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By L. C. Michels, O. G. Ricketts

The disorientations between s?nall grains, whose growth has been arrested at an early stage of recrys-tallization, and the deformed matrix in cold-rolled aluminum single crystals were determined using transmission Kikuchi line and electron diffraction patterns. The orientations of the recrystallized grains were found to be random, and the disorientations of these grains with the matrix weve found to be intermediate to large. This leads to the conclusion that the observed vecrystallization began in small areas of large disorientation present in the cold-worked structure. heavily cold-worked thin sections of aluminunz single crystals and of polycrystalline aluminum and nickel were produced directly by a mechanical technique. The specinlens thus prepared were heated with the electron beam to bring about vecrystallization during observation in the electron microscope. Motion pictures taken du.ring heating and the electvon, microg.raphs taken both before and aftev heating allowed the recrystallization process to be traced to its ovigin. Re cvystallized grains originated in very s,mall regions of the cold-worked structure and developed through rapid migration of high-angle boundaries. The boundaries either were present as such in the matrix or were formed out of dense dislocation networks. SIGNIFICANT advances have been made in recent years in the study of nucleation of recrystallization using the technique of transmission electron microscopy of thin metal foils. Bollman1 in a study of heavily rolled polycrystalline nickel found support for the Cahn-Cottrell2,3 theory of nucleation. According to this theory nuclei form by the initially slow growth of subgrains formed through polygonization. During this initial period of slow growth (the incubation period) the migrating boundary of the subgrain increases its disorientation with the cold-worked matrix and thereby increases its mobility to become a rapidly migrating high-angle boundary. Bailey4,5 investigated the annealing behavior of several metals deformed both in tension and by rolling and concluded that recrystallization took place through the migration of high-angle boundaries. With low deformations these boundaries were present in the metal before deformation. With high deformation it was not possible to tell whether the boundaries were pieces of the original grain boundaries or were produced either during deformation or by polygonization during ameal- ing. Direct observation during heating of metal foils indicated that subgrains form by polygonization and grow at an uneven rate. The grain size obtained decreased with decreasing foil thickness indicating that the foil surface resists boundary motion. Votava,6 in heating stage experiments on rolled copper, observed nuclei to appear suddenly and grow in jumps of differing magnitude. However, he found no special dislocation configurations where the nuclei appeared. Fujita,7 as a result of a study of subgrain growth in heavily worked aluminum, concluded that the boundary of a recrystallized grain initially forms from the boundary of a group of subgrains. This occurred by a process of deposition of vacancies and dislocations in the group boundary as the boundaries within the group disappear. HU8,9 directly observed a similar process in heating stage experiments on 70 pct rolled Si-Fe single crystals. The growth of subgrains appeared to proceed by a coalescence mechanism. The observed fading away of the boundary between two subgrains was explained by the moving out of dislocations from the disappearing boundary into the connecting or intersecting boundaries around the subgrains. The subgrain size and degree of disorientation with the surrounding structure were thus increased. With the increase in disorientation occurred a corresponding increase in boundary mobility, which eventually allowed the boundary to migrate rapidly. This process was observed to occur within "microbands" consisting of parallel narrow segments disoriented by a few degrees present in the as-rolled structure. The conclusion of Rzepski and Montuelle10 that growth is preceded by the coalescence of blocks through disappearance of their common boundaries supports this view. In contrast to Hu's coalescence model for nucleation were the conclusions of Walter and ~och.""~ Working with the same material as Hu, of the same orientation and rolled to the same reduction, they concluded that nucleation occurred by the Cahn-Cottrell mechanism. They observed, in agreement with Hu, that recrystallization began in the "microband" regions which they referred to as "transition" bands. Bartuska13 studied subgrain growth in heavily rolled nickel using a beam heating method in the electron microscope. He concluded that nuclei for recrystallization form from the largest most perfect subgrains present in the cold-worked structure by rapid intermittent migration of parts of subboundaries. In rare instances he observed subgrain growth by coalescence. EXPERIMENTAL PROCEDURE The materials used in this study were 99.999 pct A1 supplied by A.I.A.G. Metals, Inc., and 99.999 pct Ni supplied by Johnson and Matthey and Co., Ltd. The Hitachi HU-11 electron microscope, with uniaxial

