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Advancing Materials Research Part 2 THE STATUS OF SELECTED SCIENTIFIC AND TECHNICAL AREAS
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Advancing Materials Research This page in the original is blank.
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Advancing Materials Research Progress and Prospects in Metallurgical Research MORRIS COHEN The idea of interrelationships among processing, structure, property, and performance in materials—a concept that now forms the backbone of modern materials science and engineering1—began to take shape in metallurgy well over a century ago with the advent of the metallographic microscope.2 Processing, properties, and performance of metals and alloys had been known to mankind in one way or another for millennia, but the dawn of metallurgical science as we view it today might well be identified with the emergence of microstructure. That scientific event forged a connecting link to the practice of metallurgical engineering and technology, and all these elements progressively coalesced into what has become the discipline of metallurgy. Accordingly, to highlight some of the significant accomplishments and anticipations of metallurgical research, we shall regard metallurgy as the science, engineering, and technology of metallic materials. In other terms, it covers the study, production, manipulation, and use of metals and alloys; it is the part of materials science and engineering that not only inquires into the nature of metallic materials but also attempts to harness such knowledge for societal purposes. The following summary reflects the diverse inputs of some 80 contributors in metallurgical research whose multifaceted cooperation in this task is gratefully acknowledged. To the extent possible, the numerous suggestions and viewpoints have been unilaterally blended into topical themes, specifically selected because of their promise for new ferment in metallurgical research. It should be emphasized at the outset that virtually all phenomena in metallurgy are incompletely understood, but this is likewise the case for most aspects of any science or technology. Indeed, arguments can be made for needing further research and insight on just about any identifiable subject in
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Advancing Materials Research materials science and engineering. Nevertheless, some gaps in metallurgical knowledge are long-standing and costly to society. For example, the damage from metal corrosion and other types of failure in service amounts to more than $200 billion per year in 1982 dollars,3 a loss that is commensurate with the annual federal deficit! One may certainly wonder why the nation puts up with this appalling extravagance. The probable answer is that metals, as reflected in Figure 1,4 exhibit combined ranges of properties not enjoyed by other classes of materials (in stiffness, strength, toughness, and thermal FIGURE 1 Some important properties of engineering materials, arranged to compare metals, ceramics, and polymers. For the indicated normalized toughness, G is in units of J/m2, E is Young’s modulus in corresponding units, and a is the atomic radius. From Ashby.4
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Advancing Materials Research FIGURE 2 Relationship between through-thickness ductility of steel plate and the sulfur content. From Billingham.5 Reprinted with permission. characteristics, as well as in economic availability) that make them incredibly useful to humanity. In a similar vein, society seems to “accept” the loss of some 50,000 lives per year on U.S. highways because automobiles are so useful. Metals, like automobiles, have become engrained in our way of life; neither should be taken for granted. PROCESSING Every step in the materials cycle involves processing. In metallurgy, processing comprises all operations that produce, shape, and control the properties of metallic materials to make them perform effectively in service. Metal processing ranges from large scale, as in steel production and refining, to small scale, as in thin-film formation. Steel Refining Advances in steel refining, particularly through ladle metallurgy involving the injection of calcium, rare earths, and fluxes as well as vacuum degassing, have led to remarkable improvements in compositional control and steel cleanliness. Figure 2 shows the pronounced increase in through-thickness
