Search Documents

By F. Keller, A. H. Geisler

Some of the important changes that take place in the structure of aluminum alloys are largely submicroscopic in character. This is especially true of the changes that accompany age-hardening and recrystallization. Although improved metallographic practices have been helpful in indicating some of these changes indirectly, results are limited by the resolving power of optical microscope lenses. The electron microscope, however, provides a new and important means for investigating the fine structure of metals in a range not possible heretofore. It is anticipated that this microscope will yield new and useful information to metallurgists when suitable techniques are developed to utilize its very high resolving power. Several unique methods have been devised to permit the examination of the structure of opaque samples by the electron microscope. These are all based on the principle of producing a very thin film that represents the prepared surface of the opaque metal sample, which can be examined in the transmission microscope; however, the available methods fall into three categories, depending on the manner in which the film is obtained. With the oxide-film method developed by Mahl3-17 a very thin surface layer of the metallographic sample is converted into oxide by a thermal, chemical or anodic treatment. After removal from the metal sample by one of the special techniques described by Evans,19 this oxide film is examined in the electron microscope, and is found to portray the structure of the original metal surface. Films formed on aluminum,8,17 nickel,7 iron 3,8,9 and a nickel-beryllium alloy" by heating; films formed on iron 8.9 by chemical action; and films formed on aluminum3-16 by anodic oxidation, have been examined. The other two are more recent methods developed principally in this country; they consist in making the thin film in the form of a mold or replica of the surface contour of an etched sample. With the negative replica method the thin film is made directly on the sample; with the positive replica method it is made on a negative reproduction of the sample. With both methods, variations in the contour of the metal sample, caused mainly by etching, produce variations in the thickness of the surface replica. Because of the direct dependence of the amount of scattering of the illuminating electrons upon the thickness of the film penetrated, only the variations in thickness of the replica are responsible for tones in the electron image. Negative replicas are made by forming a thin film of some suitable material directly on the polished and etched metal sample, and derive their name from the fact that after removal from the sample they have the high and low points in the sample reversed, as shown in Fig. I. Nega-

Jan 1, 1944

By Charles Y. Clayton

The shank, to give good service, should not upset nor should it cause excessive wear on the various parts of the machine. To fulfill these requirements, the steel must have a certain hardness—that is, resistance to deformation. The hardness is expressed in conventional terms, as Brinell or scleroscope number. Before deciding on the hardness that is proper, several factors should be considered; the most important are: type of drill steel used (whether hollow or solid), length of steel, nature of ground, air pressure, and hardness of hammer or anvil block (usually 600 Brinell). These are not all, but they are the most cogent ones. Without doubt, it would be better to use a shank that is too soft than one that is too hard. Most satisfactory service will be obtained from shanks that are just hard enough to resist any tendency to upset. To express the proper hardness in terms of Brinell numbers would mean a great many tests over a long period of time, or a study of drill steels that are being used successfully in the various districts. To obtain some information along this line, the persons making this investigation requested, a number of companies to send to them drill shanks that had been given the usual treatment at that mine. The results of the hardness tests are given in Table 1. The hardness values given in Table 2 are those of shanks known to have been quenched in water and drawn. These results show that where water quenching and drawing are in vogue the hardness is rarely under 400 Brinell.

Jan 1, 1923

By Charles Y. Clayton

The shank, to give good service, should not upset nor should it cause excessive wear on the various parts of the machine. To fulfill these requirements, the steel must have a certain hardness—that is, resistance to deformation. The hardness is expressed in conventional terms, as Brinell or scleroscope number. Before deciding on the hardness that is proper, several factors should be considered; the most important are: type of drill steel used (whether hollow or solid), length of steel, nature of ground, air pressure, and hardness of hammer or anvil block (usually 600 Brinell). These are not all, but they are the most cogent ones. Without doubt, it would be better to use a shank that is too soft than one that is too hard. Most satisfactory service will be obtained from shanks that are just hard enough to resist any tendency to upset. To express the proper hardness in terms of Brinell numbers would mean a great many tests over a long period of time, or a study of drill steels that are being used successfully in the various districts. To obtain some information along this line, the persons making this investigation requested, a number of companies to send to them drill shanks that had been given the usual treatment at that mine. The results of the hardness tests are given in Table 1. The hardness values given in Table 2 are those of shanks known to have been quenched in water and drawn. These results show that where water quenching and drawing are in vogue the hardness is rarely under 400 Brinell.

Jan 1, 1923

The Mine Safety and Health Administration (MSHA) defines an independent contractor as "any person, partnership, corporation, subsidiary of a corporation, firm, association or other organization that contracts to perform services or construction at a mine." Contractors that perform specific types of work are required to report the number of employees who perform work at coal mines and at noncoal mines.

