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Introdaction ALBERT R. C. WESTWOOD The dramatic and well-publicized advances in computers and bio- technology of recent years have overshadowed those in structural ma- terials. But the materials community has not been resting on its laurels. Instead, during the past decade it has generated a variety of mechanistic insights and innovative ideas, some of which are now beginning to emerge . . . .. . in engineering applications. For example, although the potential usefulness of composite materials has been evident for some time, it was necessary to learn how to produce complex shapes from them and how to join these together before en- gineering structures could be produced. These steps have now been accomplished. To illustrate: welding silicon-carbon (SiC)-reinforced alu- minum initially proved to be difficult, because hydrogen trapped in the metal concentrated into voids in the weld zone producing a weakened, Swiss-cheese-like structure. But treatments to remove this hydrogen prior to welding have been devised, and now, stiff, lightweight, metal- matrix composite structures, such as the yoke for a shipboard satellite communications antenna, are beginning to be produced. Fiber-reinforced plastics (FRP) also are emerging as major structural materials. The new Lear Fan Turboprop, for example, makes extensive use of carbon-fiber-reinforced epoxy to save 40 percent of the weight of a similar structure in aluminum. Without engines this airplane weighs only 1,275 pounds. With engines it can travel 2,300 miles at up to 400 miles per hour on 250 gallons of fuel.2 Industry sources project that up to 65 percent of the structures of commercial aircraft will be made from composite materials by the mid-199Os. 79
80 ADVANCES IN STRUCTURAL MA TERIALS In the case of the Lear Turboprop, FRPs serve both aesthetic and structural purposes, but these functions are separated in the new Pontiac Fiero automobile. Glass-flake-reinforced, injection-molded poly- urethane is used for the vertical surfaces of the Fiero, and this can bend and snap back on minor impact, reducing body damage. The easily removed plastic panels simplify body repair and reduce corrosion.3 Progress has also been made with inorganic materials. Portland ce- ment has been the most extensively used and cheapest material of con- struction for more than 100 years, but unless it is reinforced with steel it can be used only in compression because of its poor tensile properties, a consequence of the presence of pores that act as crack nuclei.4 How- ever, by controlling ingredient particle size, adding surface-active agents to improve the rheology of the paste and polymers to fill in the pores, and working the mixture to remove entrapped gas bubbles, a Macro- Defect-Free (MDF) cement can be produced, the bend strength of which approximates that of aluminum and from which springs can be fabricated (Figure As Future developments along these lines could markedly in- fluence both architecture and the durability of roads. F~GuRE 1 Spring made from Macro-Defect-Free (MDF) cement in (a) relaxed and (b) extended positions.5 Reprinted with permission.
INTRODUCTION 81 FIGL-RE 2 Structure of tough, strong abalone shell, consisting of platelets of CaC03, ~0.2-~m-thick, interleaved with layers of protein. From Note 4. Reprinted with permis- slon. . The notion that ceramics can be tough is intriguing, since ceramics are usually thought of in terms of rather fragile pottery. However, by incorporating crack arrestors (e.g., fibers) and crack energy dissipators (e.g., phase-transforming particles) into the structure, ceramics and ce- ramic composite materials exhibiting quite useful measures of toughness (20 megapascals square-root meter [MPam1'23) can now be produced. Early applications for such materials include scissors (such as those made from partially stabilized zirconia by Toray Industries of Japan) and surgical saws. Of course, tough inorganic structures are really nothing new. Figure 2 illustrates a section through an abalone shell. It consists of 0.2-mi- crometer M-thick platelets of 99 percent pure CaCO3, interleaved with thin layers of protein. Its tensile strength also is about that for a conventional aluminum alloy, and it is as tough as Plexiglas.4 Inorganic glasses have been with us for millennia, but metallic glasses are a relatively recent arrival, and they represent one of an increasing group of materials whose useful and different properties are a conse-
82 ADVANCES IN STRUCTURAL MATERIALS quence of the application of very high rates of cooling from the molten state.6 Amorphous iron-base alloys can provide magnetic losses 10 times smaller than those of the traditional Fe-3% Si materials, and half those of the permalloys. Amorphous metals also exhibit remarkable strengths and resistance to corrosion, the latter property being attributed to the absence of second phases and grain boundaries. However, of currently more practical value as structural materials are the rapidly solidified (RS), crystalline alloys of aluminum (Al), nickel (Ni), and cobalt (Co). Produced using quenching rates of order 106°C per second or more, they are characterized by a very fine grain size, reduced amounts of segregation, and extended solubility of alloying elements. RS aluminum alloy components are now entering service, e.g., on Boeing 757 aircraft, providing improved strength-to-weight ratios and MgZn2 particles Grain boundary CO2AIg particles FIGURE 3 Structure of rapidly solidified cobalt-containing aluminum alloy. CO2A19 particles are small in size and well distributed through- out matrix and grain boundary.7