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Advancing Materials Research ductility of plate steels that results from the reduction of sulfur content and, hence, of sulfide inclusions.5 This development, growing out of investigations on the multicomponent thermodynamics of sulfur in liquid steel, has spawned the reliable use in pipeline steels and oil-drilling rigs of heavy sections resistant to laminar tearing. The achievement of such metallurgical control in liquid steel is all the more prodigious when one considers the 100- to 200-ton scale of the ladle-refining operations. Of comparable importance is the potential removal of phosphorus. Rare-earth additions have promise in this respect6 and deserve the kind of research attention that has been given to calcium in its affinity for sulfur. Phosphorus increases the sensitivity of high-strength steels to embrittlement, and its elimination would constitute a major advance in ferrous metallurgy. Controlled Rolling of Steel An example of solid-state processing that has reached commercial scale in steelmaking is the controlled rolling of steels that are microalloyed with relatively stable carbide- and nitride-forming elements such as niobium, vanadium, and titanium. Here, the precipitation of carbonitride particles is induced by plastic deformation of the austenitic phase during the rolling schedule, and recrystallization of the austenite is then inhibited because the grain boundaries are pinned by the precipitated particles. Further rolling to low finishing temperatures (but still substantially in the austenitic range) leads to a flattening of the austenitic grains, the thin dimension of which contributes to a very fine-grained ferritic structure on further cooling. The resulting grain refinement, thus attained without the expense of separate heat treatment, contributes both strengthening and toughening, as shown in Figures 3 and 4.7,8 This kind of double benefit in structure-property relations is an unusual circumstance in alloy systems, and it forms the basis of the high-strength, low-alloy (HSLA) steels that have been developed during the past quarter century. An additional advantage of these steels is that their beneficial properties are obtainable with low carbon contents and therefore are compatible with excellent welding characteristics for construction purposes. One measure of the technological success of HSLA steels is their increasing utilization in automobiles, despite the general downsizing for fuel efficiency. According to Figure 5, the amount of HSLA steel used per vehicle has been increasing even more rapidly than the lower-density aluminum alloys, plastics, and composites.9
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Advancing Materials Research FIGURE 3 Strengthening of low-carbon steel with decreasing ferritic grain size (Hall-Petch relationship). Additional strengthening contributions are illustrated schematically for solid-solution, substructural, and precipitation effects. From Baird and Preston.7 Reprinted with permission. Rapid Solidification Processing The pioneering publications of Pond10 in 1958 and Klement, Willens, and Duwez11 in 1960 ushered in a dynamic era of research and development on rapid solidification processing (RSP). The major part of this research has been directed to metallic materials, although the phenomena at play are FIGURE 4 Toughness (Charpy impact test) of two low-carbon steels versus test temperature; comparison of plain-carbon hot-rolled steel with microalloyed controlled-rolled steel. From Hansen.8
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Advancing Materials Research FIGURE 5 Increasing automobile use (actual and projected) of certain materials, including high-strength, low-alloy steels, in the United States, despite the overall trend toward downsizing. From Materials Modeling Associates.9 operative in other classes of materials as well. Cooling rates up to 109 K/s have been reported, but most of the basic effects are observed with cooling rates of 104 to 106 K/s attained by splat quenching, melt spinning, planar-flow casting, atomized droplet solifidication, or self-quenching after surface melting. The corresponding solidification rates, in terms of liquid-solid interfacial velocities, can range up to tens of meters per second, in contrast to about 1 cm/s for a typical mold casting. The microstructures resulting from RSP are characterized by enhanced compositional uniformity, refinement of the microconstituents, high degrees of supersaturation, and retention of metastable phases including metallic glasses. The aluminum-manganese phase with anomalous fivefold symmetry12 (see Cahn and Gratias, in this volume) is a startling example of metastability brought to light by RSP. The existence of such a pentagonal structure is distinctly forbidden by long-standing crystallographic theory, thus posing a fundamental dilemma. One rationalization is to regard the strange structure as quasi-periodic instead of truly periodic. The retention of metastable glassy states in certain alloy systems by RSP offers a classic example of a novel processing method that has paved the way to new regimes of structure, property, and performance relationships. For melt compositions whose glass transition temperatures are about half the respective melting points, a cooling rate of approximately 106 K/s is sufficient to avoid crystal nucleation and thus allow glass formation. Metallic glasses
