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By Reynold Q. Shotts

Nitric acid oxidation rate analyses of three coals, previously studied microscopically by the Bureau of Mines, revealed three components. Relative quantities agree with those found for the four components given by the Bureau and results are consistent with current ideas of coal constitution. Possible multi-component composition for bright coal and a reactivity-rank relation are suggested.

Jan 5, 1953

By M. Muskat, R. D. Wyckoff, H. G. Botset

The flow of liquids through porous media is known to follow Darcy's law which states that the velocity of flow is proportional to the pressure gradient. This law is but a statement of the facts obtained from experimental studies and is, therefore, uncontroversial. It should be men-tioned, however, that it applies only when the flow is viscous in nature and that at excessive velocities which tend to produce turbulence, the velocity is proportional to some power of the pressure gradient, the exponent ranging from 1 to 0.5 as the degree of turbulence increases. However., experiments with media having permeabilities corresponding to those of actual producing horizons show conclusively that the actual flow of liquids in a producing horizon must be viscous except perhaps in the immediate vicinity of a well producing at a high rate. Therefore, one is justified in assuming Darcy's law to hold for cases of practical interest, as will be implicitly assumed in this discussion Given this basic law of flow it is possible, theoretically to obtain solutions to any problem of viscous flow of dead liquids1 by the usual methods of potential theory. However, problems of practical interest often present such complex geometrical configurations of sources and sinks (driving sources and output wells) as to make the analytical solutions extremely difficult or even impossible. It is useful, therefore, to note that Darcy's law is precisely equivalent to the law of electrical conduction, so that the pressure distribution in steady state porous flow of liquids is exactly the same as the potential distribution in an electrical conducting medium.2 This analogy permits the use of electrical models wherein the potential distributions may be measured with far greater facility and accuracy than is possible in actual fluid-sand models. Such electrical models were used in determining the pressure distribution about complex arrays of wells, an example of which will be shown later in the discussion of the 5-spot flood. One phase of the porous flow problems which is of considerable practical interest involves the tracing of an advancing fluid front. Thus,

Jan 1, 1933

By L. K. Oliver, Marshall

The use of model studies for the development of invasion patterns for cycling is .illustrated by model studies obtained with a recently developed apparatus in the solution of actual cycling problems. Introduction Depletion of a gas reservoir by cycling under pressure maintenance may be divided into three general phases. In the first phase, injection of dry gas is used to move all possible wet gas to producing wells prior to breakthrough of appreciable quantities of dry gas into producing wells. In the second phase, injection moves additional wet gas to producing wells while moderate quantities of dry gas are being produced. The third phase consists of production without injection to deplete the commercial gas reserve which remains after the cycling operation. Thus, in the first phase, a portion of the reservoir is invaded by dry gas, and the dry gas usually displaces something less than 100 pct of the wet gas from the invaded portions of the reservoir. The second phase enlarges the invaded portion of the reservoir and, at the same time, increases recovery from that zone invaded by dry gas in the first phase of the cycling operation. The result is an early yield of a substantial quantity of the liquefiable hydrocarbons in the reservoir with a minimum of the retrograde losses which occur in some reservoirs. The overall recovery from such an operation is usually high. The summation of the paths taken by dry gas in a cycling operation can be termed an invasion pattern. The use of electrical models to predict invasion patterns has been the subject of several contributions to the literature.1-3 The results of such model studies as applied to actual cases has been discussed by Kelton4 and Miller and Lents.= Recently, an improved apparatus for conducting potentiometric model studies has been developed, as described by Lee.V he purpose of this paper is to present model studies or invasion patterns obtained with this instrument in the solution of actual cycling problems. The percentage of displacement of wet gas within the invaded portion of the reservoir in cycling, increase in recoveries of wet gas by continuing cycling after dry gas breakthrough, recoveries of wet and dry gas by production after cycling, or economic factors in cycling are beyond the scope of this paper. Discussions which follow will be limited entirely to the development of suitable invasion patterns for cycling under pressure maintenance and the use of model studies therein. Discussion OF Results In using model studies to predict invasion patterns in cycling, some factors which influence the results are the following:

