Volume 09 - Metallography and Microstructures Part 14 pot

Fig. 26 Graphite- epoxy composite (Thornel 300 graphite fibers in 5208 epoxy matrix), unidirectional. Standard autoclave cure of lay- up made from prepreg tape. Scanning electron micrograph of a failed tensile surface, which is tiered, as is typical of this particular graphite epoxy. Considerable resin adheres to the fibers, indicating good interfacial bond strength. 500×. (P.R. Lee) Fig. 27 Graphite-PPS composite (AS4 graphite fibers in thermoplastic polyphenylene- sulfide matrix), film stacked and hot pressed. Scanning electron micrograph of a failed tensile surface. The high ductility of the thermoplastic matrix results in the "tails" of drawn matrix materials shown. 500×. (A.C. Lou) Fig. 28 Graphite-Al composite (Thornel 50 fibers in 6061 Al matrix), unidirectional. Liquid- metal infiltration and consolidation in a liquid-phase press. SEM of a failed surface. The older version (~ 1973) of this graphite fiber, with its crenelated cross section, has debonded and pulled out of the Al matrix. Pull- out holes are also seen. 1000×. (L.W. Davis) Fig. 29 B-Al composite (25% B fibers in 6061 Al matrix), unidirectional. Fiber on foil and diffusion bonded. SEM of a fairly flat, failed tensile surface, characteristic of this material. Little matrix adheres to the fiber surface, indicating fairly low interfacial bond strength. The Al matrix shows good ductility. The tungsten cores of the vapor-deposited fibers are evident. 105×. (R. Moss) Fig. 30 Graphite-silver copper composite (Thornel 300 fiber in 70Ag- 30Cu eutectic matrix), unidirectional. Liquid- metal infiltration of fiber bundles followed by diffusion bonding. Scanning electron micrograph of a failed tensile surface. The matrix shows good ductility, but the lack of matrix adhering to the fibers (~ 1977 version) indicates low interfacial bond strength. 3000×. (W.C. Harrigan) Fig. 31 Silicon carbide-glass ceramic comp osite (unidirectional silicon carbide fibers in glass ceramic matrix). Ply lay- up and hot press densification. Scanning electron micrograph of a failed tensile surface. Although the ceramic matrix has failed in a brittle manner, the long pull-out length of the fibers indicates high composite toughness. 20×. (K.M. Prewo) Introduction to Structures in Metals Michael B. Bever, Professor of Materials Science and Engineering, Emeritus, Massachusetts Institute of Technology Introduction FOR MORE THAN A CENTURY, dating back to the pioneering contributions of Henry Clifton Sorby, metallurgists have not been satisfied merely to describe their metallographic observations, but have striven to explain them and to understand their implications (Ref 1, 2, 3, 4). In addition, new techniques of structural investigation have yielded new observations and posed new problems. The quest for meaningful and precise explanations of metallurgical structures has been the primary driving force in the development of the science of physical metallurgy (Ref 5, 6, 7, 8). Physical metallurgy now comprises a very broad spectrum. That portion of the spectrum dealing with the structure of metals is the subject of the articles in this Section. This article will develop the sequence in which the articles in this Section are presented and establish some connections among them, provide background for the subject matter explored more fully in the specialized articles, and furnish general references. It will also treat important topics, such as grain structure and substructure, that are not covered systematically and comprehensively in the other articles. Finally, it will describe the scale of structural features and introduce the concept of hierarchical relations among them. The term structure, as used here, refers primarily to the study of those microstructural features that can be investigated using optical (light) and electron microscopy (Ref 9, 10, 11, 12, 13, 14, 15, 16). The results of investigations using other techniques, such as x-ray diffraction, are included when pertinent (Ref 17, 18). Macrostructural features, which can be observed with little or no magnification, will also be considered. The purpose of the articles in this Section is to assist in the interpretation of microstructure. Such interpretation requires an understanding of the processes by which various structures are formed; therefore, the articles are organized according to the major processes that produce characteristic structures. A special article describes textures that can result from several of these processes. The principles applicable to various types of structures are illustrated by micrographs in the respective articles; references are also made to micrographs that appear in the Sections "Metallographic Techniques and Microstructures: Specific Metals and Alloys" in this Volume. Several works that treat the interpretation of microstructures systematically are cited in Ref 9, 10, 11, 12. Acknowledgement The author gratefully acknowledges contributions by several of his colleagues to this article, particularly Professor Samuel M. Allen. References 1. R.F. Mehl, A Brief History of the Science of Metals, American Institute of Mining and Metallurgical Engineers, Warrendale, PA, 1948 2. C.S. Smith, A History of Metallography, University of Chicago Press, 1960 3. C.S. Smith, Ed., Sorby Centennial Symposium on the History of Metallurgy, Gordon and Breach, 1965 4. R.F. Mehl and R.W. Cahn, The Historical Development of Physical Metallurgy, in Physical Metallurgy, Part I, 3rd ed., R.W. Cahn and R. Haasen, Ed., North-Holland, 1983, p 1-35 5. R.W. Cahn and P. Haasen, Ed., Physical Metallurgy, Parts I and II, 3rd ed., North-Holland, 1983 6. A.G. Guy and J.J. Hren, Elements of Physical Metallurgy, 3rd ed., Addison-Wesley, 1974 7. W.F. Smith, Structures and Properties of Engineering Alloys, McGraw-Hill, 1981 8. R.E. Smallman, Modern Physical Metallurgy, 4th ed., Butterworths, 1985 9. R.H. Greaves and H. Wrighton, Practical Microscopical Metallography, 4th ed., Chapman & Hall, 1957 10. H. Gleiter, Microstructure, in Physical Metallurgy, Part I, 3rd ed., R.W. Cahn and P. Haasen, Ed,, North- Holland, 1983, P 650-712 11. G.F. Vander Voort, Metallography: Principles and Practice, McGraw-Hill Book Co., 1984 12. W. Rostoker and J.R. Dvorak, Interpretation of Metallographic Structures, 2nd ed., Academic Press, 1977 13. J.W. Edington, Practical Electron Microscopy in Materials Science, Van Nostrand Reinhold, 1976 14. P.J. Goodhew, Electron Microscopy and Analysis, Wykeham Publications, 1975 15. M.H. Loretto and R.E. Smallman, Defect Analysis in Electron Microscopy, Chapman & Hall Halsted/Wiley, 1975 16. G. Thomas and M.J. Goringe, Transmission Electron Microscopy of Materials, John Wiley & Sons, 1979 17. C.S. Barrett and T.B. Massalski, Structure of Metals, 3rd ed., Pergamon Press, 1980 18. B.D. Cullity, Elements of X-ray Diffraction, 2nd ed., Addison-Wesley, 1978 General Features of Structure The structure of metals comprises features of various magnitudes. Major structural features, listed generally in increasing size, are: • Atomic structure: nuclei, atoms • Electronic structure • Crystal structure: perfect crystals, crystal imperfections • Substructure: subgrains, other cellular structures • Microstructure: grains of single-phase metals and alloys, shapes and sizes of micro- constituents and their configurational arrangements in multiphase systems • Textures • Structural features related to composition • Structural gradients • Porosity and voids • Macrostructure The structure of nuclei and atoms and the electronic structure are beyond the scope of this Volume, but are covered in texts on general physics and in specialized presentations (Ref 19, 20, 21). Some texts apply the fundamentals of crystallography to metals (Ref 22, 23). Crystal structures often found in metallic phases are listed and described in the article "Crystal Structure of Metals" in this Section. Crystal imperfections include point defects, such as impurity atoms, vacancies and vacancy aggregates, and interstitial atoms; line defects (dislocations); and area defects, for example, stacking faults, twin interfaces, subboundaries, and grain boundaries. They are described in specialized texts on the theory of dislocations and other crystal imperfections (Ref 24, 25, 26). Examples of various crystal defects are presented throughout this Volume. In the article "Transmission Electron Microscopy," dislocations (Fig. 21 to 24 and 26 to 29), dislocation dipoles (Fig. 30), dislocation net-works (Fig. 31 to 33), and dislocation loops (Fig. 34 to 36) are shown. Dislocations are also shown in Fig. 3 to 6, 11, and 12 in the article "Solidification Structures of Pure Metals" in this Section. Stacking faults are shown in Fig. 44 to 47 in the article "Transmission Electron Microscopy." Subgrains and cellular structures are formed by subboundaries (low-angle boundaries). The simplest of these boundaries consists of periodically spaced dislocations. In more complex instances, particularly in structures resulting from deformation, dislocation tangles can form cellular structures. Crystal imperfections of all kinds, including subboundaries, may occur in single crystals and within the grains of polycrystalline metals. Grain structure of single-phase polycrystalline metals, which is the most characteristic feature of their microstructure, will be discussed below. Twins, which occur within grains, are special imperfections that may originate during growth processes, for example, the annealing of cold-worked metal, or during deformation. Antiphase domain boundaries occur in solid solutions with long-range order, reducing the perfection of the order. Ferromagnetic domains are characteristic of ferromagnetic materials, as described in the article "Magnetic and Electrical Materials" in this Volume. Unlike typical metallurgical processes, a change in ferromagnetic domain structure requires a variation in magnetic field. Antiferromagnets also have domain structures. Multiphase Structures. As discussed below, the shapes, sizes, and configurational arrangements of two or more microconstituents in a multiphase system produce a variety of typical microstructures. Textures combine the crystallographic feature of lattice orientation with the microstructural feature of grain structure. In a metal having a texture, or preferred orientation, the crystal lattices of the grain are arranged in a correlated and organized manner. Chemical composition affects structure through its influence on phase relations. Composition is also involved in such structural features as microsegregation in solidified metals and solute-enriched regions at grain boundaries and other crystal imperfections (Ref 27). Structural gradients reflect changes of structural features with position. For example, a plate can have a grain-size gradient from the surface toward the interior. Composition gradients can cause structural gradients, as in case-hardened metals. Composites present special opportunities for establishing structural gradients by controlling the spatial arrangements of the reinforcing phase for example, fibers (Ref 28). Additional information is provided in the article "Fiber Composite Materials" in this Volume. Porosity and voids are structural features that are characterized by a large range of sizes. Macrostructure is discussed below and is also considered in the articles "Solidification Structures of Steel," "Solidification Structures of Aluminum Alloy Ingots," "Solidification Structures of Copper Alloy Ingots," and "Plastic Deformation Structures" in this Section. References cited in this section 19. H.W. King, Structure of the Pure Metals, in Physical Metallurgy, Part I, 3rd ed., R.W. Cahn and P. Haasen, Ed., North-Holland, 1983, P 37-79 20. D.G. Pettifor, Electron Theory of Metals, in Physical Metallurgy, Part I, 3rd ed., R.W. Cahn and P. Haasen, Ed., North-Holland, 1983, p 73-152 21. W.A. Harrison, Electronic Structure and the Properties of Solids, Freeman, 1980 22. A. Kelly and G.W. Groves, Crystallography and Crystal Defects, Addison-Wesley, 1970 23. E. Prince. Mathematical Techniques in Crystallography and Materials Science, Springer-Verlag, 1982 24. H.G. van Bueren, Imperfections in Crystals, North-Holland, 1960 25. J.P. Hirth and J. Lothe, Theory of Dislocations, 2nd ed., John Wiley & Sons, 1982 26. D. Hull, Introduction to Dislocations, 3rd ed., Pergamon Press, 1975 27. R.W. Balluffi, Grain Boundary Structure and Segregation, in Interfacial Segregation, W.C. Johns on and J.M. Blakely, Ed., American Society for Metals, 1979, p 193-236 28. M.B. Bever and P.E. Duwez, Gradients in Composite Materials, Mater. Sci. Eng., Vol 10, 1972, p 1-8 Origins of Structures The characteristic structures of metals and alloys are produced by (1) transformations in which one or more parent phases are converted into one or more new phases, (2) deformation processes, (3) thermal processes, (4) thermomechanical processes, or (5) diffusion processes that do not result in a transformation, such as sintering. A typical deformation process is cold working. Examples of thermal processes are the annealing of a coldworked metal and the homogenization of an alloy with microsegregation, The principles underlying and governing these processes are the province of physical metallurgy (see Ref 5, 6, 7, 8, 29, 30, 31). The transformations and processes that result in the production of typical structures involve characteristic basic mechanisms. The transformations that produce solidification structures and solid-state transformation structures involve several such mechanisms. The most important of these are diffusion, nucleation, and growth; more complex mechanisms operate in martensitic and bainitic transformations. Basic deformation mechanisms include slip, twinning, and grain-boundary sliding. Annealing processes leading to recovery, recrystallization, and grain growth proceed by the mechanisms of polygonization, nucleation and growth, and grain-boundary migration, respectively. Processes developed in recent years, such as rapid solidification, mechanical alloying, ion implantation, deformation of superplastic alloys, and laser annealing, have introduced new structural morphologies. For example, a structure without dendritic or cellular microsegregation was produced in an Ag-5Cu alloy that was electron beam melted and rapidly resolidified at 600 mm/s ( see Fig. 21 and 22 in the article "Solidification Structures of Solid Solutions" in this Section). In addition, rapid solidification techniques, such as melt spinning and splat cooling, can produce metallic glasses, that is, amorphous (noncrystalline) metals, as described in the article "Amorphous Powder Metals" in Volume 7 of ASM Handbook, formerly 9th Edition of Metals Handbook. References cited in this section 5. R.W. Cahn and P. Haasen, Ed., Physical Metallurgy, Parts I and II, 3rd ed., North-Holland, 1983 6. A.G. Guy and J.J. Hren, Elements of Physical Metallurgy, 3rd ed., Addison-Wesley, 1974 7. W.F. Smith, Structures and Properties of Engineering Alloys, McGraw-Hill, 1981 8. R.E. Smallman, Modern Physical Metallurgy, 4th ed., Butterworths, 1985 29. J.W. Christian, The Theory of Transformations in Metals and Alloys, Pergamon Press, 1965; 2nd ed., Part I, Pergamon Press, 1975 30. D.A. Porter and K.E. Easterling, Phase Transformations in Metals and Alloys, Van Nostrand Reinhold, 1981 31. R.W.K. Honeycombe, The Plastic Deformation of Metals, 2nd ed., St. Martin's Press, 1982 Single-Phase Microstructures The major types of microstructures solidification structures, solid-state transformation structures and deformation and annealing structures are shown in , Fig. 1, 2, and 3 . The characteristic structural features of single-phase metals and alloys, such as grain structure and substructure, are discussed below. Some of the features of single-phase metals are also found in multiphase structures (Ref 32, 33). Fig. 1 An outline of solidification structures Fig. 2 An outline of solid-state transformation structures Fig. 3 An outline of deformation and annealing structures Grain Structure. Grains are small crystals (crystallites) that form a three-dimensional aggregate; they are normally viewed in sections, which by their nature are limited to two dimensions. The main characteristics of a grain structure are grain size, grain shape, and grain-shape anisotropy. Types of Grain Structure. Typical grain structures include impingement structure, columnar structure, equiaxed grain structure, mature grain structure, deformed grain structure, inhibited recrystallization structure, and duplex grain structure. Impingement structure forms when grains grow until they meet or impinge, producing characteristic ragged interfaces. This type of structure is rarely observed, because the interfaces usually are smoothed while the specimen remains at elevated temperature. Impingement grains have been observed after secondary recrystallization (Ref 34). Columnar structure forms by unidirectional growth processes, especially during solidification, and by a growth process involving diffusion accompanied by a solid-state transformation. A columnar structure is shown in Fig. 2 in the article "Solidification Structures of Steel" in this Section. Equiaxed grain structure may form by several processes, such as solidification (see Fig. 2 in the article "Solidification Structures of Steel") and recrystallization (see Fig. 80 to 87 in the article "Copper and Copper Alloys" and Fig. 1, 9, and 17 in the article "Zirconium and Hafnium and Their Alloys" in this Volume as well as Fig. 1 in the article "Textured Structures" in this Section). Mature grain structure forms when the interfaces for example, those resulting from impingement-adjust themselves under capillarity driving forces. Deformed grain structure is the product of cold working. In such a structure, the grain shapes are anisotropic (see Fig. 60 to 68 in the article "Carbon and Alloy Steels" in this Volume and Fig. 1 in "Textured Structures" in this Section). Inhibited recrystallization structure forms when second-phase particles arranged in a nonrandom pattern inhibit the motion of grain boundaries and impose their nonrandom pattern on the resulting recrystallized structure (see Fig. 29 and 30 in the article "Refractory Metals and Alloys" in this Volume). Duplex grain structure (see Fig. 47 in the article "Carbon and Alloy Steels" in this Volume) consists of discrete regions of larger and smaller grain sizes, that is, a bimodal distribution of grain sizes. This structure is not related to microduplex alloys, which have characteristic duplex structures involving composition of two coexisting microconstituents rather than grain size (see the section "Multiphase Microstructures" below). Three-Dimensional Grain Structure. Grain structures exist in three dimensions. In a typical structure, two grains are separated by an interface; three interfaces join along a line or edge, and four edges join at a point or junction. Six interfaces and four grains join at a junction in addition to the four edges. Junctions of four grain edges are the basic units of a mature grain structure; these junctions can be connected in innumerable ways without structural symmetry or exact repetition of detail (Ref 34, 35). The major factors controlling grain structure are the requirement of space filling and the tendency toward minimum interfacial energy. Space filling implies that adjoining grains interact to determine each other's shapes. The problem of filling space with regular geometrical bodies has been studied over the past 100 years, beginning with Lord Kelvin in 1887 (Ref 34, 35). These studies have contributed to the understanding of grain structure, although actual grains may have irregular shapes. The tendency toward minimum interfacial energy operates by reducing the grain-boundary area as much as possible or, when applicable, by rotating the grain boundary into low-energy orientations. The reduced grain-boundary area is an essential characteristic of mature grain structures. Topological relations for three-dimensional grain structures, such as the average number of sides of a grain face, have been analyzed. The relations applicable to metal grains resemble those for certain nonmetallic materials, such as biological cell structures and foam structures (Ref 34, 35, 36) Crystallography of Grain Boundaries. Various models have been proposed for the grain-boundary region, ranging from simple models for low-angle tilt boundaries to complicated transition regions in high-angle boundaries (Ref 37). Coincidence and twin boundaries are discussed in the article "Solidification Structures of Pure Metals" in this Section. Two-Dimensional Grain Structure. Sectioning of a three-dimensional grain structure presents the grain structure in only two dimensions for observation. In a typical grain structure, the following simple relations between the three- dimensional and the two-dimensional structures can be established: • A volume three-dimensional cell or spatial grain becomes an area, that is, a two- dimensional cell or planar grain. • An interface in a three- dimensional structure becomes a line or a grain boundary in a two dimensional structure. • An edge becomes a point. • A corner or junction (zero- dimensional cell) has an infinitesimal probability of being intersected by the plane of observation. • The true dihedral angle becomes an apparent dihedral angle, as discussed below. In the transition from a three- to a two-dimensional grain structure, another basic relation is that a structure consisting of uniformly sized three-dimensional, or spatial, grains becomes a two-dimensional structure in which the planar grains are not of uniform size. This is because a random plane cuts grains at random positions, ranging from a corner to the largest cross section. However, the resulting two-dimensional distribution of a grain structure of uniform three-dimensional grain size has definite statistical regularity. In general, the true three-dimensional grain size is more nearly uniform than the apparent two-dimensional grain size. The problems of grain-size measurement and grain-size statistics are covered in the article "Quantitative Metallography" in this Volume and in Ref 38, 39, 40. The topological relations of grains in two dimensions (planar grains) have been observed, demonstrating that the average planar grain in a mature structure is a hexagon. Consequently, a seven-sided grain in a microsection must be balanced by a five-sided grain, a nine-sided grain by a three-sided grain, or by three five-sided grains, and so on. In addition, correct sampling for polygon distribution ensures better sampling for size (see Ref 34, 35, 36). Grain Shape. In three dimensions, the average shape of equiaxed grains may, for some purposes, be approximated by a sphere. Similarly, nonequiaxed grains may be represented by ellipsoids. When viewed in two dimensions, nonequiaxed grains have extended shapes, as shown in Fig. 1 in the article "Textured Structures" in this Section. The quantitative determination of grain shape has been discussed (Ref 41). Dihedral Angles. In three dimensions, the true dihedral angle is the angle between two faces of a grain measured in a plane normal to the edge at which the faces intersect. In any actual section, the faces are intersected by planes oriented randomly at all angles. Therefore, the apparent angle in two dimensions generally differs from the true angle in three dimensions. Stated differently, the apparent or observed angle is the angle between the traces of grain faces in the plane of a random section. The angles in a two-dimensional section are statistically random in the absence of any orientation effect or preselection. Quantitative relations exist between the true angle in three dimensions and the apparent angle observed in two dimensions. If the true angle is 120°, as in a mature grain structure, the probability of finding an angle within 5° of the true angle is greater than the probability of finding an angle in any other 10° range (Ref 42). In fact, four angles out of five are expected to be within 25° of the true angle. However, in actual grain structures, the true angles and, to a greater extent, the observed angles will have a distribution range. In two-phase structures, the true dihedral angles may differ from 120° even if the structure is equilibrated. The extent to which the true angles differ depends on the relative interfacial tensions between grains of the two phases present. It has been suggested that the true angle can be found by matching calculated and observed frequency plots. The most probable angle is in every instance the true dihedral angle (Ref 43). A simpler procedure for finding the true angle uses a cumulative distribution curve. The median angle differs only slightly, and correctably, from the true angle. In addition, fewer measurements perhaps 25 instead of several hundred are sufficient (Ref 44). Errors in measurement have been systematically analyzed, and dihedral angles with nonunique values have been considered (Ref 45). References cited in this section 32. R.D. Doherty, Stability of Grain Structure in Metals, J. Mater. Educ., Vol 6, 1984, p 845 33. A.P. Sutton, Grain Boundary Structure, Int. Met. Rev., Vol 29, 1984, p 377 34. C.S. Smith, Some Elementary Principles of Polycrystalline Microstructure, Met. Rev., Vol 9, 1964, p 1-62 35. C.S. Smith, Grain Shapes and Other Metallurgical Applications of Topology, in Metal Interfaces, American Society for Metals, 1952, p 65-133 36. C.S. Smith, Microstructure, Trans. ASM, Vol 45, 1953, p 533-575 37. R.W. Balluffi, Ed., Grain Boundary Structure and Kinetics, American Society for Metals, 1979 38. F. Schückher, Grain Size, in Quantitative Microscopy, R.T. DeHoff and F.N. Rhines, Ed., McGraw- Hill, 1968 39. E.E. Underwood, in Quantitative Stereology, Addison-Wesley, 1970, Chapters 4 and 5 40. H.E. Exner, Analysis of Grain- and Particle-Size Distributions in Metallic Materials, Int. Met. Rev., Vol 17 , March 1972, p 24-52 41. E.E. Underwood, in Quantitative Stereology, Addison-Wesley, 1970, p 228 42. D. Harker and E.R. Parker, Grain Shape and Grain Growth, Trans. ASM, Vol 34, 1945, p 156-195 43. C.S. Smith, Grains, Phases and Interfaces: An Interpretation of Microstructure, Trans. AIME, Vol 175, 1948, p 15 44. O.K. Riegger and L.H. Van Vlack, Dihedral Angle Measurement, Trans. Met. Soc. AIME, Vol 218, 1960, p 933-935 45. C.A. Stickels and E.E. Hucke, Measurement of Dihedral Angles, Trans. Met. Soc. AIME, Vol 230, 1964, p [...]... continuous phase and isolated particles of a second phase (the matrixplus-dispersed-phase structure) are the most varied of the multi-phase structures Among their characteristic variables are the relative volumes of the two phases, the size of the particles of the dispersed phase, the interparticle distance, the shape of the dispersed particles, and any special orientation of the dispersed particles with... and the matrix Some of these variables are interdependent; all of them can be measured Examples of the matrixplus-dispersed-phase structure are rod-shaped particles embedded in a matrix and cellular precipitates The development of high-strength steels has introduced the dual-phase microstructure in which a ferrite matrix contains small islands (approximately 20 vol%) of dispersed martensite A dual-phase... freeze, equation: CS = kCo(1 - f s)k-1 where CS is the solid composition formed, k is the equilibrium partition coefficient as defined earlier, Co is the initial alloy composition and f s is the volume fraction solidified Figure 17 shows an example of the use of this equation for an Al-4.5Cu alloy for which Co is 4.5% Cu and k is 0.17 Figure 17(a) shows part of the aluminum-copper phase diagram The equation... compact was electropolished at -3 0 °C (-2 0 °F) in 950 mL methanol, 50 mL HClO4, and 15 mL HNO3 6300× (Ref 15) References cited in this section 6 J.H Perepezko and J.J Paike, Under-cooling Behavior of Liquid Metals, in Rapidly Solidified Amorphous and Crystalline Alloys, B.H Kear, B.C Giessen, and M Cohen, Ed., North Holland, 1982, p 49 7 W.J Boettinger, D Shechtman, R.J Schaefer, and F.S Biancaniello, The... transmission electron micrograph of cellular structure of α-aluminum seen in a melt-spun Al12Mn alloy Small particles of another phase decorate the cell walls Electropolished at -3 0 °C (-2 0 °F) in 950 mL methanol, 50 mL HClO4, and 15 mL HNO3 16,000× (Ref 8) Fig 20 Thin foil transmission electron micrograph of elongated cellular structure in a melt-spun Al-15Mn alloy The contrast between some cells indicates... Dislocations in a small-angle twist boundary in gold Thin-foil transmission electron micrograph See also Fig 10 24,000× (R.W Balluffi) Large-angle boundaries contain regions of good fit and of bad fit Low-energy, small-angle boundaries usually represent less than 10° to 15° of misorientation This range is a small fraction of the total range of possible misorientations Therefore, most random boundaries,... solidified microstructures is described in Ref 10 Microsegregation-Free Structures A particularly dramatic microstructural change that occurs in some rapidly solidified crystalline alloys is the complete absence of dendritic or cellular microsegregation Figures 21 and 22 show Ag5Cu alloys solidified using electron beam surface melting and resolidification at speeds of 300 and 600 mm/s (12 and 24 in./s),... very rapidly in a partitionless manner The interface speed is reduced as the liquid-solid interface crosses the particle due to the release of latent heat of fusion and warming of the powder particle Because of this reduction in interface speed, the solidification front becomes cellular Fig 26 Electrohydrodynamic (EHD) atomized Al-6Si powder Transmission electron micrograph of unthinned particle mounted... 10 μm) hypereutectic Al-8Fe powder particle (Ref 15) A thin foil was prepared by electropolishing a 3-mm (0.12-in.) diam green powder compact (Ref 16) In this alloy, however, the zone to the left where nucleation occurs contains a very fine cellular structure, and the zone to the right contains a coarse cellular structure of α-aluminum with Al6Fe between the cells Larger powder particles of the alloy... this subject and the field of solidification can be found in Ref 4 and 5 References cited in this section 2 R Trivedi, Interdendritic Spacing: Part II A Comparison of Theory and Experiment, Met Trans A, Vol 15, 1984, p 977 3 M.C Flemings and R Mehrabian, Segregation in Castings and Ingots, in Solidification, American Society for Metals, 1971 4 M.C Flemings, Solidification Processing, McGraw-Hill, 1974 . Part I, 3rd ed., R.W. Cahn and R. Haasen, Ed., North-Holland, 1983, p 1-3 5 5. R.W. Cahn and P. Haasen, Ed., Physical Metallurgy, Parts I and II, 3rd ed., North-Holland, 1983 6. A.G. Guy and. Cahn and P. Haasen, Ed., North-Holland, 1983, P 3 7-7 9 20. D.G. Pettifor, Electron Theory of Metals, in Physical Metallurgy, Part I, 3rd ed., R.W. Cahn and P. Haasen, Ed., North-Holland,. Edition of Metals Handbook. References cited in this section 5. R.W. Cahn and P. Haasen, Ed., Physical Metallurgy, Parts I and II, 3rd ed., North-Holland, 1983 6. A.G. Guy and J.J. Hren, Elements