Showing papers by "Michigan Technological University published in 1977"

Scavenging of volcanic aerosol by ash: Atmospheric and volcanologic implications

Abstract: The initial concentrations of S (1,600 ppm) and Cl (1,100 ppm) in the high-Al2O3 basaltic magma from the 1974 eruptions of Fuego Volcano, Guatemala, were inferred from trapped glass inclusions in phenocrysts. During the explosive eruptions, as much as 33% of the S and 17% of the Cl fell quickly back to Earth as acid aerosol particles absorbed on the ash. An additional 5% of the S and 20% of the Cl was trapped in the silicate ash. The remaining S and Cl was released to the atmosphere. By estimating the volume of ash and applying the above values for S and Cl, 2.2 × 1011 g of S and 1.6 × 1011 g of Cl were calculated to be the atmospheric contribution of the 1974 Fuego eruption. These figures are minimum values because an undetermined amount of intrusive magma may have contributed volatiles to the eruption. Airborne in-the-plume measurements, together with the existing approaches, are the best way to eliminate this uncertainty. The absolute concentrations of scavenged elements on ash are seen to be a function of plume flux and particle trajectory, both of which vary greatly during an eruption. Intense pyroclastic activity produces higher S/Cl ratios in the coating acquired by the ash particles; this implies that there are higher S/Cl ratios in volcanic gas during more explosive phases of an eruption.

The elastic strain energy of coherent ellipsoidal precipitates in anisotropic crystalline solids

Abstract: The elastic strain energy of coherent ellipsoidal precipitates (ellipsoids of revolution) in anisotropic crystalline solids has been calculated as a function of ellipsoid aspect ratio using the method of Eshelby. When the precipitate is eithermuch softer or harder, elastically, than the matrix, the results are similar to those previously obtained using isotropic elasticity. When this condition is not met, however, anisotropic elasticity can yield quite different results which vary markedly with the orientation relationship between precipitate and matrix. When the precipitate has a non-cubic crystal structure, the elastic strain energy often passes through a maximum or a minimum at shapes which are neither thin discs nor spheres. During this study, the isotropic elasticity result that the strain energy associated with a disc-shaped precipitate is independent of the matrix elastic constants was also shown to hold under the conditions of anisotropic elasticity, and in such circumstances it depends only on the elastic properties of the precipitate in the direction of the principal directions of the disc. Incorporation of the anisotropic elastic strain energy into the calculation of ΔG *, the free energy of activation for the formation of a critical nucleus for the basic case of homogeneous nucleation with boundary-orientation independent interfacial energy, showed that the ratio of the strain energy to the volume free energy change must usually be somewhat larger than 3/4 in order to cause the shape of the critical nucleus to differ from that of a sphere.

Growth kinetics of grain boundary ferrite allotriomorphs in Fe−C alloys

Abstract: The kinetics of lengthening and thickening of grain boundary allotriomorphs of proeutectoid ferrite were measured at several temperatures in high purity Fe−C alloys containing 0.11 pct, 0.23 pct and 0.42 pct C. These measurements were conducted by measuring the length of the longest and the width of the widest allotriomorph in each specimen. All specimens were austenitized so as to make the grain boundaries perpendicular to the plane of polish. This measurement technique appreciably reduced the scatter in the parabolic rate constant data previously encountered in thermionic emission microscopy measurements. Parabolic rate constants for lengthening and thickening were calculated, using the experimental aspect ratio, by means of the Atkinson analysis for oblate ellipsoids. The ratio of the measured to the calculated constants was in all cases less than unity. The previously made suggestion that these slow growth kinetics are due to faceting was supported through the observation of facets on allotriomorphs by means of scanning and transmission electron microscopy. The aspect ratio of ferrite allotriomorphs was shown to beca 1/3, independent of reaction time temperature and carbon content. The dihedral angle of the allotriomorphs was found to be 100±5 deg, as compared with a published angle for recrystallized and equilibrated specimens ofca 115 deg. Several possible explanations for the aspect ratio and dihedral angle findings are considered.

Geochemistry of tholeiites of the basic igneous complex of northwestern South America

Abstract: Chemical analyses of basaltic rocks from the Basic Igneous Complex of the Pacific coastal areas of northern South America clearly demonstrate a consistent tholeiitic affinity for this belt and a geochemical similarity of samples from Ecuador, Colombia, and Panama. The rocks are more like ocean-ridge tholeiites than other varieties, but they have higher than expected K, Sr, and Rb contents. Most probably, alteration has caused these anomalies, but a firm conclusion has been frustrated by the lack of correlation of concentration of these elements with the degree of three types of microscopically observable secondary mineralogic changes in the rocks. The rocks form a bimodal geochemical population, about two-thirds basalt and one-third basaltic andesite; both groups are geographically widespread. The increased geochemical information confirms the correlation of this complex with similar rocks in the Cordillera Occidental of Colombia and Ecuador and the Nicoya Peninsula of Costa Rica.

On the existence of the β→α m transformation in the alloy systems Ti-Ag, Ti-Au, and Ti-Si

Abstract: The existence of the β (bcc) to αim (hcp) massive transformation is demonstrated in three titanium-base eutectoid systems: Ti-Ag, Ti-Au, and Ti-Si. Confirmation is accomplished through optical microscopy and phase composition analysis by EDAX microprobe, supplemented by transmission electron microscopy. In these three alloy systems, the characteristics of the massive transformation were found to be quite similar, with very irregularly shaped αm grains consuming most or all the parent grains during moderate to rapid quenching from the β region. The transformation occurs at compositions well into the hypereutectoid region in the Ti-Ag system but is confined mainly to hypoeutectoid alloys in Ti-Au and Ti-Si.

The stereology of grain boundary allotriomorphs

Abstract: Grain boundary allotriomorphs have been modeled as two abutting spherical caps of equal radius. The effects of the distance from the center of the allotriomorph, х, and the angle with respect to the plane normal to the grain boundary, ψ, at which a plane of polish sections the allotriomorph have been investigated. Expressions were derived relating the apparent to the true thickness, length, aspect ratio and dihedral angle. The effects of х and ϕ upon measurements of the lengthening and thickening kinetics of allotriomorphs were found to be significant, particularly at larger values of ψ. Analysis of published high-temperature measurements of allotriomorph growth kinetics indicated that an appreciable portion of the scatter in this data may have been due to nonzero values of х and ψ. A room temperature technique for making these measurements which minimizes such effects is concluded to be stereologically more reliable.

Microstructural evolution during liquid phase sintering: Part I. Development of microstructure

Abstract: During liquid phase sintering, numerous solid-solid particle contacts can be generated by particle motion within the fluid. It is shown that, somewhat surprisingly, Brownian motion can produce such contacts. If such contacts are accompanied by particle adherence, the particles can then subsequently fuse into one (i.e., coalesce) by the liquid state analog of the evaporation-condensation mechanism of sintering. An isolated microstructure will develop if the time for particle coalescence is much less than the time between contacts. A highly skeletal arrangement of particles will form under the converse condition. Using these principles, a “microstructure map” is calculated in which the expected morphology of microstructure (i.e., skeletal or isolated) is related to the solid particle volume fraction, the kinetic and thermodynamic parameters affecting particle coalescence, and the frequency of particle contacts by Brownian motion. Some discussion of the thermodynamic and morphological factors affecting the probability of particle adherence after contact is presented.

A reanalysis of the kinetics of neck growth during liquid phase sintering

Abstract: During liquid phase sintering, solid particles make contact and can subsequently coalesce into one particle. This coalescence phenomena can affect the type of microstructure formed and its subsequent coarsening behavior during liquid phase sintering. The mechanism of particle coalescence is assumed to be the liquid state analog of the evaporation-condensation mechanism of sintering. In this work, a detailed study of the geometry appropriate for analysis of the coalescence phenomena during liquid phase sintering is made. It is found that in the early stages of particle coalescence, the neck between the particles acts as a geometrical barrier to diffusion and the neck between the particles grows approximately ast1/5,i.e., the same kinetics appropriate for solid state sintering are obtained. At longer times, the neck area no longer restricts diffusive flow and at1/6 dependency of neck growth is obtained. The use of numerical techniques also allows the analysis to be carried out with fewer geometrical restrictions than in the original analysis of the evaporation-condensation mechanism.

Microstructural evolution during liquid phase sintering: Part II. Microstructural coarsening

Abstract: During liquid phase sintering, microstructural coarsening takes place. One mechanism by which this occurs is Ostwald ripening. Alternatively, particle coalescence also leads to a concomitant reduction in the solid particle surface area per unit volume. In isolated structures in which particle-particle contacts are made, the rate of coarsening by coalescence is limited by the time between particle contacts, for this is long compared to the time to fuse two particles together. In skeletal structures the “coalescence time” limits coarsening by coalescence since this is long in comparison to the time between contacts. Expressions for the rate of particle coarsening are developed for the different mechanisms and different particle morphologies. The results of these calculations are combined with the microstructure maps developed in Part I of this paper to refine these maps so that they predict both the morphology developed and the dominant mechanism of coarsening in liquid phase sintered systems.

Development of salix and populus mycorrhizae in metallic mine tailings

Abstract: Roots of two year old willow and poplar planted as cuttings in copper and iron mine tailings were stained intact and examined microscopically for mycorrhizal development. Incidence of mycorrhizae was related to tree vigor and to chemical and physical properties of the tailings. Results showed that no mycorrhizal development occurred on roots of these tree species in the copper tailings; inoculations with natural forest soil extract failed to initiate this symbiosis. Tops and roots of the willow and poplar were stunted and survival was poor. Roots in the iron mine tailings developed extensive ectomycorrhizae and trees showed good growth. Presence of mycorrhizae was confirmed and external morphology examined with the scanning electron microscope. Aseptic jack pine and naturally mycorrhizal white pine roots were used in microscopic comparisons with the hardwoods. re]19760220

Characterization of gas bubbles injected into molten metals under laminar flow conditions

Abstract: Velocity and volume measurements of gas bubbles injected into liquid metals under laminar flow conditions (at the orifice) have been achieved. A novel experimental approach utilizing noises generated by bubbles was used to collect the necessary data. Argon gas was bubbled through tin, lead, and copper melts, and gas bubble formation frequencies (and hence bubble sizes) were determined. It was found that the bubble size generated for a particular orifice diameter was dependent upon the magnitudes of the orifice Froude and Weber numbers. Maximum formation frequencies increased slightly with decreasing orifice diameter, and the transition point from varying to constant frequency occurred at an orifice Weber number of approximately 0.44. Velocities of gas bubbles rising through the metals were greater than those previously reported for studies in which only one bubble was in the melt at any time. Effective drag coefficients of the rising bubbles were found to agree with data previously generated in aqueous systems.

Strengthening of copper by dislocation substructures

Abstract: Substructural strengthening in pure copper deformed by cold rolling is shown to depend on both the mean free path and the level of nonuniform strain associated with the dislocation substructure. Contributions from the nonuniform strain are particularly important because the mean free path becomes constant after deformations of ∼50 pct R.A., whereas the material continues to work harden. The behavior of the nonuniform strains at large deformations is also found to be closely related to dynamic recovery. Development of an empirical expression for the strengthening in terms of the substructural parameters is shown to lead to a modified Hall-Petch type relation.

Effects of austenitizing temperature on the kinetics of the proeutectoid ferrite reaction at constant austenite grain size in an Fe-C alloy

Abstract: Austenitizing an Fe-0.23 pct C alloy at 1300°C and further at 900°C prior to isothermal transformation was found to increase the growth kinetics of grain boundary ferrite allotriomorphs while decreasing their rate of nucleation. A scanning Auger microprobe was used to establish that sulfur segregates to the austenite grain boundaries and does so increasingly with decreasing austenitizing temperature. A binding free energy of sulfur to these boundaries of approximately 13 kcal/mole (54.4 kj/mole) was calculated from theMcLean adsorption isotherm. The kinetic results were explained in terms of preferential reduction of the austenite grain boundary energy decreasig nucleation kinetics, and adsorption of sulfur at α:γ boundaries increasing the carbon concentration gradient in austenite driving growth.

Beneficiation of lithium ores by froth flotation

Abstract: A high grade fraction is recovered from lithium-containing ores by conditioning a finely-ground aqueous pulp of the ore with a conditioning reagent formed by incorporating a water-soluble polyvalent metal salt into an aqueous solution of an alkali metal silicate and with an anionic collector and then subjecting the conditioned pulp, without desliming, to a froth flotation operation whereby a concentrate containing lithium and a tailing containing gangue are produced.

Copper deoxidation kinetics utilizing carbon monoxide

Abstract: Gaseous deoxidation of liquid copper with carbon monoxide as the reducing gas has been examined. The reducing gas was introduced at a submerged orifice so that well characterized, single bubbles were formed. The deoxidation kinetics are determined primarily by the mass transport of oxygen in liquid copper. No effect of sulfur over the range of 10 to 200 ppm was observed. Increasing the temperature from 1113 to 1173°C slightly inhibit the kinetics of oxygen removal.