Jan 1, 1948

By Marshall, L. K. Oliver

The use of model studies for the development of invasion patterns for cycling is .illustrated by model studies obtained with a recently developed apparatus in the solution of actual cycling problems. Introduction Depletion of a gas reservoir by cycling under pressure maintenance may be divided into three general phases. In the first phase, injection of dry gas is used to move all possible wet gas to producing wells prior to breakthrough of appreciable quantities of dry gas into producing wells. In the second phase, injection moves additional wet gas to producing wells while moderate quantities of dry gas are being produced. The third phase consists of production without injection to deplete the commercial gas reserve which remains after the cycling operation. Thus, in the first phase, a portion of the reservoir is invaded by dry gas, and the dry gas usually displaces something less than 100 pct of the wet gas from the invaded portions of the reservoir. The second phase enlarges the invaded portion of the reservoir and, at the same time, increases recovery from that zone invaded by dry gas in the first phase of the cycling operation. The result is an early yield of a substantial quantity of the liquefiable hydrocarbons in the reservoir with a minimum of the retrograde losses which occur in some reservoirs. The overall recovery from such an operation is usually high. The summation of the paths taken by dry gas in a cycling operation can be termed an invasion pattern. The use of electrical models to predict invasion patterns has been the subject of several contributions to the literature.1-3 The results of such model studies as applied to actual cases has been discussed by Kelton4 and Miller and Lents.= Recently, an improved apparatus for conducting potentiometric model studies has been developed, as described by Lee.V he purpose of this paper is to present model studies or invasion patterns obtained with this instrument in the solution of actual cycling problems. The percentage of displacement of wet gas within the invaded portion of the reservoir in cycling, increase in recoveries of wet gas by continuing cycling after dry gas breakthrough, recoveries of wet and dry gas by production after cycling, or economic factors in cycling are beyond the scope of this paper. Discussions which follow will be limited entirely to the development of suitable invasion patterns for cycling under pressure maintenance and the use of model studies therein. Discussion OF Results In using model studies to predict invasion patterns in cycling, some factors which influence the results are the following:

Jan 1, 1948

IN NOVEMBER 1933 I had the pleasure of receiving an autographed copy of a small brochure entitled "Birth of the Porphyry Coppers." The author, James Colquhoun, a distinguished British mining engineer, had been the dominant figure in the Arizona Copper Company, Limited, which, until its acquisition by the Phelps Dodge Corporation in 1921, had been the principal producer of copper in the Morenci district. In addition to the highly productive Humboldt and Longfellow mines it owned the Clay property, which, of course, has now become Arizona's No. 1 copper mine. Except for Utah Copper it is the premier Porphyry copper enterprise in the United States. Mr. Colquhoun's little volume was inspired by THE PORPHYRY COPPERS which had appeared early that year. In a most courteous and friendly way he adduces data to show that "we (Arizona Copper Company Limited) were successfully treating low-grade porphyry ores in quantity for at least four years (1901 to 19W) before the Utah Copper Company did anything at all." This last remark is based on the figures in THE PORPHYRY COPPERS showing that Utah's first production came from the Copperton pilot plant in 1905. Mr. Colquhoun traces the story of Morenci beginning in 1883 when he first arrived there. He tells how the rich self-fluxing ores (containing lime- stone, silica, and iron in proportion to permit direct smelting to produce black copper) were gradually exhausted. High-grade siliceous ore could be mixed in limited quantity with the self-fluxing ores; and oxidized ores were leached to contribute substantially to the production of the district. But

Jan 1, 1957

By Joseph Kuti

ROOF SUPPORT DEVELOPMENTS IN LONGWALL (1) European longwall mining technology developed towards its modern form in relatively deep mines. With increasing depth the stability of openings driven for ventilation, travel or transport becomes paramount in mine design considerations. As a result the number of roadways and entries, as well as their width must be a minimum. (1) Another important aspect of European mining is the high density of mineable seams, as opposed to the wide lateral extension of a single seam in the U. S. A. Multi-seam mining requires strict planning and near 100% extraction, since with increasing depth, the effect of remnant pillars can be devastating. As a result the face layout of a modern European longwall panel is based on single panel entries of moderate width and great height. The mode of extraction is generally a mixture of the advance and retreat system which is called a 2-system. It is obvious, therefore, that the supports developed for the entries and for the intersection areas are more complex than those in the U. S. A.