Feb 1, 2005

FRank FirmstoNe, Easton, Pa.: The effect of lead on brass seems to have been recognized by Berthier, and the facts published by him as early as 1818 (Ann. des Mines, 1st ser., iii., 1818, p. 347 et seq.). He there says expressly that copper containing lead makes " dry " (see) brass, and that, for the same reason, blast-furnace

Jan 1, 1898

The process design of a metallurgical plant follows logical steps, one built on another, to define the circuit requirements. The methodology used and documentation produced is the same for a pre-feasibility study, feasibility study or a complete design, regardless of the process complexity or the commodity. The only difference is the amount of detail developed at each stage. SRK Consulting, an independent, international consulting practice that provides focused advice and solutions to clients, mainly from earth and water resource industries, has consulted on many mining projects around the world. The company was formed in 1974 and now employs more than 1,600 professionals in more than 50 offices on six continents. According to SRK experts, the first element in plant design is collecting metallurgical testwork or operating data from an existing plant. The testwork identifies the process, the metallurgical recoveries and product qualities and establishes the process parameters for economic recovery of the commodity. Test samples must represent both the whole orebody and the feed over the life of the mine, showing the mineral?s variability. With this data available, the process design can be developed.

May 1, 2013

Since this paper was written, some changes have taken place, materially reducing the cost of iron-making as estimated in the text. These are chiefly reductions in the prices of ore and fuel. The charcoal used is now made in bee-hive ovens, and the price is con-

Jan 1, 1895

The Mine Safety and Health Administration (MSHA) defines an independent contractor as "any person, partnership, corporation, subsidiary of a corporation, firm, association or other organization that contracts to perform services or construction at a mine." Contractors that perform specific types of work are required to report the number of employees who perform work at coal mines and at noncoal mines.

Feb 1, 2005

By D. Hurrington

For severlrl 1e1.rs the Bureau of Mines has studied dust and ventili in metal mines a.s a.tfeoting the hea.l th, safety, and efficiency of underground workers. From its beginning this work haa been directed by G, s. Rice, chief mining engineer, A. cooperative agreement was ma.de in 1915 with the u. s. Publi Health Service; since that time tield obmervations 1n metal mines in Arizona, California, Idaho, Montana, Michigan and Nevada, and less extensive observation, in several other States have been made by mining engineers of the U, s. Bureau of Mines, and doctors of the u. s. Pu.blic Heal.th Service. Most of the analyt:J:eal work has been done by chemists, petrographers, and other specialists in the Pittsburgb station ot the Bureau of Mines, Numerous articles have been printed by the otticial and the technical press on this important in•estigation; 1n response to the need for a btiet bibliography of this literature. it has been compiled to facilitate accp1,intance with published data ot1. the subject,

Jul 1, 1922

By S. Terry, W. Callender, R. J. Cornelius

When a dry ball-mill grinding circuit was in- stalled at the pellet plant of Hamersley Iron Pty. Ltd., it was something of a pioneering venture, since this was one of the first instances where such a circuit had been applied to iron ore on a major scale. The Hamersley pellet plant is located in Dampier on the coast of Western Australia. The feed for it consists of -8mm fines produced as a screened product at the Mt. Tom Price mine 186 miles inland from Dampier. It was these fines that encouraged the choice of a dry grinding facility for the plant. At the outset, tests had shown that the fines could not be filtered satisfactorily with a wet grinding system. At the pelletizing plant, the fines are stockpiled, reclaimed, dry ground in ball mills, formed into pellets on balling discs, hardened on an indurating line, and stockpiled for shipment overseas. These operations are shown in Fig. 1.

