Search Documents

By Thomas C. Wilder, Walter E. Galin

Measurements of the electromotive force of the cell at 995°C have shown that the cell may be used to detennine the zinc content of molten Cu-Zn alloys to the nearest 0.05 wt pct. The cell is used for brass melts to which no ZnO is added intentionally, because essentially all oxygen in the Cu-Zn-O system is present in the form of ZnO. The cell is also used to detev,nine the thernodynamic activity of zinc for Cu-Zn al1oys of the brass composition range, from which the equilibrium partial pressure of zinc for such alloy may also be calculated. The electrochemical measurement of the concentration of the most active metal in other engneering- alloy systems by a similar technique is also considered. IN recent years there have been many studies concerning the direct electrochemical measurement of oxygen content in molten engineering metals.' These investigations employ galvanic cells in which the electrolytes are various solid mixed-oxides which conduct current by movement of oxide ion vacancies under the influence of an oxygen potential gradient. Similar cells have also been used to measure the thermodynamic properties of mixing of solid alloys,' one component of which has a much greater affinity for oxygen than the other component(s). One alloy system of obvious engineering importance comprised of two metals of greatly differing affinity for oxygen is copper-zinc. At the casting temperature of molten brasses, the zinc concentration is very difficult to control not only because its vapor pressure is very high, but also because of its high affinity for oxygen to form ZnO. Inasmuch as brass castings may be required to have a zinc concentration within very narrow limits, it would be advantageous for the brass industry to have a means for quickly measuring the zinc content in the molten alloy just prior to pouring. A calculation based on earlier thermodynamic studies of oxygen and zinc in dilute solution in binary Cu-O and CU-Z' alloys indicates that the oxygen concentration of molten 70 wt pct Cu-30 wt pct Zn brass must be much less than 1 ppm for the separate phase ZnO not to exist in the ternary Cu-Zn-O system. Thus it can be assumed that all molten brasses, both in the laboratory and in the foundry, are sufficiently saturated with oxygen for the separate phase ZnO to be present. In view of the foregoing reasons, the cell investigated at 995°C for the purpose of measuring the zinc concentration in molten copper-zinc alloys, particularly those of brass compositions. In this cell the left hand electrode is a reference electrode comprised of a mixture of the pure powders of nickel and nickel oxide, to which a platinum contact is made. The electrode of interest is a molten Cu-Zn alloy of unknown zinc concentration with a tantalum wire contact. The electrolyte is the commercially available (Zircoa) calcia stabilized zirconia. The ZnO in the Cu-Zn electrode compartment is not intentionally added, but is naturally present as a result of the infinitesimal amount of oxygen required for its formation in this alloy system. In cell [A] nickel oxide is reduced at the cathode where O= represents the electrolyte. At the anode dissolved zinc of the Cu-Zn alloy is oxidized to form ZnO(s) in its standard state Thus the overall reaction is for which the molar free energy change is where 5 is the Faraday equivalent (23,063 cal per v equivalent), is the electromotive force of the cell in volts after correction for the Pt-Ta thermocouple, FOf (ZO) and AFF (NO) are the standard molar free energies of formation of the respective oxides, and a2n is the thermodynamic activity of zinc in the molten Cu-Zn alloy. The reference state of zinc in this case is taken to be the pure liquid. At constant temperature all the terms of Eq. [I] are constant except and azn Thus since all other participants in the electrochemical reaction are in their standard states, the change in the electromotive force of Cell [A] is represented only by a change of the thermodynamic activity of zinc at constant temperature. The zinc concentration is related to the activity by sampling the alloy for chemical analysis after cell measurements are taken. EXPERIMENTAL A typical cell construction is shown in Fig. 1. This and other experimental details were similar to that described earlier with the exception of some minor modifications which are detailed below.

Jan 1, 1970

By Myers, J. F.

The purpose of the mill tests reported herein, was to determine the relative power efficiency of fast and slow rod mill speeds on the ores of the Tennessee Copper Co. The tests were carried out at the lsabella Mill of the Tennessee Copper Co. under normal operating conditions. Every effort was made to keep the ore supply and other conditions constant, but it will be noted that some slight variation did occur. The open circuit 6 by 9 rod mill, and ore, used in these tests have been previously described.' Four motor drive pinions were available and the tests were performed in the following manner: each pinion was used to drive the mill for a period of one week. Constant sampling prevailed during the test run, and a composite sample for each speed test was made. Realizing that some slight liner wear took place during the four-week test-run, and that some slight variation of the ore may have taken place, a second and then a third series of four-week test- runs were made with the four different pinions. The screen analysis of each of the thee-week run samples at the various speeds were composited and the results are shown in Table 1. The authors feel that the 12 weeks of operation consumed during these test-runs would represent fairly the effect of each mill speed. The data of line 37 show that within the error of sampling and plant operation, the tons of minus 65-mesh material generated per horsepower hour was the same at all four speeds.