Jan 1, 1981

By Philip B. Dettmer

Up to now we have spoken optimistically of the many potential savings in capital and operating costs to be obtained from the selection of larger diameter and horsepower grinding mills. Such mills may well be the correct choice for one grinding plant but could be absolutely wrong for another plant. It is emphasized here that many significant factors other than the capital cost, labor, grinding media and power savings are involved in the sizing and selection of the optimum grinding circuit. The following discussion covers some of these factors and their relationship to larger mill design. The first consideration is: What is the grinding circuit supposed to do? In most instances it is one or both of the following: 1. Grind the feed material fine enough to liberate the desired minerals but with a minimum of over- grinding. Examples are preparation of ore for flotation, gravity separation, etc. 2. Grind to some pre-specified particle size or particle surface area. Examples of this are cement manufacture or the preparation of feed material for pelletizing; other examples are preparation for chemical reactions or ceramic manufacture.

Jan 5, 1965

By Louis A. Panek

Ground movements in the zone adjoining an active cave were measured at four sites in the San Manuel Mine. Measurements were made to detect extension and inclination, basic components of displacement, which can readily be duplicated by mine personnel using relatively simple equipment. The displacements demonstrated the development of a zone of strain relief surrounding the active cave, within which the ground tends to migrate toward the cave, and extension cracks develop tangential to the cave. This expansion of the ground surrounding an active cave can be exploited to promote future caving; it also creates special problems with respect to the design of an effective ground support system for mine openings in the production area.

Jan 1, 1981

By John W. Weeton, Max Quatinetz, Robert J. Schafer

Nickel powders of 1-, 0.4-, and 0.2- average particle size were combined with 0.05- p MgO powder. Oxide was added in quantities of 4, 12, and 20 vol pct. The mixtures were vacuum hot pressed and extruded at 2100°F. In spite of a general tendency to growth of oxide particles during processing, dispersions of fineness comparable to those of Sintered Aluminum Powder, SAP, were achieved. A small amount of oxide considerably improved 1800°F stress-rupture strength, but further oxide addition resulted in a decrease of strength. RECENTLY there has been much speculation concerning the requisites for strong and stable dispersion-strengthened alloys. The nature of the dispersed phase, the stability of the alloys, and variables influencing the mechanical properties of alloys have been given consideration. Such topics have been reviewed by Bunshah and Goetzel. Of the many parameters related to the mechanical properties of dispersion alloys, mean free path between dispersed particles (a measure of the fineness of the dispersion), particle size, and vol pct of the dispersed phase have been shown to be very important. Generally, strength increases with decreasing mean free path, and there is an optimum vol pct of dispersed phase. Just how fine a dispersion ought to be is somewhat open to debate, but Irmann, Lenel, Backensto, and Rose, Gregory and Grant,4 and others have shown in the original SAP-type alloys, aluminum plus aluminum oxide alloys, that better strength is generally obtained with finer dispersions and the best strengths were obtained when the mean free path was less than . Further, Cremens, and rant' showed that in nickel plus aluminum oxide alloys, stress-rupture strength at 1500°F was definitely increasing with decreasing mean free path. In their investigation, however, the finest dispersions that were produced were about 2.0 mean free path. To determine the potential of the nickel plus oxide type of product, it would be neces- sary to obtain dispersions analogous to those of SAP alloys; namely, ones where the mean free path between oxides is less than . Unfortunately, however, it is very difficult to produce extremely fine dispersions of oxides in metals other than those which naturally form refractory oxide skins. There are several approaches to this problem including such procedures as mechanical mixing, internal oxidation, reduction of mixed oxides, and metal deposition from solution. Work at the Lewis Research Center has shown that metal powders 0.1p or finer could be produced by ball milling, when selected grinding aids were utilized.' Because these unusually fine powders were available, it was an initial objective of this study to determine if by mixing of these fine Ni powders with various amounts of MgO (0.05) a fine dispersion of MgO in Ni comparable to those in Al-Al203 might be achieved. A second objective was then to attempt to relate the parameters of structure to 1800°F stress-rupture strength of the alloys. Nickel powders of 1-, 0.4-, and 0.2-p average particle size were combined with 0.05- average particle size MgO powder. This oxide was added in quantities of 4, 12, and 20 vol pct. The mixtures were hot pressed and extruded at 2100°F.