Jan 7, 1969

By R. H. Richman, F. Borik

A 0.3 pct C-12 pct Cr-6 pct Ni steel was rolled to 93 pct reduclion in area as austenite at 510°C, and then partially transformed as desired to ~rlartensite by qnenching to - 196°C. Pole figures for the austenitic matrix and for the martensitic product were separately determined by an X-ray transmission method. The deforitration texture of' the warm-worked austenite is characlerized by (110)(225) components, and is thus closely similar to those produced in a brasses. The pole jigure of the martensite in partially transformed material agrees well with that which can be constructed by transfortnation of the {110)(225) orientations according to either the Kuvdjuniov- Sacks or the Nishi-yatuu relatiotship. Howeuer, an important result of this construction is that me-third of the predicted orientations are missing. A graphical analysis can then be used to show that in deformed austenite certain crystallographic variants of martensite (related to the most probable austenite slip systems) are suppressed, resulting in this preferred transformation. The evidence for preferred transformation is corroborated by the measured elastic anisotropy of warm-rolled and fully transformed H-11 steel. EXTENSIVE plastic deformation of a polycrystal-line aggregate in a manner that causes flow predominantly in one direction results in a preferred orientation of the constituent crystallites. The particular orientations that are produced depend upon the crystal structure and composition of the material, as well as upon the temperature, mode, and degree of deformation; in any case, the preferred crystallo-graphic orientations, or textures, are reflected in directionality of mechanical properties. Although such anisotropy may be exploited in certain specialized applications, it is more commonly diminished or eliminated by heat treatment lest it interfere undesirably in subsequent forming operations or in structural design. In the recently developed thermomechanical treatments that significantly enhance the strength of some steels,1,2 considerable deformation of the metastable austenite prior to the martensite transformation is essential to the strengthening process. If the austenite is textured by the deformation, and if the transformation to martensite proceeds according to one of the relationships established for transformation in annealed austenite, then it must be expected that the martensite will also possess a preferred orientation even though the multiplicity of martensite orientations possible in a given austen- ite crystal will tend to restore some degree of randomness. The existence of a residual anisotropy, both mechanical 3-6 and crystallographic,' has been substantiated. In the latter crystallographic investigation, preferred orientations were determined for the martensitic structure of an SAE 4340 steel rolled 72 pct as austenite at 833°C and then quenched. However, the choice of a composition that transformed almost completely to martensite during the quench to room temperature did not permit direct measurement of the prior austenitic texture. In fact, when the "ideal orientations'' associated with well-known fcc rolling textures were converted, alone or in combination, to martensite according to the Kur-djumov-Sachs (K-s)' or Nishiyama8 relations, the agreement obtained with the observed martensite texture was only fair at best. Recently a pertinent aspect of the austenite to martensite transformation was reported by Bokros and parker,10 who found that certain habit-plane variants of martensite were suppressed by tensile deformation of Fe-31.7 Ni single crystals prior to the necessary subzero cooling. It might be anticipated that the consequences of such preferred transformation are sustained during the formation of martensite in warm-worked austenite that has a well-developed deformation texture. The present investigation was undertaken first to establish more firmly the relation between preferred orientations in plastically deformed austenite and in the resulting martensite, and second to examine the textures for evidence of deformation-induced preferred transformation. EXPERIMENTAL PROCEDURES An alloy containing 0.3 pct C, 12 pct Cr, 6 pct Ni, and the balance iron, was selected because the mar-tensite-start temperature (M,) of about -100°C allowed convenient experimental manipulation of either austenite or martensite at room temperature. Furthermore, this composition can be readily deformed as metastable austenite at moderately elevated temperatures without intervention of appreciable isothermal or athermal decomposition products. The alloy was austenitized at 1150°C, aircooled to 510°C, rolled unidirectionally at this temperature to 93 pct reduction of cross-sectional area, and finally oil-quenched to room temperature. Partial transformation to martensite was accomplished by quenching to -196°C as needed. The rolled stock was reduced in thickness from 0.067 to 0.010 in. by etching in a solution of 5 pct HC1, 45 pct HNO3, and 50 pct water, and further thinned by careful mechanical polishing to maintain the two sides of the sheet parallel within 0.0003 in. After mechanical polishing to 0.005 in., electropolishing in 1:9 perchloric-acetic acid solution produced a final thickness of 0.002 in. The preferred orientations were determined from

Jan 1, 1968

By P. G. Shewmon, H. E. Collins

We have studied the effect of adsorbed sulfur on the surface self-diffusion of copper using eight diflerent surface orientations and the grain boundary grooving method. The eight orientations studied were the four lying near the low-index surfaces—(loo), (Ill), and two directions in the (110)-plus four higher-index surfaces. Surface-diffusion measurements were made over a range of HZS concentrations (in Hz) from 3 to 1500 ppm between 830°and 1050°C. The results can be divided into two groups—Group 1 contains the two (110) surfaces while Group 2 contains the remaining six surfaces. In Group 1, increasing the temperature increases the effect of Hz S on DS for the Hz S range 5700 ppvn. Qs and Do increase with increasing H2S concentration in this Hz S range. Beyond this range, increasing the temperature decreases this effect on D,; also Q, and Do decrease. In Group 2, increasing the telnperature decreases the effect of H2 S on D, for the H2S range studied, and Qs and Do decrease with increasing Hz S concentration. In any study of surface phenomena, there invariably arises the question of the possible presence of and effect of adsorbed impurities. Such questions are well-founded since the presence of adsorbed atoms can sometimes produce marked changes in the kinetics of surface-energy-driven processes. In the last few years, values of the surface self-diffusion coefficient, D,, have been determined on a variety of metals by studying the decay of scratches or the growth of grain boundary grooves.L~3-L0~L3 Yet there has been relatively little work done in which the concentration of an adsorbed impurity was systematically varied and the effects observed. Work of this sort would provide some basis in fact for the assertions often made about the ro1.e of adsorbed impurities in the differences between the results of different workers in different atmospheres and on different metals. It also is relevant to those cases in which surface monolayers produce profound effects in commercially important processes. The most marked example of such effects is the ability of nickel or palladium to increase the sintering rate of tungsten by many orders of magnitude.' The aim of this work was to study the effect of sulfur partial pressures on the surface self-diffusion of copper. It was felt that this in conjunction with a study of the degree of adsorption and type of active sites involvedL8 would provide a wide range of data for one system and hopefully lead to some insight into the mechanism by which sulfur adsorption influences copper diffusion. The main reasons for choosing the Cu-S system were, first, faceting was reported not to accompany the adsorption of sulfur. This is required if our experimental technique is to work. Second, Oudar has determined a high temperature adsorption isotherm for this system, an event which puts the Cu-S system almost in a class by itself.I4 EXPERIMENTAL PROCEDURE Initially, we considered studying the effect of an adsorbed impurity on surface self-diffusion of copper using isolated (or single) scratch smoothing as the technique and oxygen as the impurity. Copper was chosen as the material because the effects of orientation and anisotropy of the surface self-diffusion coefficient, D,, of copper in a dry hydrogen atmosphere had been studied extensively by Gjostein' and by Shewmon and ~hoi.~,~ The isolated scratch technique was chosen because both the effects of surface orientation and anisotropy of D, in a given surface could be easily studied with this method.~ Oxygen was tried as the impurity because Robertson and Shewmon""~ had studied its adsorption on copper at 1000°C over the range of oxygen partial pressures of 10"22 to 10-l3 atm. After several preliminary runs, it became evident that neither the scratch technique nor the impurity oxygen would be satisfactory for this work. Scratching deforms an annealed surface so that the region near the scratches recrystallizes, thereby disrupting the scratch profiles. One can avoid this by deforming the specimen sufficiently before scratching to give complete recrystallization on subsequent heating.4 However, as a result of Gjostein's success in scratching and annealing undeformed gold single crystals without local recrystallization,13 we attempted something similar with copper single crystals using a 0.7-mil diamond phonograph needle mounted in a Tukon hardness tester. All specimens recrystallized upon being annealed. Also, some copper specimens were sent to Gjostein to be scratched using his technique. The results were the same. As a result, the scratch technique was dropped in favor of the grooving of symmetric grain boundaries. Preliminary work using oxygen showed that faceting began to occur before oxygen adsorption had any measurable effect on D, at 938°C (at Pbo/P, = 0.12). Since heavy faceting would interfere with the measurement, we decided to use a sulfur-containing atmosphere (H2S/H2). Work by Oudar and Benard' and Robertson" showed that sulfur absorbed on copper and that faceting was not observed.

Jan 1, 1967

By J. D. Livingston, H. E. Cline

Cu-Pb alloys in the vicinity of the monotectic composition have been directionally solidified under a high temperature gradient at rates up to 2 X l0-' cm per sec. Over a wide range of compositions, high growth rates yield a composite structure consisting of continuous rods of lead in a copper matrix. This range of compositions increases with increasing growth rate, in agreement with arguments based on the relative velocities of composite growth and the growth of copper dendrites or lead drops. The breakdown of the composite structure at slow growth rates is explained in terms of the relative interphase surface energies. The observed interrod spacings of the composite structure are large compared with the predictions of the Jackson-Hunt equations of eutectic growth. ThE directional solidification of many eutectic alloys produces fine composite structures of parallel lamellae or rods. There has been considerable interest not only in the fundamentals of this two-phase solidification process,'-3 but also in the interesting physical properties produced by such regular and aniso-tropic microstructures. Composite structures can be grown only over a limited range of composition, beyond which coarse primary dendrites of one phase appear. In organic eutec-tics, this composition range of composite structures has been shown to increase with increasing growth rate.7"10 These results were explained in terms of the relative velocities of composite (coupled) growth and dendritic growth. Although similar arguments should apply to metallic eutectics,11-13 suitable experimental results are lacking. In contrast to the work on eutectics, the directional solidification of monotectic alloys has received little attention. (The monotectic reaction is similar to the eutectic reaction, except that one of the resulting phases is a liquid, which subsequently solidifies in a separate reaction at a lower temperature.) Directional solidification of some monotectic alloys'4715 yields regular rodlike microstructures, whereas in other cases macroscopic separation of solid and liquid phases occurs.16 chadwick17 rationalized these results in terms of the probable relative magnitudes of the various interphase surface energies. A recent study of chill-cast Cu-Pb alloys18 revealed a fine rodlike microstructure in alloys near the monotectic composition. It was decided to investigate the directional solidification of such alloys, and to determine the range of composition and growth conditions yielding composite structures. The Cu-Pb system is convenient for such a study, both because it is simple metallurgically, with no compound formation and negligible solid solubilities, and because its basic properties are well-documented. Recent literature on the Cu-Pb system includes studies of bulk thermo-dynamic properties,'g surface energies,20"21 densi-ties,25 and diffusion constants.a6 A similar study of the directional solidification of Cu-Pb alloys was recently undertaken, independently, by Kamio and Oya." EXPERIMENTAL Alloys were prepared by melting 99.999 pct Cu and 99.999 pct Pb in a graphite crucible, stirring well, and pouring into a cold graphite mold. Rods 0.175 in. in diam were machined from the ingots. Alloy compositions studied ranged from 25 to 55 wt pct Pb. Samples were placed in graphite crucibles 5 in. long with 4 in. OD and 0.035-in. walls. They were melted under flowing argon in a vertical, two-zone. platinum -wound furnace. A voltage stabilizer was used to minimize fluctuations in input power. The narrow specimen diameter minimized convection. Directional solidification was achieved by driving the crucible downward into a +-in. hole in a water-cooled copper toroid. The toroid was located immediately below the narrow end zone of the furnace. The end zone was separately powered to maintain high local temperature. Therefore a high temperature gradient (approximately 300 deg per cm) was maintained in the specimen throughout the run. The crucible motion was screw-driven. and a wide range of drive speeds were available. The limited rate of heat removal caused a thermal lag in the specimens at high drive rates. However. temperature-time curves from thermocouples imbedded in a representative sample indicated that the average growth rate still approximately equaled the drive rate. Although the specimens were initially homogeneous, melting and re solidification redistributed the lead. producing composition variations of several percent along the specimen length. (During melting. lead melted first and ran down the sample surface. Rapid freezing tended to reproduce the resulting composition gr~dient, but slow freezing did not because a slow-moving interface tended to reject lead. as discussed later.) To determine local composition. ;-g samples were cut from regions exhibiting various microstructures and were chemically analyzed for lead content. Micrographs were taken on as-polished or lightly etched surfaces. Three-dimensional structure of the lead network was viewed with a scanning electron microscope after removal of some of the copper matrix with nitric acid. RESULTS Several different microstructures are observed, depending on composition and drive rate. Because melting and resolidification produced composition gradients, results are best presented in t&ms of final local composition, rather than initial or average composition. The ranges of local compositions and drive

Jan 1, 1970

By C. S. Giggins, F. S. Pettit

The oxidation of Ni-Cr alloys in 0.1 atm of oxygen has been studied at temperatures between 800" and 1200°C. For alloys with 30 wt pct or more Cr, continuous layers of Cr2O3 are formed during oxidation. In the case of alloys with chromium concentrations between approximately 5 to 30 wt pct, external scales of Cr203 are formed over grain boundaries whereas internal precipitates of Cr2O3 and external layers of NiO are formed at other areas on the alloy surface. When such conditions are present on the alloy surface, chromium diffuses laterally from those areas covered with a continuous layer of Cr2O3 to areas where a Cr2O3 sub scale exists and it is possible for the sub-scale zone to become separated from the alloy by a continuous layer of Cr2O3. Whether such a state will be attained depends upon the initial grain size of the alloy and the oxidation time. When the concentration of chromium in the alloy is less than 5 pct, Cr2O3 is formed internally both at grain boundaries and within the interior of grains and the alloy is covered with an external layer of NiO. MECHANISMS which describe the growth of oxide scales on nickel-base superalloys are complex and the effects produced by the various elements in these alloys on the oxidation behavior of superalloys are not clearly understood. In order to determine the influence of the different elements on the oxidation behavior of superalloys, it is first necessary to examine the oxidation properties of binary nickel-base systems which contain the principal elements present in the superalloys and then progressively more complex systems until compositions typical of the superalloys are attained. Chromium is present in virtually all nickel-base superalloys and the purpose of the present studies was to examine the selective oxidation of chromium in Ni-Cr alloys. The oxidation characteristics of Ni-Cr alloys have been extensively studied1-" to date principally as a result of the high oxidation resistance exhibited by some of these alloys. Ni-20Cr* has long been known *All compositions are given as wcight percent unless specified otherwise. to be oxidation resistant and is commonly used as resistance heating elements for service temperatures up to 1100°C. This alloy cannot be used for extended periods of time at higher temperatures because of the apparent reaction of the external scale with oxygen to form gaseous CrO3. In spite of the considerable work cited above some important aspects of Ni-Cr oxidation still remain unresolved. Virtually all of the previous studies agree that small additions of chromium to nickel, e.g., <10 wt pct Cr, result in increased oxidation rates as compared to that of pure nickel, whereas larger additions, e.g., 20 to 30 wt pct Cr, form alloys with substantially lower oxidation rates. The controversial aspects of the oxidation mechanisms for these alloys that still remain unresolved are as follows: 1) A description of the oxidation mechanism for the low chromium alloys. 2) A description of the oxidation mechanism for the high chromium alloys, particularly with respect to the composition of the external scale which results in the lower oxidation rates. 3) The specific alloy compositions at which the oxidation mechanism changes from that obtained for low chromium contents to that of the high chromium alloys and the reason for this transition. EXPERIMENTAL The Ni-Cr alloys listed in Table I were prepared from high purity metals by nonconsumably arc melting and casting as buttons. These alloys were then given a preliminary annealing treatment in argon at 815°C for 100 hr to promote homogeneity. Each button was cut into 0.250 in. thick sections that were subsequently cold-rolled to 0.050 in. thicknesses and annealed in argon at 815°C for 48 hr to provide a twinned, equi-axed grain structure. The grain size for these alloys was not uniform and the limits, within which the average grain size lies, are given in Table I for the single-phase alloys. All the alloys were single phase with the exception of the Ni4OCr alloy in agreement with the Ni-Cr phase diagram.&apos;&apos; Rectangular specimens were cut from the sheet to provide surface areas of approximately 2.5 sq cm. Exact areas were determined with a micrometer after surface preparation was completed. All of the specimens except the Ni-40Cr alloy and pure chromium were polished through 600-grit Sic abrasive paper, ultrasonically agitated in ethylene trichloride, rinsed with ethyl alcohol, and electro-polished. The specimens were electropolished in a 10 vol pct H2SO4 (conc), 6 vol pct lactic acid, methyl alcohol solution at 70" to 80°C for 2 min at a current density of 0.8 to 1.2 amp per sq cm. This electro-polishing procedure did not produce acceptable surfaces on the Ni-40Cr alloy nor on pure chromium and the oxidation properties of these materials were obtained for specimens polished through 600-grit Sic

Jan 1, 1970

By Yuri D. Tretyakov, Robert A. Rapp

The stoichiometry ranges ofNiOl+y and LiFe,O,-d were established by high-temperatwe electrochemical meas7rements in a stabilized-zirconia electrolyte cell. The results were consistent with doubly ionized cation vacancies in NiO,+y and interstitial lithium or iron ions in . The defect structure of the ternary ferrite was derived from the consideration of equilibration with respect to oxygen between the solid and the gas phase. The absolute magnitudes of defect concentrations were calculated. Pavtial molar enthalpies of oxygen in the compounds were calculated and interpreted in terms of the enthalpy of defect formation in these crystals. NICKEL oxide (NiO,,?) is a metal-deficient, p-type, extrinsic semiconductor whose properties are consistent with a structural model based on the presence of cation vacancies as the predominant ionic defect at sufficiently high temperatures and oxygen activities. A survey of previously reported conductivity studies and the presentation of some more recent conductivity measurements will be given in a later paper.&apos; The absolute magnitude of the equilibrium vacancy concentration in NiO has been reported from combined conductivity and thermogravimetric data of &apos; However, disagreement exists concerning the state of ionization of the nickel vacancies in NiO. Some authors3-&apos; have proposed that the predominant defects in NiO are singly ionized nickel vacancies and positive holes (h&apos;) formed by the reaction where, accorhng to the notation of Kroger and Vink,6&apos;7 Oq represents an oxygen ion on its normal lattice site. Other authors"27E have proposed that doubly ionized nickel vacancies (V{i ) and positive holes are predominant and are formed by the reaction One purpose of the present investigation was to establish the nature of the predominant defect in NiO,+? as well as its equilibrium concentration and thermo-dynamic properties at elevated temperatures and known oxygen activities. To accomplish this purpose, the coulometric titration of oxygen into and out of NiO was accomplished using a galvanic cell involving the calcia-stabilized zirconia electrolyte. The ternary oxide, lithium ferrite (LiFe50,-6) is ferromagnetic and has the inverse spinel structure LiFe,O,-d Thus, in the ideal stoichiometric XY204 lattice, Fe3 ions occupy one-eighth of the tetrahedral (A) sites, and i&apos; and Fe3&apos; ions share at random one-half of the octahedral (B) sites within the fcc sublattice of oxygen ions.g The structure and thermodynamics of the spinel structures have been comprehensively decribed.&apos;-&apos; A recent compilation of literature for lithium ferrite is also available. 15 Oxygen-excess LiFe50sis not expected to exist because both i&apos; and 17e3& ions are in their highest normal valency states (positive hole formation is not favorable). As will be discussed, equilibration of the ferrite crystal with oxygen of a surrounding gaseous phase will result in the introduction of equilibrium concentrations of ionic and electronic defects. In the Results and Discussion section a defect model for LiFe50,-d is proposed. This defect model is tested by high-temperature coulometric titration experiments. EXPERIMENT The oxygen activities in nonstoichiometric NiOl+, (and also LiFe,O,-d) were measured by means of coulometric titration with the high-temperature galvanic cell The critical characteristic of a suitable experimental cell is the complete isolation of the phase to be investigated in a minimum sized chamber which is free from extraneous sources and sinks for oxygen (leakage). Then oxygen is only admitted to the chamber or removed from it in known amounts by coulometric titration, which involves the passage of oxygen ions through the solid electrolyte with electrochemical oxidation and reduction reactions at the platinum contacts to the electrolyte. The experimental cell is shown in Fig. 1. The cap of the cell was the tip from a closed-end alumina tube, which was found to be leak-free from a helium leak-detector test. This alumina cap was about 1.2 cm OD and about 1.5 cm high. A Zircoa calcia-stabilized zirconia tablet (crucible lid) of 1.5 cm diam and 0.3 cm thickness served as the solid electrolyte. A Pyrex ring of about 0.1 cm thickness was placed between the electrolyte and the cap. The electrolyte tablet was painted with platinum paste on the entire outer face and on that part of the inner face which would be within the enclosure; these electrodes were further prepared by heating in air at 1000°C for 2 hr. Nickel oxide powder, listed as 99.999 pct pure, was purchased from Leico Industries, Inc. Pills of the NiO were cold-pressed and sintered at 1050°C for 3 hr in a platinum crucible. This poorly sintered NiO was crushed, and chunks were wrapped in 52-mesh Pt gauze (to catalyze the solid-gas exchange) for place-

Jan 1, 1970

By Drew GJ

Smelting is a process in which heat is applied to ores to produce metals. Impurities are removed by segregation in the slag, or converted to gases or sublimates and discharged up the flues. Mate- rials delivered into the smelter have generally been previously concentrated or processed at the mine, and the size of their particles, and also their chem- ical composition, greatly affect the smelting proc- ess. Some impurities are more difficult to remove than others, and are more objectionable in the finished product. The first metal mine in Australia was opened near Glen Osmond in South Australia in 1841, for silver-lead ore. It was followed by the Kapunda Mine for copper in 1844 and by the very signifi- cant Burra Mine, also for copper, in 1845 (Figure 1). A major part of their ores could not be exported profitably for smelting in England and Wales where the large smelters were then located. In addition to treating ores from Cornwall these also smelted ores from recently opened mines in Cuba, Chile, and Mexico. The Welsh smelting process (Figure 2) was developed over a period of years to utilise Welsh coal to treat copper ores particularly from Corn- wall. The first step consisted of a calcination to remove sufficient sulphur from sulphide ores so that subsequent melting operations would produce copper matte, a copper-iron-sulphide which melted at about 1050¦C. Subsequently refined to white metal with the addition of carbonate and oxide ores when these were available, and often through an intermediate step of blue metal, this matte was finally oxidised to pure copper, or as pure as it could be made in those days. The movement of metal towards greater purity was matched by a reverse flow of slag, carrying reducing quantities of copper, so that the slag rejected from the first melting process carried the minimum quantity of copper. Chemical analysis was virtually unknown. Materials were assayed in the laboratory by a small scale smelting process, and the purity of the final refined copper was judged b} the ease with Mhich it could be hammered into thin sheets.