Jan 1, 1949

By G. P. Conard, E. H. Rennhack

The creep behavior of hot-rolled, hypoeutectic Zn-Ti alloys was investigated in the temperature range from 0.43 to 0.53 TM. Secondary flow was found to originate primarily from strain-induced gvain growth where grain boundary )nigvation served to relieve the strain energy of distortion introduced by slip, grain boundary sliding, and subgvain formation. The extent to which this recovery mechanism operated was determined by the ratio of grain width to the spacing between planar fibers of TiZn,, compound particles generated in these alloys during rolling. When this ratio was unity, creep resistance demonstrated a marked improvement. In this condition, which was fulfilled by annealing following rolling, structural stability was enhanced with decreasing grain size below the equicohesive temperature (-0.5Tm), while the reverse was true above this temperature. TITANIUM concentrations approaching the eutectic composition of 0.23 wt pctl have been shown to promote a significant increase in the creep resistance of rolled zinc,2 The alloying effect created with titanium is somewhat unique; a structure closely resembling that of a fiber-reinforced metal composite can be developed which selectively modifies creep strength in preference to other mechanical properties. In an earlier investigation,~ the present authors found that, while the fiber network, composed of individual TiZn,, compound particles, had a distinct influence on rolled texture, the crystallographic variations produced were of minor importance with respect to creep. Rather, creep resistance seemingly increased when the grain size appeared to coincide with the in-terfiber spacing. The work described here was undertaken to explore this effect in greater detail. EXPERIMENTAL PROCEDURE Three zinc-base alloys containing 0.05, 0.12, and 0.16 wt pct Ti were prepared from CP zinc and iodide titanium in the form of 4 by 2 by f in. chill-cast ingots. The melting and casting procedures for these alloys have been detailed el~ewhere.~ Individual ingots of each alloy were hot-rolled at 200°C (392°F) to total reductions of 10, 25, 50, 75, and 90 pct in from one to five passes, respectively, employing a 10-min reheat prior to each rolling pass. With grain, tensile-type creep specimens with a 1-in.-long, -in.-wide gage section were machined from the rolled strips for test purposes. Annealing studies to explore the influence of grain size on secondary creep flow were carried out at 400°C (752°F) in argon for times extending up to 60 min. The grain-size effect was evaluated in terms of average grain width and length values statistically derived from lineal intersection measurements.4 A similar method was applied in establishing the average interfiber spacing, i.e., average perpendicular distance between adjacent planar fibers. The creep characteristics of the alloys were investigated by means of constant-load and constant-stress creep tests. The former tests were conducted at 25°C (77°F) under an initial stress of 10,000 psi, while the latter were performed in the range from 25°C (77°F) to 90°C (194°F) at stress levels varying from 8000 to 22,000 psi. Total specimen strain, as determined with Budd HE-1161-B strain gages, was in excess of 0.10. Maintenance of constant stress was achieved through periodic load reductions made at 0.01 strain intervals to compensate for the attendant incremental reduction in specimen cross-sectional area. The maximum indicated error in the applied stress at these strain intervals was less than 3.0 pct. RESULTS AND DISCUSSION Constant-Load Creep. In an effort to clarify the in-terrelation between interfiber spacing and grain size with respect to the creep resistance of the Zn-Ti alloys, their separate effects on secondary creep rate were determined as a function of titanium content and rolling reduction. These results are set forth in Figs. 1 and 2, respectively. The average grain diameter plotted in Fig. 2 was resolved from average grain width and length values. No data are presented for reductions of less than 50 pct because of the inability to obtain consistent measurements on these strips. The curves of Fig. 1 indicated that, for a given titanium content, a decrease in interfiber spacing, as produced with increasing reduction, promoted a decrease in creep rate. Depending on titanium content, however, wide variations in creep rate occurred at the same interfiber spacing suggesting that interfiber spacing, by itself, has little or no influence on creep resistance. Grain size, on the other hand, decreased progressively with both increasing rolling reduction and titanium content, the effect of which led to a pronounced decrease in creep rate, particularly when the average grain diameter became smaller than 3.0 x 10"4 in., Fig. 2. The continuity of this relationship tended to support the view that grain size rather than interfiber spacing was predominant in controlling secondary creep. Annealing Effect. The observed dependence of creep flow on grain size suggested that a further contribution to creep resistance would result when the alloys were annealed to effect a coincidence between grain width and interfiber spacing, see Fig. 3(b). ~eiides creating an immediate barrier to grain boundary movement, annealing offered the possibility of providing increased structural stability by eliminating many high-energy, mobile grain boundaries.= To test this hypothesis, specimens from the Zn-0.16 Ti strips reduced 75 and

Jan 1, 1967

By David R. Mitchell, B. M. Bird

DENSE-MEDIA processes utilize the familiar laboratory float-and-sink procedure on a commercial scale. Just as wood chips float on water and sand sinks, so coal floats and refuse sinks when placed in a medium intermediate in specific gravity between the coal and refuse. Liquids intermediate in specific gravity between coal and refuse used in commercial plants are of four types: (1) organic liquids of high specific gravity-halogenated hydrocarbons; (2) dissolved salts in water; (3) pseudo liquids obtained by suspending solids, such as sand or magnetite, in water, and, (4) fluidizing a mass of sand or other solids by air currents. Those most widely used for cleaning coal are solutions of calcium chloride in water and a suspended sand or magnetite pseudo liquid. Laboratory float-and-sink testing is an intermittent operation during which every opportunity is given for particles lighter than the medium to float and particles heavier to sink. Commercial processes are continuous; vertical and horizontal currents of the separating medium occur as either part of the process or unavoidedly produced by the introduction of feed and withdrawal of products, 'so that the exact separation of the laboratory technique is not duplicated by commercial processes. However, separations close to the theoretical are obtained in commercial machines by careful technical control of the process. Most machines in commercial installations are crowded to get as much out of the equipment as possible, so that careful technical control is necessary is get the recoveries and grade of product desired. Sizes usually cleaned are plus ¼ in. or plus 1/8 -in. in the United States and plus 1/16-in. in Great Britain. Coal as small as 1 mm. has been cleaned in some experimental installations. The separation of coal from refuse in sizes finer than about 1/8-in. is made difficult by viscosity effects and hence at the present time cleaning is confined to the coarser sizes. Technical advances in the art of cleaning coal by dense-media processes is progressing rapidly and it is likely that equipment, media, and procedures will be developed to clean sizes down to 0.5 mm. efficiently and at low cost.