INTRODUCTION 83 resistance to stress corrosion. To illustrate, cobalt has very limited sol- ubility in aluminum, and concentrations of only 0.5 percent or so in ingots produced by conventional metallurgical processes result in the precipitation of large cobalt intermetallic phases that reduce fracture toughness. However, when aluminum alloy powders (7000 series) con- taining 0.4 to 0.8 percent of cobalt are prepared by rapid solidification and then consolidated by compaction and extrusion, the CO2Alg inter- metallic compound appears as fine particles dispersed throughout the structure (Figure 3), modestly enhancing strength, but markedly in- creasing resistance to stress corrosion cracking (SCC). In fact, the critical stress for the onset of SCC is increased by about 25 percent in the 0.4 percent Co alloy and 50 percent in the 0.8 percent Co alloy. This im- provement is thought to be caused in part by the CO2Alg particles acting as hydrogen recombination catalysts, reducing the likelihood of hydro- gen atoms entering the aluminum alloy and causing embrittlement.7 Other developments include treatments to change the chemical com- position or structure of the surface of a solid to improve its mechanical properties or durability. With wear and corrosion costing U.S. industry an estimated $80 billion to $90 billion annually, this is clearly a research . . . in. area 0 economic slgnl~lcance. One approach involves improving near-surface properties without changing the composition of the material. This can be achieved, for example, by the intense local heating of a laser beam. Figure 4 shows a carbon dioxide (CO2) laser being used to harden the wear surface of a crankshaft.8 A second approach involves changing the near-surface F~GuRE 4 CO2 laser beam being used to harden lobe surface of crankshaft. From Note 8. Reprinted with permission.
84 ADVANCES IN STRUCTURAL MA TERIALS chemistry or microstructure of the solid, for example, by means of ion implantation. Using this approach the surface of an iron component can be implanted with chromium and made as corrosion resistant as that of stainless steel. Likewise, it has been found that the wear resistance of a diesel fuel injection pump can be increased a hundredfold via yttrium implantation.8 Chemical treatments that alter the structure of oxide films also are proving useful. Such films can be made porous and then filled with Teflon, graphite, or MoS2 to produce hard, corrosion-resistant, and lubricious surfaces. The morphology of oxide surface films also deter- FIGURE 5 Oxide cell and whisker structure produced on aluminum by an acid pretreatment. Such structures permit the development of strong and reliable adhesive bonds in aircraft manufacture.9 Re- printed with permission.
INTROD UCTION 85 mines the strength of structures made by adhesive bonding, as aerospace structures increasingly are. A smooth oxide film on aluminum produces weak and unreliable bonds, but an acid pretreatment that produces a porous oxide film with a cell and whisker structure, such as that shown in Figure 5, to which the adhesive can become mechanically as well as chemically attached, produces a strong and reliable bondment.9 The durability of adhesively bonded aluminum structures can be further improved by first spraying the oxide film with a dilute aqueous solution of NTMP (nitrilo-tris-methylene phosphoric acid). The adsorbed mol- ecules of this and similar organic substances inhibit the transformation of the strong oxide whiskers in humid, salt-containing environments into weak, stress-raising, hydroxide platelets.~° The result is a severalfold increase in bond life. In summary, new types of stronger, more durable, and increasingly cost-effective structures are beginning to emerge, with the intrinsic prop- erties of all classes of solids metals, ceramics, polymers, and com- posites being more efficiently utilized. Indeed, as R. B. Nicholson recently commented, the future role of materials science lies not so much in developing new materials per se as in developing new and more efficient ways of processing existing materials so that they exhibit the properties of which they are theoretically capable. Such efforts will provide us with strong and tough ceramics, extremely corrosion-resistant metals with outstanding strength-to-weight ratios, more durable concretes for road use, electrically conductive and self- reinforced polymers, and "designer" composites with a wide variety of combinations of desirable properties. NOTES 1. Materials Highlights, Naval Surface Weapons Center, White Oak, Md., Feb. 1982. 2. Mater. Eng., p. 43, May 1982. 3. Mater. Eng., p. 30, Oct. 1983. 4. J.D. Birchall and A. Kelly, Sci. Am., p. 104, May 1983. 5. J.D. Birchall, A.J. Howard, and K. Kendall, J. Mater. Sci. Lett., 1:125, 1982. 6. Amorphous and Metastable Microcrystalline Rapidly Solidified Alloys, National Ma- terials Advisory Board Rep. No. 358, National Research Council, Washington, D.C., May 1980. 7. J.R. Pickens and L. Christodolou, unpublished work, Martin Marietta Laboratories, 1983. 8. C. Rain, High Tech., p. 59, March 1983. 9. J.D. Venables et al., Appl. Surf. Sci., 3:88, 1979. 10. J.S. Ahearn et al., in Adhesion Aspects of Polymeric Coatings, Plenum, New York, 1983, p. 281. 11. R.B. Nicholson. Reported in Chem. Eng., p. 36, March 1983.