Jan 1, 1962

By Donald L. Katz

The equilibrium constants developed by Souders, Selheimer and Brown1' have been very useful tools in predicting vapor-liquid equilibria during the past five years. Their equilibrium constants were computed by assuming ideal solutions and recognizing the deviations of vapors from ideal gas laws. The same fundamental concepts were used by Lewis and co-workers8 for predicting vaporization phenomena by means of fugacities. The equilibrium constant is defined by equation 1: y\x =K [1] in which K = equilibrium constant, y = mol fraction of a constituent in the vapor phase, x = mol fraction of the constituent in the equilibrium liquid phase. The equilibrium constants are a function of temperature and pressure and might be considered as replacing the ratio of the vapor pressure to the total pressure of Raoult's law. They may be used to compute the vapor or saturation pressure of a liquid or the dew point of a vapor for any pressure or temperature from the analysis of the mixture much as Raoult's law is used. Likewise, the quantity of a mixture that will vaporize at any equilibrium temperature and pressure and the composition of the vapor and liquid may be calculated. The purpose of this paper is to review the available equilibrium data of interest in petroleum production, to discuss the auxiliary physical properties of the hydrocarbons, and to present typical uses of the equilibrium constants. The particular applications chosen are the computation of the results of stage separation of crude oil and natural gas, the calcula: tion of a pressure-volume curve of a crude oil-natural gas mixture, and information gleaned from a consideration of reservoir equilibria.

Jan 1, 1938

By Andrew Button

INTRODUCTION This review deals with the controls exerted by unconformities on the stratigraphic and areal distribution of gold (and uranium) in Precambrian conglomerates. The relations described are drawn mainly from Precambrian basins in southern Africa, particularly the Witwatersrand Basin. The mining industry of South Africa started in earnest with the discovery of diamonds in 1866. Gold was first discovered in 1870, and in March of 1886, two out-of-luck prospectors stumbled across what was to become the world's greatest goldfield, in the vicinity of Johannesburg. George Harrison and George Walker sold their claims for a pittance and were heard of no more. The Witwatersrand goldfield has been in continuous operation since that time, some 150 mines having operated at one time or another. Deep mining technology developed here has allowed operations to proceed to depths approaching 13,000 feet. The Witwatersrand has produced some 55% of all the gold ever mined, and currently produces 60% of the world's supply, some 650,000 kg Au per year. To 1971, the Witwatersrand had produced some $32 billion of gold and $2 billion of uranium (when average prices were less than $35/oz. for Au and $10/lb. for U308). At present prices, the annual Witwatersrand production of close to 700,000 kg would be worth about $10 billion. Uranium was discovered in the Witwatersrand in 1923. In the $15-30/lb. bracket, the basin contains a reasonably assured plus

Jan 1, 1983

By A. J. Garnier, N. H. van Lingen

Rock downhole is known to be lesc. drillable than when brought to the surface. This must be ascribed mainly to the presence under downhole conditions of a pressure differential across already made chips, which hinders their being lifted. The pressure differential has partly a static and partly a dynamic origin. Balling-up of bits is another consequence of this pressure differerztial. Reduction in penetration rate owing to an increase in the strength of the rock is governed by the difference between the mud pressure and the pressure of the for/nation pore liquid. Rotationally symmetric geostatic stresses have 170 effect on drillability. Fracture of rock when drilling will be brille in most cases. The above is supported by laboratory drilling experiments with drag bill and roller bits an elevated mud, pore, and confining pressures On rocks differing in strength and permeability. INTRODUCTION In oil well drilling drillability of rock is found to decrease with increasing depth of the hole. Naturally deep rock will be more compacted and, therefore, harder to drill than shallow rock of the same type. However, apart from this the drillability of a sample of deep and compacted rock brought to the surface is generally much higher than in its original location downhole. In view of the economic implications of this reduction in drillability, it seems worthwhile to analyze its causes. The origin clearly has to be sought in the difference of environment. The only conceivable factors would seem to be the presence of mud under pressure, the pressure of the formation pore liquid, and the overburden of the rock. Down the borehole the rock is compressed triaxially by mud pressure and overburden. It is well known that the strength of rock is increased when confined by external pressure1. Various authors have, therefore, ascribed the difference in drillability mainly to the strengthening of rock by triaxial compression. Another factor, mentioned by Bobo and Hoch5, is that forces. including "pressure differential forces", tend to hold a dislodged particle in place. However, the conditions determining their magnitude are not clarified nor is their effect on drilling rate assessed. In the laboratory, drilling experiments on pressurized samples of rock yielded evidence that pressure differential forces holding the chips down are the major factor in reducing rate at depth. This paper describes the experiments and shows how the results enable both a qualitative and a quantitative interpretation of the factors determining the effect of chip hold-down on drilling rate. The implication with respect to balling-up and jet action is also discussed. The greatcr part of the experiments were performed in the pressure vessel shown in Fig. 1. The spacc between the sample of rock and the vesscl is divided by "0" rings into three separate chambers. The rock sample is confined laterally by pressurized oil in thc middle chamber. Penetration of oil into the sample is prevented by partly jacketing it with brass foil. Thc pressure of the drilling fluid in the hole can be adjusted via the upper chamber. Permeable rock specimens arc water saturated before being drilled. With a properly plastering mud as drilling fluid the pressure of the pore water can be adjusted independently via the lower chamber.