Jan 1, 1987

By William G. Kegel

Apart from a usable product and good mining conditions, the greatest asset for a profitable coal mining organization is an effective mine maintenance program. The first step in achieving this is to have good mechanics-or men who want to be mechanics and are interested in knowing ,more about the design and control of the machines in their charge. A good mechanic&apos;s knowledge and ability result from ambition, curiosity, and determination. Starting with these qualities, the aspiring mechanic can launch his career by learning all he can about mechanics, electricity, and hydraulics. Though the path is rough, the end result is rewarding. Mining machinery probably receives as much, if not more, abuse as any other heavy equipment in use today. Considering the conditions under which much of the repair work must be done, it is essential that the mine operator has the best men and tools to perform the work. Mining machines, from their inception, have utilized the electric motor ; and down through the years of mechanization, this power source has undergone many changes to better adapt it to the job. The first motors used were direct-current (d-c) motors since d-c power was the most economical power source for mining at the time of early mechanization. These motors were large open-type machines which are seldom used in modern mining. Because of the mine environment, the d-c motor has been reduced in size to better suit the application in mining machines and is now manufactured with dust-tight or explosion-proof enclosures. The d-c motor is constructed with either (or both) series and shunt fields located on the stationary part of the motor (stator) and an armature coil wound on the rotating part of the motor (rotor). Power is supplied to the armature by brushes which are held stationary and make sliding contact with the armature. The brushes requires regular and careful maintenance because worn or defective brushes will cause heat or arcing which can destroy the motor. D-c motors can be used as d-c generators when mechanical power is supplied to them by the rotating

Jan 1, 1981

By Alexander R. Troiano, Taylor Lyman

Studies of the transformation of aus-tenite at constant subcritical temperatures have been numerous since the work of Davenport and Rain.&apos; Considerable information has been obtained on low-alloy steels and on high-carbon alloys containing undissolved carbide or graphite at the austenitizing temperature. Little is known of the isothermal transformation of aus-tenite containing in solution large amounts of both carbon and a single alloying element. In this investigation four steels containing one per cent carbon and with chromium in the range 3 to 9 per cent have been examined with the view to establishing the fundamental character of the isothermal reactions when all carbon and chromium are initially in solution in austenite. Experimental Procedure Alloys The chemical compositions of the steels investigated are given in Table I. The 4 per cent Cr alloy is a commercial steel. The other steels were made by the Carpenter Steel Co. as 50-lb., induction-furnace heats and rolled to 5/8 -in. round bars. Bars were softened at 650&apos; to 700°C. before the specimens were cut. Most of the lines on the transformation diagrams presented here have been placed on the basis of the microscopic examinatioll of partially reacted specimens, Thees specimens were approximately 1/2 by 1/4 by 1/8 in. ill size, specimens of the 9 per cent cr steel were held 10 to 12 hr, at 1200°to 1225°C. before quenching to the various subcritical temperatures. The other steels were held for 2 to 2 1/2 hr. at 12ooOC. before quenching. These treatments, carried out in an atmosphere of purified nitrogen, produced an austenite grain size larger than A.S.T.M. So. I, some grains being 1 to 2 mm. in diameter. There were a few undissolved carbide particles in some specimens of the 9 per cent Cr steel. It is believed, however, that any errors in the results due to incomplete solution of carbon and chromium in austenite are negligible; that is, austenite compositions may be considered as being those listed ill Table I. X-ray Methods Constituents were identified and mar-tensite axial ratios determined from Debye

Jan 1, 1945

By R. L. Jones, H. Conrad

The effects of gain size on the room-temperature tensile properties of commercial(A- 70)-purity and iodide-purity titanium were investigated by concurrent tensile testing and thin-foil electron transmission studies. The mechanical tests showed that the effect of grain size on the yield, flow, and fracture stresses could be expressed by relations of the Hall-Petch type. The magnitude of the grain size effect on the yield stress was larger for commercial-purity than for high-purity titanium, but in both materials it was intermediate between that typically observed for fcc and for bcc metals. The effect of strain on the gain size sensitivity of the flow stress was found to reflect the grain size dependence of strain hardening. The electron transmission study indicated that the tensile behavior could be accounted for in terms of the dislocation densities and distributions formed during deformation. It was concluded that the effect of grain size on strength could be attributed to a relationship between grain size and the dislocation densities generated in initiating and sustaining plastic flow, the difference in stress levels being due to strain hardening. The relationships between recrystallized grain size, uniaxial tensile properties, and dislocation substructure have not been explored in any detail for a titanium. With titanium and its alloys finding increasing applications in aerospace systems, such information has become essential. The research described in this paper had two objectives: first, to document the effect of grain size on the yield, flow, and fracture stresses of unalloyed a titanium by the tensile testing of recrystalliied specimens at room temperature; second, to attempt an interpretation of the mechanical test behavior in terms of the dislocation structures formed on deformation, as observed by transmission electron microscopy of thin foils prepared from deformed tensile specimens. The observations reported here are restricted to room-temperature deformation; the effect of temperature on the yield and flow stresses of a titanium and the athermal nature of the contribution of grain size to the flow stress have been discussed elsewhere.1"3 EXPERIMENTAL DETAILS Two purity grades of unalloyed titanium have been investigated: Crucible Steel A-70, a typical commer- cial-purity product, and a high-purity iodide grade supplied by the Foote Mineral Co. Chemical analyses for interstitial impurity contents are given in Table I, together with values for the total interstitial atom fractions in terms of oxygen equivalents, which were calculated by the method outlined previously.4 ,5 According to the manufacturer&apos;s nominal analysis, the A-70 grade also contains about 3700 ppm of Fe. The A-70 grade was supplied as 1/4-in.-diam centerless-ground rod. The iodide grade was supplied as crystal bar which was consolidated to approximately 1/4-in.-diam rod by a single, rapid, molten zone pass, using electron beam heating. The 1/4-in. rods were cold-swaged to 0.078-in.-diam wires and 2-in. lengths were recrystallized in a vacuum of -5 x 10-5 torr, giving grain sizes in the range 0.8 to 30 . The thermal treatments and resulting grain sizes are summarized in Table 11. Since there was in no case any obvious sign of a bimodal dis tribution of grain sizes, the grain size values quoted in Table I1 are those obtained from mean linear intercept measurements involving >I00 grains. For the two coarse grain sizes the measurements were made using conventional optical techniques. It was noted during these determinations that the etchants used (10 pct HF; 30 pct HN03; 60 pct H20 and 2 pct HF; 2 pct H202; 96 pct H20 were both tried) etched grain boundaries very nonuniformly and optical polarization effects had to be employed to reveal boundaries which might otherwise have been missed. It was found that this problem became more acute with the finer grain sizes as the usefulness of polarization diminished.

Jan 1, 1970

By T. G. Nilan

Steels containing 0.4 and 0.8 pet C have been transformed isothermally at pressures up to 34 kbuv. Decomposilion mechanisms are so intimately related to phase equilibvia that, as the equilibria shift under high pressure, the microstruclures of the decomposition producls change, maintaining at pressure the same correspondence between a given phase equilibrium and a given microstructure as a1 1 atm. A bainitic unicvo-structure occurs in these steels at high pressure that IS observed only at 1 aim abole 1.4 pct C. This strrrcl111,e 1s ruliorzalized in terms of the effect of pressure on the transformation to lower bainite. The pressuve depetlderlce of the kinetics of pearlile formation is descrihed by an absolute rule theory amalysis. The activation volume for the transformation is 7 cu cm per mlole, which is indicative of a phase-interface transformation-rate cotntrol mechanism. This fortmulation, when expressed in terms of known solule effects on the energelics of tvansformation, gives promise of explaining the effects of alloying elemenls on hnrden-abilily, particularly that of &apos;coball. KINETIC processes in solids are functions of the primary thermodynamic variables of state: composition, temperature, and pressure. Until the development of high-pressure techniques&apos; that permitted the generation of sustained high pressure at high temperatures, metallurgical studies of such reactions were largely limited to isobaric conditions at atmospheric pressure. The data so obtained, and theoretical deductions based on these data, were necessarily incomplete For the particular case of austenite decomposition, a knowledge of the phenomenology of the reaction in terms of all the state variables should lead to a clarification of the process. This deeper understanding should facilitate a decision between conflicting theories that have been proposed to rationalize these phe- nomena. An analysis, derived from an understanding of the transformation mechanisms, may aid in the optimization of alloy steel development. The first studies of the effect of pressure on transformations in steel were made by Kulin et al. in 1952.&apos; In a study of the influence of applied stress on mar-tensite formation in a 30 pct Ni steel, they found that the martensite start (M,) was depressed 8°C per kbar* by hydrostatic pressure. Jellinghaus and Friedewold3 were apparently the first to investigate the effect of pressure on isothermal transformations above the Ms. They found that the bainite transformation rate in a 1.2 pct C, 3.8 pct Mn steel was reduced by a factor of 3 by a hydrostatic pressure of 4 kbar. In both of these investigations,2,3 the M, of the steels studied was below room temperature. Austenitizing and quenching were done at atmospheric pressure, followed by the decomposition of the metastable austenite under pressure. The maximum temperature at pressure in the bainite study3 was 350°C. Until the advent of the present high-pressure devices, it was not possible to conduct isobaric high-pressure heat treatments from austenitizing through isothermal decomposition and quenching. The exploitation of high pressure as a variable in metallurgical studies was greatly advanced by the apparatus and techniques developed by Hall4 and co-workers. Utilizing these techniques, Radcliffe et a1.5 and Hilliard6 etermined the pressure dependence of phase equilibria in the Fe-C system. Hilliard and cahn7 examined the pearlite transformation rate in an AISI 1080 steel and also in a high-purity 0.92 C Fe-C alloy at 1 atm and at 34 kbar and found a 700-fold reduction in rate at pressure in the 1080 steel but only a fivefold reduction in the high-purity alloy. In agreement with the shift of phase equilibria under pressure, the microstructures were hypereutectoid at 34 kbar pressure, whereas they are eutectoid at 1 atm. Determination of the effect of pressure on carbon diffu-

Jan 1, 1968