Jan 1, 1950

By W. van der Knapp

A theoretical and experimental ana1ysis is given of the change in volume of a porous medium due to changes in external and internal pressures. The result enable one to deduce directly the effect of large increlents in stresses. It is shown that the three-dimensional representation of the volume of a nonlinear porous system is a cylindrical rules surface, the generating lines of Which have a slope according to the solid rock compressibility. Analytical expression for such a ,surface is very simple. It follows that the bulk and pore compressihilities depend only on the difference between pore fluid and external hydrostatic pressure, i.e., the effective rock frame pressure. From measurements on a number of sandstones as well as limestones it can be conduded that for practical values of this effective frame pressure the pore compressibility falls in the range between the compressibility of water and that of undersaturated crude oil. In general, pore compressibility is found to be higher the lower the porosity. For a particular limestone reservoir pore compressibility and porosity could he related by means of a simple empirical formula. Results from the described study find a direct applicxtion in material balance calculations and in prob1ems of liquid flow through porous media. Since the elastic constants of the rock bulk material enter into equatiorls for velocity of acoustic waves, the results are also of importance for calculating rates of wave propagation in these media. It is shown for sandstones with 1.5 to -10 prr cent porosity that laboratory measured porosities may differ from those lender reservoir conditions by about I per cent of their value. The difference for low porosity limestotnes can be of the order of 10 per cent. The subsidence caused by elastic deformation of reservoir rock is shown to he very small. General formulas relating pore and bulk volume to external and internal pressure can be derived, as has been shown by Biot1, Gassmann2 and Geertsma.2 Their theories describe the differential change in volume caused by a variation in pore fluid tension and external hydrostatic tension. It has been known for a long timei that the stress-strain relations of a packing of spheres are essentially noninear. Our experiments show that for almost all reservoir rocks which arc elastically deformable the relation between volume and loading is also a nonlinear one, resembling that of a packing of spheres very closely. The behavior of these media is often described by using the linear stress-strain relations for small incremental stresses and strains in the vicinity of a pre-stressed condition." The present study differs from these considerations in that it enables one to deduce the effect of large increments in the stresses directly. In analogy to the stress tensor for a homogeneous body the stress tensor for a porous body can be written in the conventional way: where the tensor components, o, and t, represent the total forces acting on the faces of a unit cube of porous material. This tensor can be split into two parts. a hydrostatic and a deviatoric part, as follows2,3,1 where the component of the hydrostatic stress system, a. equals The hydrostatic stress causes a change in volume whereas the deviatoric stress system gives rise to a change in shape only. One of the difficulties in describing changes of volume occurring in reservoir rocks is that the relations connecting these volume changes with external and internal stresses are essentially non-linear. A complete description requires the bulk volume, V,,, as well as the pore volume,V,,. to be known as a function of both the changes in pore fluid tension (v) and the hydrostatic tension on the outer boundary.

By Frederic E. Carter

There have been many attempts to prove that platinum was known to the ancients, but since no traces of the metal have been found in the relics of early times, it must be concluded that it was not known to them. As far as records go, it was actually first discovered in the 16th century in a Spanish mine on the Pinto River in South America; from which source it received its name of Platina del Pinto (little silver from the Pinto). It was more or less of a rarity and was a subject of interest to a few chemists only, until the year 1822 when the metal was first discovered in Russia. In 1819 there were noticed in the washings of the gold sands of the southern part of the Ural Mountains, grains of a heavy white substance, but it was not until three years later that this substance was found to contain platinum. While at first the amounts of platinum brought from the Urals were meager, prospecting soon disclosed rich deposits extending over a considerable area, and nuggets from 10 to 20 lb. were by no means rare. This discovery assured to chemists an ample supply of platinum for their crucibles, etc. The ore is now found in various parts of the world but the richest deposits are still found in the Ural Mountains. The platinum of commerce is obtained entirely from alluvial deposits at depths varying from 6 to 40 ft. The sand and gravel of former river beds are subjected to a series of screenings and washings to effect the separation of the ore. Actually, probably all gold-bearing gravel beds of the world contain platinum, and it is more than likely that large quantities of platinum ore have been thrown away with the black sand washings from gold placer deposits. This is especially true of the alluvial deposits of the Pacific Coast in California and Oregon. Platinum ore is usually found in the form of rounded or flattened grains of "sand," and occasionally in irregular lumps of the size of peas. Large nuggets are very rare. The ore has a metallic luster and is distinguished by its steel gray color, but shows light gray streak when rubbed on a hard white surface; it has a high specific gravity, 16 to 19; is infusible in the hottest blast furnaces, and insoluble in any single acid. Platinum ore is a complex body consisting of the metal in combination with varying proportions of the other members of the platinum group; iridium,

By Jerome F. Kuzmick

Many investigations have been made on the effect of the variables of pressure, sintering temperature and sintering time on the physical and microstructural charactcristics of metal-powder compacts. Severa1 such investigations have indicatcd that pressure is the least important of these three factors in an attempt to secure a dense. strong compact by a simple coltl-pressing and sintering procedure. However, it was felt that a further study of the effect of pressure on pressed and sintered metal powder compacts might prove interesting, for the following reasons: In work done in the past on the effect of pressure, temperature. arid time, the, metal-powder compacts were subjected to long sintering periods, and temperatures right up to the melting point of the metal were applied. It was thought that pressure might exert a more profound effect under short-time simtering and within narrower ranges of temperature. such as normally would be applied in large-scale production of metal-powder parts. 2. It was hoped that something further might be learned of the nature of the bond in pressed and sintered metal-powder compacts. 3. It was thought that interesting infor mation concerning the work-hardening effect of pressure on the crystal structure. of sintered metal-powder compacts would be obtained. Procedure Rectangular bars, 2 by 1/2 by 1/4 in. were molded from the following metal powders: Atomizcd till powder, 325-mesh Atomized aluminum powder, 325-mesh Reduced copper powder, 100-mesh Electrolytic iron powder, 100-mesh Atomized 70-30 brass powder, l00-mesh The, till powder was molded at various pressures up to 40 tolls per sq. in. This was the maximum, because at higher molding pressures there was considerable extrusion of tin into the die wall, jamming the die plungers. The tin specimens were not -in tered, as this phase of the work was- done merely to see how much a very sold metal could be compressed. The aluminum powder was compressed at various pressures up to 50 lons per sq in. Higher pressures could not be used because of extrusion into the die wall The compacts were sintered one hour at 000 in pure dry hydrogen. The copper powder was molded at vari-ous pressures up to 96 tons per sq. in. One set was sintered at 750°C., another at 050 C. for one hour in pure dry hydrogen. The electrolytic iron powder was molded at various pressures up to 00 tons per sq. in. and sintered, one set at Soo°C and one set at 1300°C for one hour in pure dry hydrogen. Extra 325-mesh specimens were also prepared to study the effect of reduced particle size on grain structure. The brass powder was molded at various pressures up to 96 tolls per sq. in. and silltered for one hour. one set at 65o°C. and one set at 800°C, in pure dry hydrogeu.