Jan 1, 1968

By R. F. Yap, N. Arbiter, C. C. Harris

A fully-instrumented driving mechanism has been constructed to study the power, aerating and solid suspension characteristics of several laboratory flotation machines. Machines operating over normal flotation speed ranges give substantially constant power numbers in liquid systems indicating that they operate close to fully baffled turbulent flow conditions. Owing to lack of geometrical scaling, power numbers for different sizes of cells of the same make are different. Even larger differences occur between cells of different manufacturers. For a given impeller, varying the tank size did not significantly affect the power consumption. Tank geometry and baffling has a slight effect on power consumption with shrouded impellers. For a given impeller speed, suspension height decreases while power increases with increasing mass of solids in suspension. While power consumption is hardly affected by particle size and clearance between the impeller and tank bottom, the suspension height significantly increases as clearance increases and as particle size decreases. Aeration was found to affect not only power but also the suspension characteristics of the machine. For closely sized particles, drastic sedimentation occurred when a critical value of the air flow number was exceeded. The critical air flow number decreased with increasing particle size. This effect was less marked with a wide particle size range. The self-aerating capacity of flotation machines increases with cell size at a given speed for the same impeller-shroud system, but decreases in the presence of frother and solids. Equations have been established for correlating all of the studied variables. Data taken from the literature were also correlated with these equations. The modern mechanical flotation machine has, over a period of 60 years, displaced other designs to become the dominant type. Once the correct chemical environmental conditions have been established, the major requirement, selective mineral-air adherence, is essentially a function of two operating variables — impeller speed and air rate. The special nature of this dependence is such that in the same apparatus, but under different operating conditions, adherence can be either prevented or destroyed. Another aspect of this observation is that flotation machines though differing in detail to a considerable extent can produce similar metallurgical results. It must be concluded that having determined the correct reagent levels, establishing the correct operating conditions are at least as important as machine design. Since about 1960, after 30 years of fairly stable circumstances, manufacturers have been developing new types of machines whose major feature is emphasis on pulp flow characteristics by virtue of new im-peller-stator designs. As far as can be ascertained, these new designs necessitate radical departures from accepted impeller speed-air rate values. Performance data are still awaited. Flotation machine development has been empirical, as is selection of operating conditions. The purpose of the work at Columbia University has been to elucidate the fundamentals of the subject so that the purely mechanical aspects of the flotation process can be understood in hydrodynamic terms.1-6 Hopefully, this might lead to intelligent selection of a particular machine and its operating conditions for a specific ore based on a minimum of testing, or even to new types of flotation machines. The present investigation is the most comprehensive to date.7,8 Earlier work, which was largely exploratory, has been repeated and extended and employes the most accurate measuring techniques available. Additionally, sufficient hydrodynamic data were obtained under flotation conditions to delineate the relevant operating ranges. Attention has been given to scaling in that several geometrically scaled tanks and impellers have been constructed and tested; it has been pointed out elsewhere that undistorted geometrical scaling rarely pertains in flotation machine design.5 This paper is divided into sections dealing with relevant combinations of the three phases: liquid, air and solid. Summaries of the literature have been given in previous publications2-4 and will not be repeated here. Recently, several reviews of mixing have appeared, which can be usefully referenced.9-11 It is of interest to note that flotation is not included

Jan 1, 1970

By John Chipman

Published data on the equilibria of carbon in binary Fe-C austenite with H2-CH4 and CO2-CO gas mixtures are used to obtain the constants of an equation repre -senting the activity coefficient of carbon throughout the stable austenite range of temperatutre mid compositiotl. Calculated values for the solubility of graphite are found to be in agreement with the best-published direct observations. Its heat of solution is 10,680 calper g-atom. The free energy of formation of cementite is culculated from the equilibrium compositions of austenite and of gas. Its entropy and heat of formation are also obtained. BASIC knowledge of the high-temperature properties and treatment of steels is contained in the Fe-C diagram and its extensions for the effects of alloying elements. The application of thermodynamics to the Fe-C system, with addition of alloying elements, can serve to simplify and systematize this knowledge. The points of beginning are the binary fee solution, austenite, concerning which our information is reasonably complete, and the binary carbide, cementite. Further studies must include the other phases of the binary system, a and liquid, and for these an accurate knowledge of the ? phase boundaries is essential. These studies should provide a firm base for systematic study of the effects of alloying elements on all phases of the system, their transformations, and precipitation phenomena. With the aid of the simplifying thermodynamic concepts of activity coefficients and interaction parameters it is to be hoped that the results of these studies will be amenable to computer solution thus making the information on multicomponent systems more readily available than would be possible in multidimensional diagrams. Studies of the thermodynamic properties of austenite have utilized equilibrium measurements on the reactions:

Jan 1, 1968

By C. S. Barrett, C. T. Haller

Twinning in magnesium is known to occur profusely under certain conditions, and when it occurs in polycrystalline materials it brings about a partial or even a complete change in the preferred orientation. The amount of research on this subject, however, has not been in proportion to its importance as a mechanism of deformation and reorientation. Much of the research has been limited to twinning in single crystals. The present study1 concerns polycrystalline sheet, and is an investigation of the twinning produced by plastic deformation as a function of the following variables: 1. Direction and type of deformation. 2. Amount of deformation. 3. Speed of deformation. 4. Temperature of deformation 5. Composition and temper of the sheet. 6. Twins previously produced. 7. Twins previously produced, then annealed. By proper control of these variables it is shown that, on a laboratory scale, samples can be produced that are free from preferred orientations. The materials used were samples of commercial magnesium-alloy sheets with nominal compositions given in Table I. X-ray diffraction was used in most of this work to detect the twins and to estimate the amount of twinning. Since twinning on (102) planes causes a re-orientation of the crystal lattice by 86", it is very easy to study with X-ray diffraction patterns, particularly if one notices the diffraction ring from the basal plane— the 002 reflection. By properly directing the X-ray beam, it is possible to produce 002 reflections from twinned material at, say, the 6 and 12 o'clock positions on the 002 ring, with the untwinned material reflecting to 3 and 9 o'clock positions; and by judging the relative intensity of the arcs on the diffraction ring at these positions, it is possible to judge the amount of twinned material relative to the untwinned. This X-ray method of estimating the amount of twinning is generally superior to metallo-graphic observation, for it was found