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Advancing Materials Research are typically high in strength but are not “glass-brittle.” They tend to undergo localized shear (due to lack of strain hardening) when stressed beyond the yield strength, and thus they resist fracture even though they are not generally deformable like crystalline alloys. The metallic glasses based on metal (Fe, Ni, Co) and metalloid (B, Si, P, C) combinations are of special interest in view of their markedly low magnetic losses, and the processing is potentially inexpensive because of direct casting of the alloy liquid to final strip form. The technological impact of this metal-processing development is discussed below in the section on magnetic alloys. The extensive refinement of dendritic structures caused by increased cooling rate or growth velocity during the solidification of crystalline alloys is shown by Figure 6.13 The reduction in dendritic-arm spacing not only improves the as-cast strength and ductility14 but also promotes compositional uniformity by decreasing the diffusion distances between the regions of microsegregation formed by solute buildup in the last pockets of liquid to solidify between the dendritic arms. Such compositional uniformity is advantageous in raising the incipient melting temperature of alloys intended for high-temperature service, as in the case of superalloys. At the same time, second phases that are likely to precipitate in the microsegregated regions tend to be finer in size and more uniformly distributed because of the rapid solidification. These second phases often appear as intermetallic compounds or nonmetallic inclusions that are embrittling when present in coarse or segregated form but that can be desirable when finely divided and well dispersed. A beneficial consequence of uniform dispersions is that they pin grain boundaries of the matrix phase and thus inhibit grain growth, as shown in Figure 7.15 The effectiveness of this phenomenon is an inverse function of d/fv, where d is the average diameter of the precipitated particles and fv is their volume fraction. This means that for the grain boundary pinning to persist at very high temperatures, the distributed phase must be sufficiently FIGURE 6 Dendritic-arm and microsegregate spacing in as-cast aluminum alloy microstructures as a function of cooling rate from the liquid state. From Cohen, Kear, and Mehrabian.13 Reprinted with permission.
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Advancing Materials Research FIGURE 7 Grain growth characteristics of high-sulfur austenitic stainless steel after conventional processing versus rapid solidification processing. The latter was produced by centrifugal atomizing and subsequent consolidation by hot extrusion. From Kelly and Vander Sande.15 Reprinted with permission. stable (minimal solubility in the matrix phase) to resist Ostwald ripening. Such phases are generally more soluble in the liquid state (e.g., oxides, silicates, and oxysulfides in liquid steel) and are then amenable to uniform dispersal as fine precipitates in the solid state by RSP. The resulting resistance to grain growth permits the consolidation of rapidly solidified particulates and subsequent heat treatment to be carried out at relatively high temperatures without undue grain coarsening. Moreover, with larger-volume fractions of stable second phases, including appropriate intermetallic compounds, dispersion strengthening can be achieved and maintained at elevated temperatures. An example of second-phase refinement by RSP is shown in Figure 8.16 Here the addition of 10 weight percent beryllium to a commercial aluminum-copper alloy leads to extremely coarse second phases upon normal ingot solidification, whereas RSP produces a very fine dispersion of the second phases. In this instance, as much as 10 weight percent beryllium can be dissolved in the liquid alloy and then precipitated by rapid solidification as a finely distributed beryllium-containing compound (approximately Be3Cu) in the solid alloy, with the maximum equilibrium solubility of the beryllium in solid aluminum being only about 0.03 weight percent. The corresponding improvements in strength and ductility given in the caption of Figure 8 are also worthy of note. The large beryllium addition made possible by RSP is of further potential value because it increases the stiffness and decreases the density of aluminum-base alloys.16 Figure 9 compares the high-temperature strength of RSP aluminum alloys with that of conventionally processed high-strength aluminum alloys.17 For these studies, the compositions of the RSP alloys were specially developed
