glasses

Abstract: A series of transparent homogeneous amorphous solids between GeO2 and GeO has been prepared by rapid quenching of liquids and by vapour deposition. Density and refractive index increase smoothly with increasing Ge/O ratio and provide a useful index of composition. Glasses in the range GeO–GeO1.85 undergo a rapid exothermic transformation when heated above 400°C accompanied by a pronounced blackening, an increase in refractive index, and slight decrease in density. Electron microscopy and diffraction results indicate that the transformation involves the precipitation on a 50–100 A scale of an amorphous tetrahedrally coordinated Ge-rich phase. The interconnected microstructure so produced is suggestive of spinodal decomposition. A transitional region near GeO1.9 is characterized by sluggish phase separation kinetics and a dispersed particle morphology suggestive of a nucleation and growth precipitation mechanism. Glasses in the range GeO1.9–GeO2 show no evidence for phase separation.

Some aspects of the variation of the strain anisotropy in titanium

Abstract: Strain anisotropy in commercial purity titanium was studied between 77° and 973°K using the technique of Rittenhouse and Picklesimer. Both longitudinal and transverse tensile specimens were used that were machined from a plate with a strong alignment of the basal planes of the grains parallel to the rolling plane. The contractile strain along the transverse plate direction was always larger than that parallel to the rolling plane normal. The strain anisotropy was greatest at the highest temperature. With decreasing temperature it became smaller at an ever increasing rate. At 973°K the strain anisotropy was strain independent. At lower temperatures it decreased with increasing strain. It is believed that deformation twinning can account for most of the temperature and strain dependence of the strain anisotropy. Quantitative microstructure studies show good correspondence between changes in the volume fraction of twins and changes in the strain anisotropy. Transverse specimens exhibited a greater degree of strain anisotropy, and this is consistent with the original texture of the titanium plate used in this investigation.