Jan 1, 1947

By C. S. Barrett, C. T. Haller

Twinning in magnesium is known to occur profusely under certain conditions, and when it occurs in polycrystalline materials it brings about a partial or even a complete change in the preferred orientation. The amount of research on this subject, however, has not been in proportion to its importance as a mechanism of deformation and reorientation. Much of the research has been limited to twinning in single crystals. The present study1 concerns polycrystalline sheet, and is an investigation of the twinning produced by plastic deformation as a function of the following variables: 1. Direction and type of deformation. 2. Amount of deformation. 3. Speed of deformation. 4. Temperature of deformation 5. Composition and temper of the sheet. 6. Twins previously produced. 7. Twins previously produced, then annealed. By proper control of these variables it is shown that, on a laboratory scale, samples can be produced that are free from preferred orientations. The materials used were samples of commercial magnesium-alloy sheets with nominal compositions given in Table I. X-ray diffraction was used in most of this work to detect the twins and to estimate the amount of twinning. Since twinning on (102) planes causes a re-orientation of the crystal lattice by 86", it is very easy to study with X-ray diffraction patterns, particularly if one notices the diffraction ring from the basal plane— the 002 reflection. By properly directing the X-ray beam, it is possible to produce 002 reflections from twinned material at, say, the 6 and 12 o'clock positions on the 002 ring, with the untwinned material reflecting to 3 and 9 o'clock positions; and by judging the relative intensity of the arcs on the diffraction ring at these positions, it is possible to judge the amount of twinned material relative to the untwinned. This X-ray method of estimating the amount of twinning is generally superior to metallo-graphic observation, for it was found

Jan 1, 1947

By Max Y. Seaton

THE scope of this paper will be limited to finished materials that contain a large preponderance (around 80 per cent or more) of magnesium oxide. The large and commercially important production of refractory dolomite and modified dolomites is not discussed here, and the production of magnesium basic carbonate for use as a heat insulator is barely mentioned. The discussion is confined mainly to magnesias produced in comparatively great quantity by direct processing of naturally occurring raw materials and hence excludes secondary or highly purified products such as pharmaceutical products or U.S.P. light and heavy calcined magnesias of commerce. NOMENCLATURE The nomenclature of the magnesia products that will be discussed is somewhat confusing, in that it is customary in the trade to designate as "calcined magnesite" the materials that result from the calcining of magnesite to temperatures that remove the contained carbon dioxide. The finished product, therefore, is not magnesite at all but simply a more or less pure magnesium oxide or magnesia. Furthermore, partly for statistical and partly for commercial reasons, it has been customary, during recent years, to classify as "calcined magnesite" products that result from the heat-treatment of magnesium hydroxide precipitated from magnesium-containing brines; products that, obviously, have no relationship whatever to magnesite. In the listing of magnesia-containing materials, tonnages of which are regularly reported by the Bureau of Mines, reports ordinarily are made under two headings: "caustic calcined" magnesite, in which all of the varieties of products treated at relatively low temperatures are lumped; and "dead-burned" magnesite, in which are included both high-purity and low-purity, and particularly both high-iron and low-iron, materials that have been so heat-treated that the magnesium oxide occurs predominantly as crystalline periclase-materials used almost entirely for refractory purposes. In 1940 the apparent domestic consumption of "caustic calcined" magnesite was approximately 17,000 tons, about 95 per cent of which was produced in this country. The apparent consumption of "dead-burned" magnesite was approximately 172,000 tons, some 82 per cent of which was domestically produced. In both total quantity and percentage of material produced within the United States, these figures are record highs for many years past. UTILIZATION As indicated by these figures, the major tonnage outlet for magnesia is the refractories industry, in which magnesia is employed principally in three ways: (I) as dead-burned grain magnesite for the construction and maintenance of the hearths or bottoms of basic open-hearth furnaces; (2) in the form of refractory magnesite bricks and shapes; and (3) as a chemical