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Advancing Materials Research FIGURE 8 Microstructure of aluminum alloy (No. 2219) plus approximately 10 wt % beryllium (a) after normal casting and hot extrusion (yield strength 43.3 ksi, tensile elongation 1.7 percent), and (b) after RSP by melt spinning and hot extrusion (yield strength 43.3 ksi, tensile elongation 15 percent). From Vidoz et al.16 Reprinted with permission. to take advantage of RSP; the intermetallic phases involved would be much too coarse and embrittling if they were to form during regular solidification. The strength retention exhibited by the RSP alloys at elevated temperatures is quite striking and is beginning to match or exceed the density-compensated strength of titanium alloys over the temperature range studied. Surely the time is now ripe to apply RSP to titanium alloys and possibly even to niobiumbase alloys. The RSP aluminum alloys are also unusually resistant to corrosion, ac-
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Advancing Materials Research FIGURE 49 Semicoherent interface for face-centered cubic to body-centered cubic martensitic transformation. The a/18 and a/16 dislocations (designated by light Ts) act to maintain coherency between the parent and product phases and accomplish the transformation during their motion by a lattice deformation, whereas the a/2  and a/2 dislocations (designated by heavy Ts and circles) are misfit dislocations that interrupt the coherency and accomplish a lattice-invariant deformation of the resulting semicoherent particle to relieve the long-range stress fields. All of the dislocations are in the form of loops around the particle. From Olson and Cohen.91 Reprinted with permission. role in the overall kinetics of martensitic transformations, particularly in the heat treatment of steel. In other words, although preexisting nucleation sites are essential for the start-up and early progress of martensitic transformations in ferrous alloys, their density is insufficient without the ongoing generation of new sites to achieve the technological hardening of steel. The description of martensitic interfaces in the form of dislocation arrays makes it possible to deal with the growth problem in terms of dislocation mobilities under the operating driving-force and obstacle conditions. The nature of such calculations is presented in the next section; and in a later section on displacive diffusional transformations, it is shown how these ideas can be carried over to other phase transformations, including precipitation processes, in which lattice deformation and diffusion participate simultaneously. This establishes a spectrum among phase transformations that have previously been regarded as different in kind. Martensitic Growth For convenience of discussion, the propagation of martensitic interfaces can be divided into two categories: thermoelastic transformations, wherein the shape deformation attending the phase change is accommodated elastically by the alloy system; and nonthermoelastic transformations, wherein the shape deformation is accommodated plastically. In the former, a thermoelastic force
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Advancing Materials Research balance prevails between the driving and retarding forces, and the interface can be made to move progressively and measurably under well-controlled experimental conditions. The interfacial mobility of thermoelastic copper-aluminum-nickel martensites has been investigated comprehensively in this way.93 It is found that the effects of temperature and applied stress on the interfacial velocity are consistent with the concept of thermally activated interfacial motion, as suggested by the kinetic interrelationships in Figure 50.93 Just as in the case of mechanically driven slip dislocations, the driving force for interfacial motion has both athermal (τµ) and thermal (τf—τµ) components; similarly, activation energies and activation volumes are derivable from the experimental data, and specified models of interfacial structure can be tested against various modes of lattice friction and obstacles to growth.93 Nonthermoelastic martensites tend to propagate rapidly, in some instances even approaching the velocity of a shear wave in the parent phase. These transformations are difficult to nucleate and hence are effectively “overdriven” when the martensitic growth process sets in. Dissipation of the latent heat of transformation then becomes a complicating factor and must be taken into account. This has been done for the calculated interfacial-velocity versus particle-size curves plotted in Figure 51.94 During the acceleration stage, the interfacial motion is assumed to be thermally activated at the lower velocities FIGURE 50 Interrelationships among interfacial driving stress, thermodynamic driving force, temperature, and interfacial velocity for martensitic growth in a copper-13.9 wt % aluminum-3.9 wt % nickel alloy. The operative driving force is calculated from the resolved mechanical stress (τf) which drives the moving interface; τµ is the athermal component of the driving stress. From Grujicic, Olson, and Owen.93 Reprinted with permission.