Jan 1, 1945

By A. W. McReynolds

One characteristic of plastic deformation which distinguishes it from elastic strain is the essential inhomo-geneity of plastic strains. Elastic strain varies continuously through a material, and average relative displacements of initially adjacent atoms are only small fractions of their initial spacing, (strains of the order of 0.01 or less). On the other hand, plastic flow corresponds to the appearance of discontinuities in strain of the lattice, such as dislocations or slip bands, where local strain, on an atomic scale, is several orders of magnitude higher. These discontinuities are visible on a microscopic scale as the familiar slip lines (Fig 1). In spite of this obvious microscopic inhomogeneity, however, macroscopic measurements almost invariably show a smooth curve of stress vs. strain (Fig 2b) even if measurements of linear strains be made to an accuracy of one part in 107. This macroscopic homogeneity of strain indicates that the discontinuities in strain on slip planes occur in increments too small or too slow to be recorded individually, and further that they occur sufficiently independent of one another so that the small increments add at random to a smooth stress-strain curve. The present paper describes observations of plastic strain in aluminum of commercial purity and in high purity Al-Cu alloys, where there exists a strong coupling between slip in various regions of the specimen such that once initiated it spreads rapidly through a large volume. The total effect is that of relatively large, rapid, and regularly spaced steps of strain followed by periods of only elastic strain. Fig 2a illustrates the type of "stair-step" stress-strain curve which results. The properties of this cooperative slip phenomenon will be described further in the section on results: in par- ticular it will be shown that each step corresponds to the propagation of a wave of plastic deformation through the specimen. Some interpretations of the mechanism by which it occurs will be made in the following section. Although the type of plastic wave phenomena to be described has not previously been reported, there are numerous cases of related effects in the plastic yielding of metals: YIELD POINT PHENOMENA The most familiar of such effects is the "yield point" observed in low carbon steels, brass, duralurninum, and the like. It consists in the sudden termination of the elastic portion of the stress-strain curve by a large plastic strain. Since the usual tensile machine is such that yielding of the specimen relieves the load, the resulting curve is as shown in Fig 3. As the strain continues, deformation occurs at a lower stress for some time, then follows a rising curve, but with no further sudden yielding. This effect has been observed in brass by Sachs and Shojil and later by many others. Edwards, Phillips and Jones2 made extensive studies of the effect in steel, and of the role of various alloying elements. Although there seems to be fairly clear evidence that the yield point is caused by a hardening of the material by precipitation of impurities, no satisfactory explanation for the sudden yielding has been given. Winlock and Leiter3 have shown that the strain. level of the upper yield point depends strongly on the rate of loading, the yield point increasing by almost a fac- tor of two as the strain rate goes from 0.002 in. per in. per min. to 4.4 in. per in. per min. This effect would seem to imply an incubation period before yielding is initiated at a certain stress. On the other hand, by going to very slow loading rates, Edwards, Phillips and Jones2 showed that the yield point does not become lower and eventually disappear as might be expected, but, on the contrary, begins to rise at loading rates below about 25 Ib per in. per min. becoming much higher than at rapid loading rates. STRAIN AGING If, instead of continuing straining of a specimen after occurrence of a yield point, the load is removed and the specimen aged, resumption of the test results in occurrence of another yield point as shown by the dotted curve of Fig 3. The new yield stress is generally higher than the previous maximum applied stress. This hardening of the material by straining and subsequent aging is undoubtedly related to quench age-hardening resulting from the aging of a specimen quenched from high temperature. Since neither effect is observed in pure metals, it is generally accepted that quench-aging in all cases is the result of hardening by precipitation of a supersaturated alloying element, and that strain-aging is probably a similar precipitation, accelerated by disruptions of the lattice by previous strain. Pfeil4 has shown that strain-aging does not occur in iron from which all of the carbon has been removed, but that only a very small carbon content, around 0.003 pct, is necessary to cause strain-aging. In accord with this observation is recent work by Dijkstra5 in this laboratory showing that the solubility limit of carbon in iron is extremely low, less than 0.001 pct at 400°C. Edwards, Phillips and Jones2 have shown that the strain-aging effect is also removed by the addition of small quantities of elements such as Mo, Mn, Ti, and the like, which readily form carbides. Their results demonstrate the

Jan 1, 1950

By D. Kuhlmann-Wilsdorf

A new theory of work hardening is developed which rests on only a few simple principles and is applicable to a wide variety of materials and dislocation structures. It explains, qualitatively, the general characteristics of the three-stage shear stress-shear strain curve of fcc metals, as well as the most important features connected with easy glide. The value of, the work-hardening coefficient in stage 11, is derived quantitatively, from first principles, and is shown to be insensitive to many changes in detailed dislocation behavior. The low rate at which energy is stored during mechanical working in stage 11, the dependence of slip line length on stress, and the so-called Cottrell-Stokes law are explained. Although the theory is primarily developed for fcc metals and alloys, it is largely applicable also to some other materiuls, and in particular apparently to poly crystalline simple steels in theh linear range of hardening. FOR pure fcc metals and alloys, after many different pretreatments and under a great variety of testing conditions, the well-known three-stage work-hardening curve is commonly observed. Moreover, the work-hardening coefficient in stage 11, 011, bears an almost constant relationship to G, the modulus of rigidity, such that K = G/Brr" = 300, within a factor of about 2 either way. The very fact that the three-stage curve and the value of K are so very persistent, can hardly be understood except on the assumption that some quite simple phenomena are responsible. In this light all available theories of work hardening must be judged unsatisfactory: Each of them is based on some quite specific dislocation model, while it is known that metals with similar values of K may have widely different dislocation structures, for example, dislocation tangles in the case of pure fcc metals, and piled-up groups of dislocations in a-brasstype alloys. Thus, while a particular theory might conceivably represent a special case with a reasonable degree of accuracy, it cannot possibly have illuminated the underlying principle. Another severe criticism applying to the presently most widely discussed theories is that they employ empirical or even quite unexplained parameters in order to arrive at a numerical value of K. This indicates that some vital aspect of the work-hardening process has remained unexplained. In the present paper, a new theory of work-hardening is developed which does not suffer from the above shortcomings. It is derived from first principles, and is founded on the realization that the underlying causes for the three-stage work-harden- ing curve and for the persistence of the numerical value of O,, must be few and simple. As a first step it is shown herein that the experimental oklservations on "easy glide" can be explained on the assumption that in this stage dislocations multiply, beginning in a number of restricted area:;, and from there penetrate into crystal regions still substantially free of glide dislocations until a quasi-uniform dislocation distribution has been established. During "easy glide", the resistance to the movement of the foremost dislocations, advancing into still largely a dislocation-free regions, is believed to control the flow stress. This resistance is determined by various factors, among them the reaction of the dislocations against bowing-out into loops, which is due to their line tension. Without such bowing-out no dislocation multiplication would occur, but the corresponding contribution to the flow stress stays practically constant all through stage I. According to the present theory, stage TI begins as soon as there are no more areas left into which newly formed dislocations have not yet penetrated, i.e., as soon :is a quasi-uniform dislocation density has been attaned. Regardless of how, in detail, the dislocations should happen to be arranged at the end of "easy glide", whether in pileups or tangles, on one glide system or on several, the stress necessary to overcome the line tension of the segments must now increase, because the dislocation density increiises while the average free dislocation length decreases. It is argued that all other contributions to the flow stress stay at about the same level as in stage I, and that the said increase in the stress necessary to bow out dislocation segments into loops is responsible for the major part of work hardening in stage 11. Stage TII of the stress-strain curve is explained by largely follouing the ideas of Seeger and his co-workers; however, it is pointed out that the start of stage m may on occasion indicate the onset of "conservative climb" instead of cross slip. Indirect evidence indicates that linear work hardening persists even under the profuse action of cross slip, but at a lower rate. Only when, in addition to cross slip, climb operates extensively, does the work-hardening rat,. drop sharply, ultimately down to zero. BRIEF SURVEY OF EXPERIMENTAL EVIDENCE The stress-strain curve of crystals of many different substances are qualitatively similar. No significant permanent plastic deformation takes place below the so-called "critical resolved shear stress". As the applied stress is raised beyond this level. yielding occurs with little or no work hardening and

Jan 1, 1962

By W. F. Campbell

One of the deepest coal operations today is the Springhill No. 2 mine of Cumberland Railway & Coal Co., subsidiary of Dominion Coal Co. Ltd. Mining is now conducted at a slope distance of 14,000 ft, with 4400 vertical ft of cover. The record of Springhill No. 2 can be said to contain the history of bumps in the Province of Nova Scotia. The Springhill coal field forms part of the Cumberland field of Carboniferous age. There are seven mineable fields in the area, numbered in the order they were discovered. Mining has been carried on in all but the No. 4 and 5 seams, but present operations are confined to the No. 2. Fig. 1 shows a vertical section through the seams. Opened in 1873, No. 2 mine was first worked from parallel slopes driven from the outcrop of No. 2 seam down to the 7700 level. As the mine went deeper, a two-place auxiliary slope was driven from the 6900 level to the present workings. A transfer level at the 7800 connects the main haulage with the auxiliary haulage slope. The No. 2 mine plan is shown in Figs. 2 and 3. The seam is bituminous, averaging 8.5 to 9 ft thick. There is a well defined parting 14 to 16 in. from the roof, and this roof coal is harder than the rest of the seam. Average pitch at the outcrop is 30°; at the 6500 level, 20°; at the 7900 level, 16°; and at the 13,800 level, 10°. Immediate roof and floor strata consist of beds of variable thicknesses of shales, grading to arenaceous shales to shaly sandstones to sandstones. A characteristic of the strata is the appearance and disappearance of sandstone bands, of considerable thickness, over distances of several hundred feet. MINE HISTORY Entrance to No. 2 mine is obtained by three parallel slopes, separated by 100-ft pillars except in the upper portion of the mine where the pillars are smaller. Main haulage levels were originally driven 600 ft apart for room and pillar extraction. A parallel drainage and intake airway was driven below each haulage level and a parallel return airway (counter level) above. Pillars 80 to 100 ft wide separated these two servicing entries from the center haulage level. Inclines up to 700 ft apart were driven off the levels up pitch to the upper levels, and at 40-ft centers level rooms 12 ft wide were driven off each side of the inclines, separated by crosscuts every 50 to 100 ft. The rooms were driven up to 350 ft or until they were holed into the room from the adjoining incline—a plan followed until the level reached its boundary. When the entire area between two levels had been divided into pillars, the pillars were extracted from the boundary back to the slope pillars. As depth of cover increased, mining conditions required larger pillars, and at the 3300 level, where NECESSARY PRECAUTIONS FOR MINING AT DEPTH In Room and Pillar Extraction: 1) Pillars must not be too small. 2) Pillar size must be uniform. 3) Extraction lines must be kept as straight and as uniform as possible. If they are irregular they should be brought back into line slowly. 4) Peninsulas of coal surrounded by gob must be avoided. 5) Pillars should not be disturbed after they have been formed. If it is necessary to increase the width of an opening, stripping or ribbing should be done gradually over a long distance. If increased width of openings is necessary at certain locations and can be anticipated, the openings should be widened during development. 6) Under no conditions should small pillars be left in the gob that would interfere with caving. In Long wall Operations: 1) Pillars formed during development for retreating longwalls should be uniform in size and shape. 2) Pillars should not be split after development, but if pillar splitting does become necessary it should not be carried out within the zone influenced by the working faces. 3) Width of levels should not be increased after the levels have been developed. If it does become necessary to widen a level it must be done gradually over a long distance by stripping or ribbing the coal ribs. 4) Extraction lines should remain uniform. If several walls are worked together—one behind the other—a uniform distance must be maintained between them and the wall faces must be kept as straight as possible. If the walls do get out of line, they must be brought back into line gradually to allow for a gradual change in the stress distribution over the area. 5) Stumps of coal, roof supports, etc. must not be allowed to remain in the gob to interfere with caving. 6) Midwalls should be built as rigidly as possible and properly maintained.

Jan 1, 1959

By G. R. Leverant, M. Gell

The high- and low-cycle fatigue poperties of MAR-M200 directionally -solidified into columnar-grained and single crystal forms were determined at 1400" and 1700°F. These results were compared with the corresponding properties of conventionally -cast MAR-M200. The low-cycle fatigue lives of the columnar-grained and single crystal materials were similar at both temperatures and were one to two orders of magnitude greater than those of conventionally-cast material. The variations in the fatigue lives among the three forms of MAR-M200 were related to the more rapid rate of intergranular muck propagation compared to that of transgranular propagation. In conventionally-cast MAR-M200, cracks were initiated in grain boundaries and crack popagation occurred rapidly along an almost continuous grain boundary path. In the columnar-grained material, crack initiation occurred on short transverse segments of grain boundaries, but crack propagation was transgranular. The fatigue lives of columnar-grained and single crystal materials were approximately the same because most of the life in both materials was spent in trans-granular propagation. For the directionally -solidified materials, the number of cycles to failure, Nf, can be related to the total strain range, , by: heT = K where n and K are 0.16 and 0.044 at 1400'F and 0.29and 0.098 at 1700, respectively. In addition to intergranular crack initiation in the columnar-grained material, initiation also occurred at we-cracked MC carbides and micropores in both directionally-solidified materials. At 1400°F, fatigue life was reduced with increasing MC carbide size, but at 1700 there was no effect of carbide size. THE creep and stress-rupture properties of conventionally-cast nickel-base superalloys can be greatly improved by directional solidification into either single crystal or columnar-grained forms. The improvement in properties results from a reduction in intergranular cavitation and crack growth in the columnar-grained materials and the complete absence of this fracture mode in the single crystals. This paper describes the effect of grain boundaries on the elevated temperature fatigue properties of the nickel-base superalloy MAR-M200. The effect of cycling frequency on cracking arid fatigue life and the role of MC carbides and micropores on crack initiation are also emphasized. The fatigue properties of columnar-grained and single crystal MAR-M200 at room tem- perature,4,5 and the change in the mode of fatigue crack propagation with temperature6 have recently been described. I) EXPERIMENTAL PROCEDURE The material used in this study was the nickel-base superalloy MAR-M200, directionally-solidified into columnar-grained and single crystal forms. The columnar grains were approximately 0.5 mm in diam. The nominal composition of these materials in wt pct was 0.15C, 9Cr, 12.5W, loco, 5A1. 2Ti, lCb, 0.05Zr. 0.015B, bal Ni. They were solutionized for 1 to 4 hr at 2250°F followed by aging at 1600°F for 32 hr. which resulted in 0.2 pct offset yield stresses of 150,000? 144,000, and 95,000 psi at room temperature? 1400°, and 1700°F, respectively. The corresponding elastic moduli parallel to the testing direction were 19.2. 15.0, and 12.5 x 106 psi, respectively. Specimen design, testing procedures and alloy mi-crostructure have been described previously and will only be summarized here. Following the 1600°F aging treatment. MAR-M200 contains an ordered, cuboidal, y' precipitate 0.3 1 on edge, which is coherent with the 1 matrix. The y' precipitate is quite stable; even after testing at 1700°F. the precipitate is only slightly enlarged and its corners somewhat rounded. The alloy also contains a small volume fraction of micropores, and MC carbides. some of which contain preexisting cracks5 formed during casting. These cracks are always parallel to the long dimension of the carbide. Measurement of MC carbide size has been described previously.5 Axial fatigue tests were conducted in air over a wide range of strain amplitudes in both the high - and low-cycle fatigue regions, with specimen lives varying from about 10' to 10' cycles. Low-cycle fatigue (LCF) tests were strain-controlled with strain varied between zero and a maximum tensile value at a frequency of about 2 cpm. High-cycle fatigue (HCF) tests were stress-controlled with the stress varied between 5000 psi and a maximum tensile value less than the yield stress at a frequency of either 10 cps or 0.033 cps (2 cpm). The temperature in the gage section was controlled to 52°F. Specimen axes were within 5 deg of the [001] growth axis of the single crystals and the common [001 ] growth axis of the columnar-grained material. Specimen gage sections were electro polished prior to testing. After the standard heat treatment, three specimens were coated with a typical aluminide coating applied as a slurry. An additional specimen was given the coating heat treatment without actually being coated. In all cases: specimens were reaged at 1600°F for 32 hr after receiving the coating heat treatment at 1975'F for 4 hr.

Jan 1, 1970

By I. R. Kramer

The effect of the specimen diameter, d, on the flow stress, cra of polycrystalline aluminunz (99.997) was studied. The increase in the flow stress could be accountedfor by the increase in the surface layer stress, with decreasing specimen diameter. Both , and a, were found to be proportional to For the smaller-dianzeter specimen (< 0.033 in.) at strains less than aboul 0.1, the work hardening of the surface layer was greater than that associated with the bulk of the specimen. At higher strains the work hardening due to the bulk appears to be independent of the specimen diameter. THE increase in the strength of metals with decreasing diameter is well-known; however, an adequate explanation for the cause of the size effect is still lacking. The earliest systematic investigation of size effect appears to be that of Onol who reported that for aluminum monocrystals the resistance to slip at low strains increased as the specimen diameter decreased. A change in the stress-strain curve beyond 0.001 strain was not found. However, Suzuki et a1 .&apos; reported for monocrystals of a brass and copper having diameters in the range of 2 to 0.12 mm that the entire stress-strain curve was raised as the specimen diameter was decreased. The effect of size was most apparent when the diameter of the specimen was less than 0.5 mm. In the discussion of this paper Honey-combe reported a size effect in copper crystals as large as % in. diam. These results are in agreement with those of paterson3 and Garstone et al.4 While the majority of the investigations on size effects was conducted in terms of the variation in the diameter of the specimen, several investigators studied the influence of the specimen geometry. For example, Wu and smoluchowski 5 reported that in aluminum monocrystals the slip system was a function of the specimen dimension in the slip direction. King-man and Green 6 studied the influence of size on the compressive stress-strain relationship of aluminum monocrystals when the ratio of length to diameter was constant. Their specimen diameters ranged from to & in. For specimens oriented for single slip the critical resolved shear stress for the smaller-size specimens increased with decreasing diameter. No effect was observed in the large-size specimens. Specimens having an orientation near the corners of the stereographic triangle did not exhibit a size effect. Apparently, the increase in strength with decrease in the diameter of the specimen is a general phenomenon and has been observed in a brass |T and cadmium as well as in aluminum and copper.&apos; In a series of investigations (for example Ref. lo), it was shown that during deformation a surface layer was formed which imposes a back stress, a,, on the moving dislocations. It is reasonable to predict that this surface layer stress, as, should be a function of the specimen diameter and could possibly account for the flow stress size effect. In fact, experimental evidence will be presented to show that this is the case; i.e., the increase in flow stress with decreasing size is equal to the increase in the surface layer stress, as, with size. In addition, data will be presented on the variation with size of and a* where is the back stress associated with the generation of dislocation obstacles in the bulk of the specimen and a* is the net effective stress acting on the mobile dislocations. A limited investigation was carried out on gold specimens to determine the influence of an oxide film. EXPERIMENTAL PROCEDURE The aluminum specimens were prepared from -in. bar stock (99.997 pct purity). The 0.350- and 0.150-in.-diam specimens were machined directly from the bars while the specimens having a diameter of 0.033, 0.020, and 0.015 in. were prepared by swaging and drawing to 0.04 in. and electropolishing almost to final size. The specimens were prepared with a 2-in. gage length. The specimens were annealed in vacuum (-10-4 Torr) at 350°C for 8 hr. The grain diameter of the specimens in the various specimen diameter groups was 0.08 ± 0.02 mm. Gold specimens of two diameters, 0.14 and 0.03 in., were prepared in a similar way and annealed at 650°C for 8 hr. The grain diameter of the gold specimens was 0.2 mm. After annealing the specimens were electrochemically polished to the final size and tested in an Instron tensile machine at a strain rate, E&apos;, of 10- 3 per min. While it was possible to determine the surface layer stress, a,, in the larger-size specimens by measuring the difference, Aa, between the stress before unloading the specimens and the initial flow stress after removal of the surface layer as outlined in detail in Ref. 10, this method is not applicable for small wires because of the difficulty in obtaining a sufficiently accurate measure of the diameter. The values at the various strains were therefore determined by measuring after the specimen had been annealed at 35°C for 4 hr. It has previously been shown" that the two methods give the same results for a provided that the annealing temperature is low enough to affect only the surface layer and not the dislocation barriers in the bulk of the specimen. For the gold specimens a treatment at 150°C for 16 hr was found to be satisfactory for the determination of by the low-temperature annealing method. EXPERIMENTAL RESULTS Determination of a,, and a,. The stress-strain curves for the various diameter aluminum specimens, plotted in terms of the logarithms of the true stress, and true strain, are given in Fig. 1. These curves represent the average data taken from at least ten specimens at each size. Over the range of strains investigated the curves follow the empirical equation

Jan 1, 1968

By D. L. Archer, W. W. Owens

Conventional waterflooding often is uneconomic in highly fractured reservoirs because of the gross bypassing of the reservoir oil by injected water. Imbibition and pressure pulse flooding have been used in at least one fractured reservoir in an attempt to achieve better oil recovery performance. This paper presents the results of laboratory flow tests conducted on large cores to evaluate the possible applicability of these methods (particularly pressure pulse flooding) to different types of reservoir systems. Test data were obtained on both water-wet and oil-wet systems, and on systems having two widely different levels of compressibility and flow capacity. Results indicate that pressure pulse recoveries from fractured reservoirs will likely not exceed 5 to 10 per cent pore space with maximum response achieved during the first pulse cycle. Improved recovery by this method is possible from both oil-wet and water-wet reservoirs. Comparable saturation distributions during imbibition and pressure pulse production suggest that an initial pressure pulse cycle to speed production response would not interfere with subsequent imbibition flooding in water-wet reservoirs. INTRODUCTION There is a steady increase in the number of fluid injection projects being initiated each year to improve oil and gas recovery over that obtained by primary production mechanisms. Experience gained in different geographical areas and with different recovery methods is teaching that reservoir anatomy is one of the more important factors controlling the success or failure of such projects. Fractured formations appear to be the rule rather than the exception, especially in carbonate reservoirs. Conventional methods of fluid injection, whether it is waterflooding, gas injection or miscible flooding, have limited applicability to highly fractured reservoirs because of the severe bypassing of reservoir fluids. The result is early breakthrough of the injected fluid and rapidly increasing ratios of injected to in-place fluid with an undesirable effect on return on investment. Thus, innovations to conventional methods must be developed if recovery from fractured reservoirs is to be optimized. One new method proposed for application to highly fractured reservoirs is pressure pulse waterflooding. Although pressure pulsing with gas has been suggested&apos; and rested in at least one reservoir; the Sohio Oil Co. is generally credited with the development and testing of pressure pulsing in conjunction with waterflooding. Publications" of Sohio&apos;s waterflood operations in the Spra-berry Trend indicate that the idea for pressure pulse flooding evolved from a critical analysis of the disappointing early performance of their flood initiated in a portion of the trend in April, 1961. This flood was planned to take advtantage of imbibiltion and high pressure gradients across the reservoir matrix blocks (unfractured Mocks of the reservoir rock which are visualized to be generally surrounded &apos;by the fracture system) evaluated in pilot tests by the Atlantic Richfield Co.5 and Humble Oil & Refining Co.&apos; However, the early recovery and pressure performance of this flood indicalted that high pressure gradients induced between the fracture system and the centers of the matrix blocks were forcing water into the periphery of the blocks and temporarily interfering with the counter-current flow of oil due to imbibition. Fill-up in the blocks was occurring with re-solution of the free gas phase as pressures climbed above the original reservoir bubble point. It appeared that cessation of water injection to permit the capillary forces to become dominant and that expansion of the rock and its contained fluids during pressure reduction might aid in expulsion of oil from the rock matrix into the fractures. Subsequent production performance. aflter cessation of injection, proved this hypotheslis correct. Thus, pressure pulse waterflooding was born. Pressure pulse flooding appears to have several advan&apos;tages over imbibition type flooding in highly fractured reservoirs. First, all wells may be used for water injection which should hasten fill-up and achieve a more rapid increase in reservoir pressure. Second, because of increased injection pressures and rates, flooding gradients will force water into the reservoir matrix more rapidly than would be achieved by imbibition alone. Third, during the pressure depletion cycle of the flood, the compressibility of the system, which has been increased by resolution of the free gas phase, provides energy for the displacement of oil at a rate greater than would correspond to countercurrent flow during imbibition. Fourth, during depletion all wells may be put on production, thus contributing to higher oil withdrawal rates on a reservoir-wide basis. One possible disadvantage of pressure pulse flooding as compared to imbibition floodling, however, is that the outer periphery of each matrix block is flooded to a residual or near residual oil saturation during the Water lnjection stage. This zone of reduced permeability to oil m&apos;ay offer greater restriction to the production of

Jan 1, 1967

By J. N. Maycock, P. W. M. Jacobs

Previous measurements of the thermoelectric power of ionic crystals are reviewed briefly. It is concluded that, while extensive measurerements are available on systems in which the electrode M has a cation in cornon with the sall MX, there is as yet little modern work on systems in which the metal electrode M&apos; does not haue a calion in common with the salt. We haue now measured the thermoelectric power of- single cryslals of KC1 and of KC1 + SrCl2 between plati.nu?n electrodes over a much wider range of temperature than before. individual ionic heats of transport, which, while probably not very accurate, are of a reasonable order of magnitude, are obtained from this data on the basis of an assumption that charge carriers do not cross the electrode/salt interface. RELATIVELY little work has been done on thermal diffusion in ionic crystals. Reinhold and coworkers1-I measured the thermoelectric power and Soret effect in silver and cuprous salts and their mixed crystals. One electrode was kept at constant temperature and electromotive force was measured as a function of the temperature gradient. This is not a satisfactory technique if the results are to be compared with theory; rather should the electromotive force be measured for small temperature differences as a function of the mean temperature of the two electrodes. They used electrodes having the same cation as the salt. Patrick and Lawsons measured the thermoelectric power of AgBr and of AgBr containing 0.25 mole pct CdBr2 using silver electrodes and temperature gradients of 10°C in the temperature range 100° to 400°C. The results were analyzed using the Teltow9 values of the ratio of the mobility of the two types of charge carrier (cation vacancies and interstitials) found from conductance measurements and the wagnerlo estimate of the heterogeneous contribution to the thermopower, Ohet They obtained values of qi = 0.017 ev and q$ = -0.385 ev for the heats of transport of interstitial Ag ions and cation vacancies, respectively. Their data has been reanalyzed by Haga using the corrections to the Wagner formula for ehet given by Howard andLidiard; Haga found ev. Christy et a1.14 have measured the thermopower of AgBr containing 0.01 to 1.0 mole pct CdBr,. They analyzed their results in terms of the theory of Howard and Lidiard and found that excellent agreement with the theory was obtained if q5 + : + hF was set equal to 0.70 ev at 175°C and equal to 0.58 ev at 250°C. Since Teltow&apos;s value for hF, the enthalpy of formation of a Frenkel defect pair, is 1.27 ev, this means that qt + q: increases in magnitude from -0.57 ev at 175°C to -0.69 ev at 250°C. This result is in poor agreement with Patrick and Lawson&apos;s value of -0.37 ev for pure AgBr (for which the corrections to the Wagner estimate for ehet are very small). christy1&apos; has also reported thermopower measurements for pure silver halides and for the systems he finds that qr + qt for AgBr increases in magnitude from about -0.55 ev at 150°C to about -1.1 ev at 400°C. The temperature dependence of the sum of the heats of transport in AgCl is very similar in magnitude. The sum qi + q cannot be decomposed into the individual interstitial and vacancy heats of transport without a knowledge of the entropy contribution from the change in the lattice vibrational frequencies around the interstitial and vacancy. Despite this it seems an inescapable conclusion from Christy&apos;s analysis that and -6 = Q Ag are quite strongly temperature-de pendent and that -q: is much greater in magnitude than the corresponding activation energy for vacancy migration, +0.37 ev. It remains a mystery as to why the data for pure AgBr are fitted so well by the assumption of temperature-independent heats of transport if association of impurity ions and vacancies is neglected.l The values for q$ and q: so obtained are also much more in line with those expected from a simple kinetic interpretation of the heats of transport due to wirtz.17 Christy and Hsueh have also measured the thermopower of CuCl between copper electrodes; 0 for pure CuCl is comparable with that for the silver halides but, owing to the limited solubility of divalent cation impurities it was not found possible to deduce values for the heats of transport. In contrast to these extensive measurements on cells of the type M MX 1 M, there have been relatively few attempts to measure the thermoelectric power of cells of the form M&apos; 1 MX I M&apos;. (We specifically exclude from discussion cells of this type in which the metal electrodes M&apos; are in contact with gas containing a controlled partial pressure of either M or X,, since in these cells the electrochemical reactions are analogous to those of the first type.) The thermoelectric power of Pb12, PbC12, TlC1, TlBr, TlI, NaNO3, and KNO 3 between platinum electrodes was measured by Thie1e.l3 In discussing these measurements Holtan20 asserts that the thermopotential initially fluctuated wildly and even changed sign before approaching a steady value. A high degree of irreproducibility is also evident in the work of Nikitinskaya and Murin21 who first studied the thermopower of NaCl and KC1 between platinum electrodes. Much better reproduci-bility was achieved in our earlier measurements on KC1;22 these were, however, confined to a relatively narrow temperature range and one of the objects of the work reported in this paper was to extend the temperature range of the measurements of the thermoelectric power 8 of KC1. The only other published

Jan 1, 1967

By Bieniawski, Z. T.

The importance of understanding the phenomena associated with rock fracture has long been fully appreciated in rock mechanics. This is clearly apparent from the special attention paid to rock fracture in the comprehensive papers published within the past four years. These papers present a picture of how the knowledge of brittle fracture of rock was gradually enriched and also show that what was often taken for granted three or four years ago is no longer considered as valid today.

Jan 1, 1972

By Paul M. Tyler

DOES mining pay? Inasmuch as the whining of minerals from Nature is one of the world&apos;s principal sources of new wealth, this question is of general economic interest but it is obviously of even more vital concern to investors and those engaged in mining. In a previous article on "Wage Costs in the Mineral Industries,&apos; published in the November issue, I attempted to show that wages paid workers on the job accounted for, roughly, 50 per cent of the mine dollar. Salaries normally range over 5 per cent - lower for large metal mines but higher in stone quarries and minor nonmetallic min. in, industries. Purchases of supplies, materials. fuel, and power generally aggregate within the range of 15 to 25 per cent of total receipts, with the average well under 20 per cent. The total of all such specified expenditures which may he loosely described as

Jan 1, 1934

By William G. Kegel

Problems involved in putting the gob back underground in modern deep mines we dealt with. In particular, the problems associated with haulage of the gob back to the mine, the blown gob method of dispersion underground, and handling the refuse as slurry are discussed. Some alternative methods of disposal are presented.

Jan 1, 1976

By G. W. Powell

ACCORDING to crank,&apos; the accumulation equations for inter diffusion in a binary system whose molal volume is a function of concentration are

Jan 1, 1965

Jan 1, 1895