Jan 1, 1942

By Henry Lloyd Smyth, J. Ralph Finlay

The most important area of the so-called Keewatin rocks of northern Minnesota is that which runs westerly from Lake Saganaga, near the national boundary, and finally disappears beneath the drift (or has not been farther traced), in the neighborhood of Vermilion Lake. With this belt we have a general acquaintance, extending back some four years. During the past year we have had the opportunity of studying in considerable detail a small part of it, embracing an area of about 12 square miles on the south and east shores of Vermilion Lake. In this paper we desire to record our observations and conclusions, which seem to us important. II.—Literature. Outside the Minnesota Geological Survey, the list of the investigators who have studied the geology of the Vermilion Lake area is not long. Whittlesey, in 1876,* mentioned the occurrence, in the area about Vermilion Lake, of rocks resembling the Canadian Laurentian. Chester,? in 1884, described the iron-ore at Vermilion Lake as occurring in connection with jasper and quartzite, intimately bedded with the country-rock, chiefly sericite schist, and standing in a nearly vertical attitude. The Vermilion iron-bearing rocks, and those of the Mesabi range, he says, are of the same age.

Jan 1, 1896

Jan 1, 1887

C. W. GENNET, JR.,* Chicago, 111. (written discussion?).-Since the Lehigh Valley accident, transverse fissures have become a source of constant anxiety to railroad officials, because such defects, only infrequently detected by the trackmen, may appear under a moving train entirely without warning. The number of actual accidents resulting from rails containing fissures is no doubt a small proportion of the total, but the serious losses that have been directly attributable to fissures, coupled with the continual possibility of repetition, creates an alarming situation which demands a full investigation of the cause of fissures; apart from other types of rail failures. Mr. Howard's position gives him a field for action and an opportunity for laboratory examination and research that are unsurpassed, and the results. of his investigations must be received with the utmost attention and respect. His admirable paper, summarizing his previously published studies and analyses of the transverse fissure problem, constitutes a convincing treatment of the subject chiefly from the standpoint of stress and strain to which rails are subjected. The intricacies of the problem are so great that seemingly any theory advanced for the cause of fissures may be attacked from some angle; and although Mr. Howard's deductions are logical in the direction followed, it is apparently necessary for considerable work to be done along other lines before his theories can be fully accepted. It is generally conceded that fissures, whether fundamentally clue to fatigue or something else, are of a progressive character, their ultimate size' being the result of growth from an originating nucleus or point of rupture. Abundant proof of this lies in the variable sizes of the fissures found, and there can be no doubt that their development or growth is the result of the strains to which rails are subjected in the track. Obviously, therefore, the most important factor in investigating the cause for fissures lies in determining the conditions that exist at the point or

Jan 4, 1918

By C. J. McHargue, J. C. Ogle

Lightly deformed columbiun single crystals which contained only parallel hoins or purullel and intersecting trains were annealed at 1000' and 1600"C. No re-crystallizntion occurred in specimens hawing only parallel twins. Only noncoherent twin boundaries nzipated at 1000°C but both coherent and noncoherent ones moved al 1600°C. Recrystallization occurred within a few minutes at twin intersections at 1000°C. The orientation 01 the recrystallized grains differed front that of both the matrix and deformation twins, but could he derired by (110) and/or(112) rotations. ALTHOUGH twinning in metals has been extensively studied, there have been no definitive studies of the annealing behavior of crystals containing deformation twins. Some effects observed after annealing deformation twins have been summarized by Cahn1 and Hall2. Any or all of these phenomena are observed: 1) The twins may contract so that the sharp edges of the lens become blunted, and eventually the twin may disappear entirely. 2) The twins may balloon out at an edge, giving rise to a large grain having the same orientation as the twin. 3) The specimen may recrystallize; i.e., new grains are nucleated and grow at the expense of the twins and the crystal immediately adjoining the twin. Such grains have orientations which are not present before. Contraction has been observed in iron,3 titanium,3, 4 beryllium,5 zinc,8, 7 Fe-A1 alloy,' and uranium.9 Long anneals at high temperatures are required to have any appreciable effect in these metals and only thin twins are absorbed. Lens-shaped twins are absorbed from the edges: the thin, almost parallel-sided twins are usually punctured in several places and each piece contracts independently. Absorption is very gradual and no sudden cooperative jumps have been observed. The expansion of a twin into a larger grain of identical orientation is unusual, but such growth has been observed in iron,"'" zinc,6 and uranium." Crystals which have been deformed simultaneously by slip and twinning recrystallize first in the area adjacent to the twin. New grains appear faster where the twins intersect: but isolated twins, especially if thick, can also give rise to new grains. This type of recrystallization occurs in zinc.6, 7, 12, 13 and beryllium.14 Reed-Hill noted, in a single crystal of magnesium, the nucleation of a recrystallized grain at a twin intersection which had the same orientation as the second-order twin and which grew into the highly strained matrix.15 Short-time annealing has been reported to cause no change in the deformation twins in vanadium,16 columbium, 17, 18 tantalum,19 tungsten,'' and zinc.7 The purpose of this investigation was to note the effects of annealing on the coherent and noncoherent boundaries of deformation twins in columbium and to locate the nucleating sites for recrystallization. The orientation relationships, which the new recrystallized grains have with the parent crystal and the deformation twins, were also determined. EXPERIMENTAL PROCEDURE Single crystals of columbium were obtained by cutting large grains from electron-beam-melted buttons which contained 10 to 50 ppm C, 10 to 100 ppm O,, 1 to 10 ppm H2, and 10 to 15 ppm N2. The crystals were hand-ground and chemically polished until all grain boundaries were removed. The specimens were mounted in an epoxy resin and a face of each crystal was mechanically polished on a Syntron polisher using Linde A and then Linde B polishing compounds. After all faces were mechanically polished, the crystal was electrolytically polished to remove all distortion due to cutting and grinding. Laue photographs were taken of all faces of the crystals to determine the quality and orientation of each crystal. The crystals were compressed about 10 pct at -196 C in a specially constructed compression cage with an Instron tensile machine. Each crystal was separated from the top and bottom anvils by teflon films which acted as a lubricant. With the specimen crystal in position, the entire cage was cooled to -196°C by being submerged in a Dewar containing liquid nitrogen. The crystals were compressed at a rate of 0.02 in. per min and the load was recorded on a strip-chart recorder. After deformation the crystals were mechanically polished on 600-grit paper and Pellon cloth with Linde A and Linde B polishing compounds. The crystal faces were chemically polished and then etched. The twin planes were identified metallographically from an analysis of the twin traces on two surfaces. Annealing was carried out by placing each crystal in a columbium bucket made from the same electron-beam-melted material as the crystal itself and suspending the bucket by a tantalum wire in a quartz tube. After a vacuum of 10-7 Torr was attained, a furnace at 1000" or 1600 C was raised into position and the crystals held for various lengths of time. The crystals were repolished and etched after annealing to remove any surface contamination. Approximately 0.010 in. was removed during this process. The resulting surface was examined metallographically for microstructural changes due to annealing. A microbeam Laue camera mounted on a Hilger Micro-focus X-ray unit was used to determine the Orientstions of the recrystallized grains. This X-ray micro-beam camera had a 0.002-in.-diam collimator and incorporated the ideas of both and and chisWik21 and Cahn.22

Jan 1, 1967

By Hans Lundberg

By electrical prospecting, orebodies that do not otherwise manifest themselves at the surface may be located. Conditions favorable to success with the methods are: Favorable geological conditions, such a proportion of minerals of good electrical conductivity in the orebody as will render the latter materially a better conductor than the enclosing rock, situation of the orebody within the range of influence of the electrical currents used and in proportion to this range a sufficient area of horizontal cross section of the orebody. The result is to permit of spotting drill holes accurately and to save the expense of drilling barren ground. THE art of prospecting, like other branches of mining, has developed rapidly during the last ten years, mainly because of the advancement of mining geology, but also because of the development of "geophysical methods," which are based on the different physical characters of minerals and rocks, e. g., specific gravity, elasticity, and magnetic and electric properties. These methods have proved of great value, especially in countries where the rock is covered by soil and direct geologic investigation is out of the question. In prospecting for orebodies, the methods based on studying differences in electrical conductivity have proved most successful; for which reason, the methods of electrical prospecting have been subject to the most energetic and systematic work, especially in Sweden. This paper gives a brief description of the methods of electrical prospecting now in use, especially the new electromagnetic methods, which have not been described before. The great differences in the electrical properties, especially in conductivity, between ore minerals and rock minerals are fundamental to all methods of electrical prospecting, for which reason, the electrical conductivity of ore minerals and rock minerals has been carefully studied in connection with and previous to the development of the various methods of prospecting with electricity. Ore minerals with a metallic luster, such as chalcopyrite, iron pyrite, galena, specular iron, magnetite, etc., also graphite and a number of cows, have considerably higher conductivity than the surrounding rock and soil (see Table 1). The figures of the table denote the specific resistance W, expressed in ohms for a cube with an edge of 1 centimeter.

Jan 7, 1925

By W. M. Robertson

The rate of crack Propagation thvough a solid meta1 in the presence of a liquid metal has been calculated. The crack is assumed to propagate by solution of the solid in the liquid under the influence of an applied stress, with volume diffusion of the dissolved solute through the liquid controlling the propagation. The calculated velocities can be tens of centimeters per second or higher. THE embrittlement of normally ductile solid metals by liquid metals has been the subject of extensive study, as summarized in the book by Rostoker, McCaughey, and arkuus.' In experimental studies of liquid-metal embrittlement it is usually concluded that the observed crack-propagation rates are too high to be accounted for by diffusion processes. Diffusion is considered to be an unimportant factor and various other mechanisms are invoked to account for the experimental observations. The similarities of liquid-metal embrittlement to stress-corrosion cracking, to hydrogen embrittlement of steel, and to low-temperature embrittlement of bcc metals are frequently used as analogies for these mechanisms.1"3 The present paper assesses how rapidly a crack could propagate by a mechanism of volume diffusion through the liquid metal. We assume that the crack propagates by solution of the solid in the liquid at the crack tip under the influence of the applied stress and diffusion of the dissolved solute away from the crack tip. The model uses only macroscopic thermodynamic, kinetic, and elastic concepts. The calculations indicate that this dissolution-diffusion mechanism can give high crack velocities. In some previous treatments4'' of crack propagation in a liquid-metal environment the fundamental step has been the breaking of atomic bonds at the crack tip. The liquid greatly decreases the energy necessary to break these bonds. The present model assumes the crack is filled with liquid metal; the presence of the liquid is essential because diffusion through the liquid is assumed to control the rate of propagation. With proper modification this model could also apply to stress corrosion in a liquid environment. THEORY Consider a crack in a homogeneous elastic continuum, ignoring all the structural details of the solid. This neglects a host of metallurgical details which can be significant in the problem of liquid-metal embrittlement;' but this simple assumption is a useful approach in evaluating the role of diffusion through the liquid. The stress distribution around the crack tip establishes a gradient in the chemical potential at the tip, which causes a diffusion flux through the liquid away from the tip. The solid-liquid interfacial tension works against crack propagation, since propagation requires creation of interfacial area. From these two considerations we derive the diffusion flux, which then yields an expression for the velocity of the crack tip. The concentration of the solute in the liquid, C, is equal to the equilibrium concentration in the presence of an unstressed, flat surface, Co, plus the excess concentration due to the stress, AC(a), plus the excess concentration due to capillarity, AC(y): We calculate AC(o) and AC(y) and then obtain the diffusion flux. Consider (Fig. 1) a long flat crack of length 2L, thickness 2r, and tip radius r, where r remains constant as the crack propagates in the positive x direction. There is no stress ay in the y direction across the flat surfaces of the crack. The magnitude of the stress at the crack tip due to the applied stress, a,, is given by the relation6 ^=^[l-2^] 12] which, for long narrow cracks, becomes The stress increases the chemical potential in proportion to the strain energy.? The excess chemical potential due to the stress, Ap(a), is given by where 52 is the atomic volume of the solid and E the Young's modulus. Inserting Eq. [3] gives Now consider the crack to be filled with a liquid metal. The excess solubility due to stress, hC(a), is given by the excess chemical potential due to capillarity, Ap(y), is given by8 where y is the solid-liquid interfacial energy, and v1 and YZ are the two principal radii of curvature of any surface point. At the crack tip, r1 = -r and vz = m, so TRANSACTIONS OF THE METALLURGICAL SOCIETY OF AIME

Jan 1, 1967

By M. J. Blackburn

The ordering reaction and its effect on dislocation awangements in alloys based on Mg3Cd has been studied by transmission electron microscopy. Ordering occurs by the growth of ordered domains until they impinge, thus producing an antiphase domain boundary (APB) network. In the disordered alloy, slip is confined to the basal plane while in the ordered condition slip occurs on the basal, prism, and pyramidal planes. Superdislocations have been identuied on these planes and their origin and expected spacings are discussed and compared with the observations. The APBs between the dislocation pairs have been resolved and have proved useful in the identification of superdislocations. Observed slip structures are related to the changes in mechanical properties produced by ordering. At high temperatures magnesium and cadmium form a continuous series of disordered hexagonal solid solutions. At lower temperatures three ordering reactions occur; two based on the compositions Mg3Cd and MgCd3 are transformations to DO19-type superlattices while alloys based on the composition MgCd transform to a B19 lattice. The ordering process in alloys near the composition Mg3Cd has been studied by several workers'-4 and it has been shown to be a first-order type transformation. The variation of mechanical properties with the degree of order and temperature have been reported in some detail by Stoloff and Davies.5 It was shown that the disordered alloy was stronger and less ductile than the ordered alloy, the differences being attributed to changes in the number of active slip planes and to the presence of short-range order in the disordered alloy. In this investigation the ordering reaction has been qualitatively followed by transmission electron microscopy. The effect of this ordering transformation on the dislocation arrangements in deformed specimens has been studied in some detail. EXPERIMENTAL Alloys were melted in a stainless-steel crucible under a flux and chill cast; both melting and casting were performed in an argon atmosphere. The ingots were hot-rolled and tensile specimens cut from the resultant strips. Some large-grained specimens were grown by the strain-anneal method. Heat treatment was carried out in inert atmosphere furnaces and in oil baths at the lower temperatures. Thin foils for transmission electron microscopy were produced by electropolishing in a cooled solution (temp = -40°C) containing 50 cu cm of nitric acid and 180 cu cm of methanol at a potential of 10 v. The ordering reaction was followed by transmission electron microscopy, X-ray, and electrical resistivity measurements but this paper will be concerned chiefly with the electron-microscopy results. Much of the work described in this paper was performed on an alloy containing 26.10 at. pct Cd although two of the micrographs are taken from work on a 25.55 at. pct Cd alloy. RESULTS i) The Quenched Solid Solution. Specimens which were quenched from above the critical temperature showed diffuse superlattice reflections. sark-field analysis on these reflections failed to reveal the presence of ordered domains and thus the superlattice reflections were assumed to arise from regions of short-range order. Quenching from temperatures of 400" to 450°C results in the formation of dislocation loops and helical dislocations. Such defects have been observed and analyzed in detail in several hexagonal metals6-' and as the crystallography and interactions of such loops appear similar in this alloy they will not be reported in detail. Fig. 1 shows a high density of loops

Jan 1, 1968

By J. B. Warriner

Stripping is the name given to the process of removing clay, rock, or other cover from deposits of coal or ore. In this paper it is intended to cover the methods used in carrying on this operation in the anthracite coal regions of Pennsylvania. In doing this, attention will be chiefly devoted to those methods wherein practice in different sections of the region differs, or is susceptible to change or improvement, though it is intended to give as complete data as possible concerning all stages of the operation. The data from which this paper has been prepared have been collected and submitted by a committee† representing the larger users of stripping methods, of which committee the writer has acted as chairman. Data and assistance have also been obtained from various mining men, engineers, contractors and manufacturers of stripping equipment, to all of whom our appreciation is due. Early History and Statistics The earliest mining operation on a commercial scale in the anthracite region was a stripping operation. This was the famous Quarry mine at Summit Hill, where this method was used only because no other method of mining a deposit of the nature found there seemed feasible to the early operators. A description of this stripping from a report published in 1821 is as follows: "The coal mine at present worked by the company lies on the top of a mountain, and appears to extend over some hundred acres of land covered by about 12 ft,. of loose black dirt resembling moist gunpowder, which can be removed by cattle with scrapers, and thrown into the valley below so as never to impede the work. The thickness of the coal is not known, but a shaft has been sunk in it 35 ft. without penetrating through. More than an acre of mine has been uncovered and presents a huge rock of coal, which is easily quarried without blasting."

Jan 1, 1918

By Parke A. Hodges

CHROMIUM, one of the most vital of the strategic metals, has constantly increasing importance as research expands potential uses in alloy steel metallurgy. All signs point to a steadily growing use for chrome ore, and the general subject has received a great deal of attention recently.1 A relatively little known country, Southern Rhodesia produced a total of 5.949 million tons, or 18.6 pct of the total world chrome output, between 1906 and 1950, according to figures from the U. S. Dept. of Commerce. According to U. S. Bureau of Mines figures, Southern Rhodesia produced 291,525 tons in 1950 and 300,267 tons in 1951, or approximately 13 and 11 pct, respectively, of world production for the two years. In 1953 the Rhodesian Vanadium Corp. produced about 85,000 tons of which probably 7,000 tons were concentrate. Production by other companies was high. Large in themselves, the figures assume even greater significance because most of this output is of metallurgical grade.

Jan 8, 1954

By L. D. Ricketts

' In recent years the metallurgical field of the copper industry has expanded greatly, the copper ores have become lean add diverse in charactcr, and we are obliged to treat such ores on a very large scale. On the commercial side, the operating and consulting staffs of the great mining companies have been enlarged and organized, and no great capital expenditure is decided upon until it has been investigated and recommended by a highly organized force of specialists as a committee. I believe that in the scientific field and in the most valuable proceedings of our -various societies the old idea of one-man point of view and one-man treatment of principle and detail together has been splendidly developed and should continue, but for the advancement of our art we should deliberately discuss subjects among ourselves in order to do away with the idiosyncrasies of the individual and to gain a composite view reprcscnting the best thoughts of many men. In the development of new ideas leading to the introduction of new methods, the societies could enlarge their present great field of usefulness if such discussion be had, and in my opinion further great good could be done if a number of men of different qualifications and experience would join in preparing a joint book, each dealing with his particular problem, but consulting the other men. In this way individualism would be preserved and various processes would be described, and if properly reviewed the paper would be palpably capable of analysis, and the student would have at his command the associated efforts of a group of qualified engineers. I have neither the time nor the qualifications to make an investigation and study in detail of hydro-metallurgical processes, but for some years I have foreseen the development of such processes, and I have a deep interest in the subject and have given some of its phases careful consideration from a long-distance viewpoint. I have assisted in directing many experiments and in reviewing the merits of many processes, and am glad to make some introductory remarks with the understanding that other gentlemen present will freely and candidly enter into the discussion and help to fill out the few bare bones that I can uncover.

Jan 1, 1916

By Thomas Fraser, H. F. Yancey

BefoRe a new coal-washing plant is installed, or an existing washery is remodelled and improved, considerable experimental work on the coal to be washed should be done. A thorough examination of a coal, to determine whether it can be washed successfully and the best and most economical process for securing a satisfactory grade of washed coal, calls for mine sampling, analytical work, and experimental coal-washing tests which may require several months' time and the expenditure of considerable money. Having collected the required data on the characteristics of the coal and the results secured with different types of machines under different sets of conditions, it is necessary to decide what method of treatment should be used. The interpretation of the experimental data is no small part of the problem. The first step in the investigative work is to determine from the characteristics of the coal and the manner of occurrence of the impurities whether it is possible to separate the dirt from the coal and produce a washed coal of the required quality. Then it is necessary to select the type of washer or the system of treatment that will produce the greatest amount of washed coal of this grade for the least cost. To secure this information, tests may be made at laboratories operated by consulting or contracting engineers, at the testing plants of machinery manufacturers, at commercial washeries, or in an experimental washery erected on the site of the proposed plant. Such tests will produce figures in which every factor, including analyses of raw coal and washed coal and yield of washed coal, may vary widely. To reduce these results to a common basis for comparison is very difficult. A number of plans

Jan 1, 1923

By Thomas Fraser, H. F. Yancey

BefoRe a new coal-washing plant is installed, or an existing washery is remodelled and improved, considerable experimental work on the coal to be washed should be done. A thorough examination of a coal, to determine whether it can be washed successfully and the best and most economical process for securing a satisfactory grade of washed coal, calls for mine sampling, analytical work, and experimental coal-washing tests which may require several months' time and the expenditure of considerable money. Having collected the required data on the characteristics of the coal and the results secured with different types of machines under different sets of conditions, it is necessary to decide what method of treatment should be used. The interpretation of the experimental data is no small part of the problem. The first step in the investigative work is to determine from the characteristics of the coal and the manner of occurrence of the impurities whether it is possible to separate the dirt from the coal and produce a washed coal of the required quality. Then it is necessary to select the type of washer or the system of treatment that will produce the greatest amount of washed coal of this grade for the least cost. To secure this information, tests may be made at laboratories operated by consulting or contracting engineers, at the testing plants of machinery manufacturers, at commercial washeries, or in an experimental washery erected on the site of the proposed plant. Such tests will produce figures in which every factor, including analyses of raw coal and washed coal and yield of washed coal, may vary widely. To reduce these results to a common basis for comparison is very difficult. A number of plans

Jan 1, 1923

By E. T. Turkdogan

This paper deals with an analysis of the conditions leading to the formation of blowholes and surface and subsurface defects in cast low-carbon steels. The theoretical analysis of the problem is based on the concept of the occurrence of deoxidation reactions during cooling of liquid steel and during its solidification. The reaction equilibrium data are given on deoxidation of steel by carbon, silicon, and manganese. The details of the calculations are given on the simultaneous deoxidation of steel by silico-manganese additions to produce a desired level of deoxidation at certain residual concentrations. Based on an idealized model, an attempt is made to calculate the enrichment of solutes in the solidifying liquid entrapped within the interdendritic cells which are known to be less than 200 µ thick. Formulae are derived for the calculation of the change in the residual oxygen content of the enriched liquid steel, controlled by Si/Mn deoxidation. It is shown that for every particular initial silicon, manganese, and oxygen concentration in steel (as residuals in liquid solution at the beginning of solidification) there is a particular carbon concentration below which carbon monoxide, leading to blowholes or pinholes, is not expected to form. There are a number of independent variables which have a decisive influence on the formation of blowholes. For example, it is shown that the position of the curve depicting the critical relation between initial silicon and carbon content for suppression of the evolution of carbon monoxide) can be shifted to lower silicon or higher carbon levels by: increasing manganese content; decreasing oxygen, hydrogen, or nitrogen content; or increasing external pressure on the solidifying mass. FOR the production of sound castings it is essential to prevent the formation of interdendritic worm-holes or pinholes brought about by gas evolution during solidification. Another major problem encountered in solidification of ingots is the formation of surface defects. These problems are thought to be closely associated with the deoxidation reaction occurring in liquid steel during cooling and subsequent solidification. As the liquid becomes richer in impurities during dendritic solidification of steel, a series of reactions may occur in the liquid phase trapped within the branches of growing dendrite platelets. Because of the strong interaction between dissolved oxygen and impurities in liquid steel, the reactions most likely to occur are of the deoxidation type. However, inherent to the extreme complexity of the problem in question, numerous simplifying assump-

Jan 1, 1965

Tooele Flue-type Cottrell Treater Discussion of the-paper of A. B. YOUNG, to be presented at the New York meeting, February, 1921, and printed to accompany MINING AND METALLURGY No. 162, June, 1920. HARRY V. WELCH,* Los Angeles, Calif. (written discussion).-The basic requirement in the electrostatic process of electrical precipitation is that gases containing solid particles in suspension are forced to pass through an electrical field consisting of an insulated high-potential wire, or other type of discharge member, and a grounded plate, or other, type .of collecting member. Commercial. operation and efficiency of precipitation are secured by electrical and structural arrangements to provide the maximum value of the variable factors modifying the degree of precipitation. That is to say: 1. A unidirectional flow, of high-potential current is vastly more effective in precipitation than a high potential alternating-current flow.

Jan 2, 1921