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Advancing Materials Research FIGURE 51 Calculated radial-growth velocities of disk-shaped martensitic particles in two Fe-Ni alloys at their respective martensite-start temperatures; thermal changes due to heat-transfer effects are taken into account. From Haezebrouck.94 and phonon-drag controlled at the higher velocities. The marked differences in martensitic growth characteristics between the two iron-nickel alloys in Figure 51 originate from differences in the respective dynamic flow stresses and drag coefficients.94 However, not yet incorporated into these calculations is the quantitative effect of local plastic accommodation, including the associated defect formation and strain hardening, on the resistance to interfacial motion. This is a crucial issue, not only because of the combination of dynamic phenomena at play here but because it may be forcing a general study of the no-man’s-land between continuum and discrete-lattice solid-state science. It is somewhat reminiscent of the crack-tip/plastic-zone problem in fracture mechanics. Furthermore, the plastic accommodation process may hold the key to a deeper understanding of the autocatalytic nucleation that enters so significantly into the overall kinetics of nonthermoelastic martensitic transformations. Transformation Plasticity and Toughening The shear-like displacive nature of martensitic transformations allows these phase changes to operate as a deformation mechanism in parallel with ordinary slip processes.95 Applied stress can promote the formation of martensite by contributing to the transformational driving force and by introducing new nucleation sites through accompanying plastic flow. At the same time,
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Advancing Materials Research the acting stress will favor the formation of those martensitic crystallographic orientations that have optimal displacive components for “yielding” to the stress. The resulting transformation-induced plasticity (TRIP) reflects a novel interplay between the kinetics of a structural change and macroscopic stress-strain behavior. Indeed, there are now cases in which constitutive relations involving transformation plasticity have been derived to predict flow stress as a function of strain, strain rate, temperature, and stress state.95 In austenitic steels having appropriate deformation-induced transformation characteristics, the uniform ductility in a tensile test has been increased about fivefold beyond that of the untransformed parent phase.96 Transformation plasticity offers an attractive mechanism for enhancing fracture toughness at high-strength levels by coming into play in the vicinity of an advancing crack, particularly in alloys that undergo shear instability before rupture. In such instances rather small amounts of mechanically induced transformation occurring in the plastic zone of a crack tip can delay the impending strain localization and thereby increase the fracture toughness substantially. Two examples are shown in Figure 52 for high-strength austenitic steels as a function of normalized test temperature.96 The maximum toughness values correspond to about KIc=250 MPa m1/2 at—70°C, which is very tough for these 1300-MPa yield-strength steels. The beneficial effect on sharp-crack toughness arises not only from the transformation plasticity as such but also from a reduction of the triaxial stress state because of the volume expansion that attends the phase change. The decrease in fracture toughness at lower test temperatures in Figure 52 is caused by the formation of too much martensite, which in itself is less tough than the parent phase. Hence, the eventual use of transformation toughening will require compositional modifications to decrease the temperature sensitivity of the transformation—e.g., by decreasing the entropy change of the transformation in order to reduce the temperature dependence of the thermodynamic driving force. Manganese is known to affect the transformational thermodynamics in that manner. It is conceivable that the greatest potential for transformation toughening lies in its applicability to the retained austenite in martensitic steels. This is an unexplored field that warrants intense study, inasmuch as the technological use of ultrahigh-strength steels could be materially advanced even by modest increases in toughness. Displacive-Diffusional Transformations One can imagine a potential martensitic transformation under conditions where the driving force is not large enough for nucleation or interfacial motion to ensue. But if some compositional partitioning is permitted at the temperature in question, the driving force may then be sufficient for the phase
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Advancing Materials Research change to proceed. The velocity of the interface, whether coherent or semicoherent, will depend on diffusion as well as on the resistance to interfacial motion, and this interplay can result in some degree of solute trapping which, in turn, will determine the extent of partitioning and diffusion taking place. It is also possible that a steady-state balance in the functioning of these phenomena may be attained such that there is just enough diffusion to supply the necessary force for overcoming the resistance to interfacial motion. This type of phase transformation is, strictly speaking, neither displacement-controlled nor diffusion-controlled; rather, it represents a coupled displacive-diffusive mechanism of interfacial motion in which both lattice deformation and diffusion participate jointly and interactively. The above transformational characteristics begin to take on some of the displacive-diffusional features of bainitic transformations, allowing for precipitation processes in the product phase behind the advancing front.97,98 This theory of bainite formation has not been definitively tested as yet, but it surely would be worth doing so, especially in view of the favorable me- FIGURE 52 Fractional increase in J-integral fracture toughness as a function of temperature normalized according to martensitic transformation characteristics of two Fe-Ni-Cr alloys. During test, no transformation takes place at temperatures above Md—i.e., fracture toughness is that of the parent austenitic phase ; plastic-strain-induced transformation takes place between Md and ; and elastic-stress-assisted transformation takes place below . From Leal.96