By Earl S. Greiner, Bradley Stoughton

As a part of the systematic study of the alloys of iron under the auspices of the Engineering Foundation, and preliminary to the commencement of comprehensive work on this larger subject, the authors have undertaken an investigation of the available literature dealing with the alloys of iron and silicon. The purpose is to collect all the data in English and foreign languages and present them in two monographs, one of a more scientific and the second of a more industrially useful nature. Of course, a great deal of information upon the constitution of the iron-silicon alloys was found specificially upon the changes of phase in the equilibrium diagram and the different phases present, such as solid solution, chemical compounds (iron silicides), etc. While no extended research was contemplated in the particular study undertaken, nevertheless the data collected proved to be so contradictory in many respects that a simple study of some of the outstanding problems involved in the constitution and equilibrium diagram was undertaken. Because research was not a primary object of the work, and also because of the extreme industrial importance of the silicon alloys between 0.80 and 14 per cent. silicon, and finally because of the unsatisfactory state of knowledge existing as to these alloys, the experimental investigation was limited entirely to alloys with less than 10 per cent. silicon. Discussion oF Continuous Series It has often been said in print that the iron-silicon alloys form a continuous series of solid solutions from 0 to 15 per cent. silicon. This claim the authors are inclined to doubt: first, because the alloys from 0 to 4.25 per cent. silicon are ductile and malleable at atmospheric temperature, while those with 5 per cent. or more of silicon are comparatively hard and brittle. Furthermore, the alloys with more than 4.25 per cent. silicon are comparatively hard and low in ductility at atmospheric temperature, but become much more ductile when heated slightly. It is well known, for example, that the alloy containing 4.75

Jan 1, 1930

By Tran K. Van, Francois E. Heuzé, Richard E. Goodman

Sound design of structures founded in or upon rock requires that the deformability of each rock member involved be characterized. This can be done by means of loading tests (plate bearing, flat jacks, borehole jacks, pressure chambers, etc.) or through convergence measurements (tunnel convergence, roof sag, etc.) in exploratory works. Analysis of data will be qualitative (degree of nonlinearity, creep behavior, scattering of results over a number of sites, etc.) and quantitative (modulus of elasticity or deformation). Some of these tests involve a very bulky apparatus and are expensive to perform. On the other hand, for the purpose of mapping and describing the various rock members which occur within a given site, many feet of diamond drill holes are frequently completed. Even with the optimum equipment and expert drillers, however, quantitative characterization of the rock units on the basis of tests on returned samples is apt to be misleading; the softer and weaker components of the rock tend to be lost and the fabric of the rock block- fracture system in situ is not sampled. Laboratory moduli of elasticity have proved to be consistently higher than in-situ values selected for engineering purposes. This led to the design of instruments which would operate in boreholes, combining the advantages of a reduced size and a deeper investigation, from the surface to several tens of feet. BOREHOLE DEFORMABILITY MEASURING INSTRUMENTS In the last 10 to 15 years, a number of testing devices have been con- structed which can be inserted into a borehole to apply a load and mea-

Jan 1, 1972

By Ben Gumpertz

THE important factors in any study of the screening of sand with a gravel enve-lope, as applied to use in oil wells, are: (r) sand size and shape, (2) gravel size and shape, (3) ratio of gravel size to sand size, (4) flow velocity, (5) viscosity of the fluid. Perhaps the first extensive investigation of the function of gravel in screening sand was performed by Coberly and Wagner,1 from which the following conclusion was drawn: "The diameter of the largest grains in a single classified sand suitable for gravel packing is approximately 23 times the grain size at the ten-percentile analysis of the formation sample." This applies to sands classified by Tyler screens and is for gravel grains of the maximum size that could be used to form a stable bridge. Field practice has shown that the screening effect of a given gravel for a given sand apparently varies with the flow velocity.2 Also, it has been assumed by some opera-tors that the thickness of the walls of the gravel envelope surrounding the well has little effect upon the screening process. It is thought by others that a narrow envelope of gravel might form an ineffec-tive screen. Thus the purposes of this work are: I. To investigate the effect of gravel-sand size ratio upon screening action under the condition of varying flow velocities. 2. To investigate the effect upon screen-ing action of varying the thickness of the gravel screen.

Jan 1, 1940

By Nelson Severinghaus

The Rock Chapel plant of Consolidated Quarries Corporation (Fig. 1) is three miles northeast of Lithonia, DeKalb County, Georgia. It was opened about eight years ago for crushed stone aggregate. This paper reviews a number of experiments that have been tried in drilling and blasting, and gives the practice in use today. It also presents several theories on the action of explosives in this type of blasting. The rock worked is a bare dome of granite gneiss about 3000 ft. in diameter, rising 150 ft. above the surrounding ground (Fig. 2). It is uniform in composition and quality, without seams, and extends downward to unknown depth below the quarry floor. A water well drilled 600 ft. remained in the same rock. The principal difficulty encountered in blasting is an easy split, which tends to break down the rock in large blocks.

Jan 1, 1938

By Nelson Severinghaus

The Rock Chapel plant of Consolidated Quarries Corporation (Fig. 1) is three miles northeast of Lithonia, DeKalb County, Georgia. It was opened about eight years ago for crushed stone aggregate. This paper reviews a number of experiments that have been tried in drilling and blasting, and gives the practice in use today. It also presents several theories on the action of explosives in this type of blasting. The rock worked is a bare dome of granite gneiss about 3000 ft. in diameter, rising 150 ft. above the surrounding ground (Fig. 2). It is uniform in composition and quality, without seams, and extends downward to unknown depth below the quarry floor. A water well drilled 600 ft. remained in the same rock. The principal difficulty encountered in blasting is an easy split, which tends to break down the rock in large blocks.

Jan 1, 1938

By Immo H. Redeker

Through close cooperation with geophysical and geological test work by Texas Gulf Sulphur Co. in Beaufort County, the Asheville Minerals Research Laboratory assisted in the evaluation of North Carolina's most valuable mineral resource, the Pungo River phosphate deposit. After a core evaluation study, the flotation process for the concentration of North Carolina phosphate was demonstrated in a 500-lb per hr pilot plant in Asheville. Tonnage samples of phosphate concentrate were produced for customer evaluation in the United States, Europe, and the Far East. Plans for a larger pilot plant to be erected at the plant site at Lee Creek were prepared for TGS. The larger pilot plant was operated by TGS in conjunction with a test dredging operation which led to the adoption of a dragline dry mining method to move 90,000 tpd of material for the production of 10,000 tpd of phosphate concentrate in the all-flotation phosphate concentrator. Calcining, dry grinding, and byproduct studies complimented the construction of one of the world's largest and most modern fertilizer raw-feed complexes on the eastern North Carolina seaboard.

Jan 1, 1972

By William E. Messner

California’s Monterey Peninsula, noted for its beaches, resort hotels, and beautiful homes, also harbors a pioneering beneficiation operation. Here for more than 35 years the Del Monte Properties Co. has provided sand products from large dunes, some over 300 ft high, located in the Del Monte Forest. Raw material-the dunes derived by wave action from Santa Lucia granite of Jurassic age-has unusually consistent chemical and physical composition. Coarse crystalline feldspar is the principal constituent, followed in abundance by quartz, with some biotite, ilmenite, garnet, zircon, and monazite.

Jan 2, 1955

By G. Baralis, P. Gondi, I. Tangerini, G. Scandola

The precipitation behavior of Zn-0.5 pct A1 alloy single crystals was studied by means of electrical resistivity measurements and by optical and electron microscopy. The single crystals for the resistivity measurements were prepared by an original method in - 100-p -thick sheets. The order of the precipitation kinetics ranged between 1 and 1.5. The dislocations play a relevant role in the first-order kinetics. Precipitation always occurs both on dispersed particles and on dislocations. Statistical examinations have shown that the first-order kinetics can have two different activation energies; i.e., the precipitation can have dz;fferent mechanisnrs which could not be identified, however, in the course of the research. During the tnetallographic exanzination of the precipitation structures a specific process of dislocation decoration was obsereed. The main purpose of this work was to study the contribution of dislocations to the precipitation. A number of authors have observed precipitation on dislocations and reference might be made to several monographs on the ubject.'' The possibility that dislocations also accelerate precipitation has been considered by Turn-bull3 and Fischer et al.4 The studies described in the present paper were carried out on zinc, chosen as a base metal owing to the ease with which dislocations can be introduced into it and because of the absence of excess vacancies after quenching in conditions where phenomena of accelerated precipitation still occur. Aluminum was preferred as alloying element because of the accelerated precipitation phenomena that resulted in a preliminary reearch. EXPERIMENTAL METHODS The observations refer to a Zn-0.5 pct A1 alloy. The zinc was 99.995 pct pure; a typical spectroscopical analysis is given in Table I. As a rule the alloy was subjected to homogenization, quenching, or slow cooling and annealing. Homogenization was carried out by heating at 390" to 410°C for 24 hr. From the homogenization temperature, some specimens were quenched and some slowly cooled at a rate of 2°C per sec. At this rate no precipitate was detectable under the optical microscope just after cooling. Quenching was carried out simply by dropping the specimens into water, aqueous ethylene glycol solution at -30" c, or liquid-nitrogen baths placed close to the homogenization oven. Vaseline oil baths were used with a thermal stabilization of 10-20 for both the aging treatments and the measurements; aging was generally carried out at 90" or 130°C. To avoid oxidation phenomena during heating, the vaseline oil baths had to be frequently renewed. The precipitation kinetics were studied by means of electrical resistivity measurements, using ans potentiometric method (reproducibility ± 5 x 10 5 v, that is 0.5 pct of the total voltage decreases on the specimens during precipitation). First, various types of specimens were tested, i.e., polycrystals, single crystals grown in capillary quartz tubes, and thin single-crystal sheets prepared by means of an original method requiring no container except for the natural oxide. Even if fully annealed, the polycrystals and the capillary grown single crystals showed resistivity in -creases, most probably due to dislocations introduced in the course of the measurements. Similar resistivity increases in pure zinc were noticed by another author. Only the single-crystal sheets showed no resistivity change; thus they were chosen for the subsequent tests. As already mentioned, these single crystals were obtained by using, as a container, the natural oxide on the zinc surface; the oxide strength is sufficient to maintain the original shape during melting with sheets up to 500 p thick. An initial zone melting and subsequent zone leveling, which led also to formation of the single crystals, were thus carried out on rolled sheets of the required thicknesses (- 100 p) and shape, lying on a flat silica surface. The resistivities were first evaluated by measurements at the liquid-nitrogen temperature. This method gave poor reproducibility, however, and this was attributed to the thermal cycles which had to be operated. To avoid cycles and handling, it was therefore decided to make measurements directly in the annealing oil baths; this required thermal stabilization at ilo-' "C. In this way only the resistance changes were measured. Specimens of pure zinc and of completely annealed alloy were always examined as controls together with those under consideration; only those measurement runs were taken into account where the reference samples showed no resistance increases. Again, the main inconvenience was due to oxidation and this was avoided by renewing the oil baths; even so data reproducibility was poor and the observations were therefore carried out on a large number (many hundreds) of specimens so as to provide indications of statistical value. For the transmission observations under the elec-

Jan 1, 1969