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Advancing Materials Research FIGURE 53 Schematic illustration of ∊-carbide precipitation via an invariant-plane strain in compositionally modulated martensite in an iron-15 wt % nickel-1 wt % carbon alloy. Arrows indicate displacement directions in the martensite and in the carbide. The designated internal shear plane in the carbide is parallel to the carbide basal plane. From Taylor.100 chanical properties of bainitic microstructures being reported for high-strength, low-alloy steels.99 However, there appears to be an even broader significance to the concept of displacive-diffusional transformations in that some typical precipitation reactions display both shape and compositional changes. It has now been demonstrated that the well-known precipitation of ∊-carbide from ferrous martensites falls in this category, as shown schematically in Figure 53.100 These martensites first develop a compositionally modulated structure of coherent high- and low-carbon bands on aging near room temperature, as discussed earlier in the section on modulated structures, and then the ∊-carbide precipitates with an invariant-plane strain, plate-like morphology composed of both homogeneous lattice and inhomogeneous internal deformations, comparable to a martensitic transformation except that carbon diffusion is also required to form the ∊-carbide. The precipitation is heterogeneous, tends to nucleate along the modulated-band interfaces, and adopts a (012) habit that is close to the (023) habit of the modulated bands. In view of the lattice correspondence between the precipitated phase and its surroundings, the carbide interface is undoubtedly semicoherent. A similar type of displacive-diffusional interpretation has been applied to the precipitation of hydrides in vanadium and zirconium solid solutions.101 It may be turning out that precipitation processes in general, to the extent that coherent or semicoherent interfaces are involved, combine both lattice-distortive and diffusion-related phenomena (although not necessarily coupled and not necessarily shear-like) with different degrees of kinetic control. In this sense, precipitation reactions are likely to comprise a spectrum of phase transformations in which martensitic transformations constitute only a very special (diffusionless) case. This possibility offers new incentives and guidelines for future research on phase transformations.
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Advancing Materials Research CONCLUSION There are many innovative areas of metallurgical research not treated in this review that should be recognized as significant opportunities for important advances in metallurgy. Among these are the electromagnetic handling of liquid metals, sensors and techniques for on-line process control, in situ observations of fast events, hydrogen embrittlement, atomic-bonding calculations, and computer modeling of processes and phenomena. In this chapter we have attempted to integrate various viewpoints and suggestions, received mainly from the metallurgical community, into a valid perspective on some notable accomplishments of modern metallurgical research and on the sustained intellectual excitement that is inspiring further research on this vital class of materials. Metals not only represent a large majority of the elements in the periodic table and therefore constitute a pervasive part of the natural world, but they are generally useful to mankind and therefore invaluable to society. This confluence gives rise to a persistent driving force for probing metals ever more deeply and for using them ever more wisely. It is both a lofty challenge and a proud responsibility for metallurgical science and engineering. ACKNOWLEDGMENTS I am extremely grateful for the outpouring of ideas and opinions, both oral and written, that came to me in response to my informal requests for advice on many metallurgical fronts. The resulting communications grapevine operated with amiable efficiency and goodwill. In instances where the submissions could not be included, I am no less appreciative of the cooperation and involvement. I am also deeply indebted to Marguerite Meyer, Miriam Rich, and John Mara at MIT, who helped so much in the preparation of the manuscript, and to NAE Fellow Dr. Peter Psaras, who waited so patiently to receive it. NOTES 1. Committee on the Survey of Materials Science and Engineering, Materials and Man’s Needs, Summary Report of the Committee on the Survey of Materials Science and Engineering (National Academy of Sciences, Washington, D.C., 1974). 2. H.C.Sorby, Report of the 34th Meeting, British Association for the Advancement of Science (Bath, 1864), Part II, p. 189. 3. J.H.Payer, D.G.Dippold, W.K.Boyd, W.E.Berry, E.W.Brooman, A.R.Buhr, and W.H.Fisher, The Economic Effect of Corrosion in the United States (Battelle Columbus Laboratories, 1977); J.J.Duga, W.H.Fisher, R.W.Buxbaum, A.R.Rosenfield, A.R. Buhr, E.J.Honton, and S.C.McMillan, The Economic Effects of Fracture in the United States (Battelle Columbus Laboratories, 1983).
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Representative terms from entire chapter: