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The attractive physical and mechanical properties of ordered intermetallic alloys have been recognized since early in this century. However, periodic attempts to develop intermetallics for structural applications were unsuc­ cessful, due in major part to the twin handicaps of inadequate low-temper­ ature ductility or toughness, together with poor elevated-temperature creep strength. The discovery, in 1979, by Aoki and Izumi in Japan that small additions of boron caused a dramatic improvement in the ductility of Ni3Al was a major factor in launching a new wave of fundamental and applied research on intermetallics. Another important factor was the issuance in 1984 of a National Materials Advisory Board reported entitled "Structural Uses for Ductile Ordered Alloys," which identified numerous potential defense-related applications and proposed the launching of a coordinated development program to gather engineering property and processing data. A substantial research effort on titanium aluminides was already underway at the Air Force Materials Laboratory at Wright­ Patterson Air Force Base in Ohio and, with Air Force support, at several industrial and university laboratories. Smaller programs also were under­ way at Oak Ridge National Laboratory, under Department of Energy sponsorship. These research efforts were soon augmented in the United States by funding from Department of Defense agencies such as Office of Naval Research and Air Force Office of Scientific Research, and by the National Science Foundation.



Basic Properties


Chapter 1. Defect Structures

In the early 1900s, many binary combinations of metals that form inter-metallic compounds were found. Since then, the mechanical properties of these compounds have been studied mainly from their theoretical implications and requirements to understand the mechanical properties of inter-metallic compounds existing as secondary phases in many alloys used as structural materials. As a result of these efforts, various new findings on the lattice defects and properties of intermetallic compounds were made. Among them were discoveries of (a) the complications introduced by heat treatment in the property/composition relationships of some intermetallic compounds, later leading to the finding of ordered structures formed from solid solutions disordered at high temperatures (Kurnakov, Zhemchuznii, and Zasedatelev 1916); (b) the brittleness of intermetallic compounds in general at temperatures lower than 0.7 Tm (Tm: melting temperature) (Tamman and Dahl 1923); (c) the effect of compositional deviation from stoichiometry on hardness and flow stress; (d) the existence of antiphase boundaries and antiphase domains (Smith 1943); (e) the existence of superlattice dislocations traveling in pairs or groups in ordered lattices (Koehler and Seitz 1947; Marcinkowski, Brown, and Fisher 1961; Marcinkowski 1963); (f) the anomalous temperature dependence of hardness of Ni3Al (Westbrook 1957).
M. Yamaguchi, Y. Shirai

Chapter 2. Grain Boundary Structure and Chemistry

The importance of understanding the atomic and chemical structure of grain boundaries in intermetallic compounds for a fundamental comprehension of their fracture behavior is the principal theme of this contribution. Since intermetallics are the prime examples of quasi-brittle materials, we first discuss general features of brittle fracture in ductile materials. Ll2 intermetallic compounds, in particular Ni3Al, have been studied most extensively; therefore, we review in detail the present state of our understanding of the atomic and chemical structure of grain boundaries in these alloys and discuss possible reasons for intrinsic brittleness of their grain boundaries. Results of recent atomistic studies of grain boundaries in Ll2 alloys are then described. First, we concentrate on comparison of the boundary structures in Ni3Al and Cu3Au, the two alloys with the same crystal structure but rather different propensities to ordering. Second, we discuss the effects of temperature and bulk nonstoichiometry on the structure and chemistry of grain boundaries in Ni3Al. At this point, the most important finding is that significantly different structures are invoked by segregation of nickel and aluminum, respectively, which may be related to the fact that only nickel-rich alloys may be ductilized by boron alloying. Finally, we discuss briefly the structure and properties of grain boundaries in NiAl and assess the possibilities and limits of further research on atomic level behavior of interfaces in intermetallic compounds.
V. Vitek, M. Yan

Chapter 3. Brittle Fracture and Toughening

This chapter reviews the intrinsic and the extrinsic factors that contribute to the brittle fracture of strongly ordered polycrystalline intermetallic compounds, and then considers methods for improving the ductility and the fracture toughness of this class of materials. A high work-hardening rate appears to be common to all brittle intermetallics and may be a major factor in limiting the energy that can be absorbed by plastic flow near the tips of propagating cracks. The incorporation of a ductile, random solid solution within the matrix is an effective toughening method.
E. M. Schulson

Chapter 4. Creep

The rotating blades of a gas turbine or a jet engine are subjected to centrifugal stresses and high temperatures, a combination that gives rise to deformation by creep. Hence, blade material requirements include, in addition to satisfactory corrosion and oxidation resistance, high creep resistance, that is, high-temperature capability. The continuous need for ever increasing turbine inlet temperatures, improved efficiency and performance will necessitate both static and rotating components that retain dimensional stability at temperatures exceeding those for current nickel-base superalloys ~ 1100°C (Miller and Chamber 1987). A number of materials of the intermetallic class have the potential to meet the requirements for elevated temperature applications in gas turbines for two main reasons: first, for their inherent higher melting point above that of nickel-base superalloys, and second, for the low density that some of the intermetallics may possess. In fact, the second reason can be a unique advantage of intermetallics over nickel-base superalloys, with respect to the design of the turbine disk. This is because the turbine disks are subjected to radial pull of the blades in addition to the body forces resulting from the centrifugal forces associated with rotation. Hence, for large rotating components, the intermetallics, with their inherent low density, can be of interest in disks design.
Mohamed Y. Nazmy

Chapter 5. Fatigue

It was first reported by Boettner, Stoloff and Davies (1966) that long-range order can enhance high-cycle fatigue resistance. However, the lack of tensile ductility manifested by most ordered alloys with potential for structural applications precluded serious attention of researchers to cyclic behavior until very recently. Rather, the emphasis has been upon increasing ductility through alloying or improved processing techniques. Therefore, only a few studies, especially on high-cycle fatigue, were reported in the literature up to about 1987. However, recent progress in producing ductile-ordered alloys has led to renewed interest in fatigue behavior and, where possible to study, to the effects of long-range order on crack initiation and propagation, respectively. The purpose of this chapter is to review the literature on cyclic deformation of ordered alloys, with emphasis upon recent work on Ni3Al, NiAl, Ti3Al and Fe3Al-type alloys.
N. S. Stoloff

Behavior of Alloy Systems


Chapter 6. The Physical and Mechanical Metallurgy of Ni3Al and Its Alloys

The physical and mechanical metallurgy of Ni3Al and its alloys are reviewed in this chapter. Emphasis is on developments within the past ten years, a period of extremely rapid growth in research on the behavior of this unusually interesting intermetallic compound. This chapter is particularly concerned with solute effects on properties such as strength ductility, fatigue resistance, and environmental stability, subjects which have received much attention. Novel processing techniques, including rapid solidification and powder processing, are shown to have played a key role in providing Ni3Al-base alloys with improved properties. An assessment of needed areas of research to hasten commercial applications of Ni3Al alloys concludes the chapter.
N. S. Stoloff, C. T. Liu

Chapter 7. The Physical and Mechanical Metallurgy of NiAl

Considerable research has been performed on NiAI over the last decade, with an exponential increase in effort occurring over the last few years. This is due to interest in this material for electronic, catalytic, coating, and especially high-temperature structural applications. This report uses this wealth of new information to develop a comprehensive description of the properties and processing of NiAI and NiAl-based materials. Emphasis is placed on the controlling fracture and deformation mechanisms of single and polycrystalline NiAI and its alloys over the entire range of temperatures for which data are available. Creep, fatigue, and environmental resistance of this material are discussed. In addition, issues surrounding alloy design, development of NiAl-based composites, and materials processing are addressed.
Ronald D. Noebe, Randy R. Bowman, Michael V. Nathal

Chapter 8. Titanium Aluminides

Excellent elevated-temperature properties and low density make the titanium aluminides attractive candidates for use in both engine and airframe applications particularly in the aerospace industry. The challenge to the materials scientist is to maintain these characteristics while building in “forgiveness.” The basic Ti-Al-phase diagram and crystal structure of both the Ti3Al and TiAl phases are reviewed, followed by a consideration of chemistry, processing, microstructure, deformation/fracture, and mechanical property relationships in monolithic material. Synthesis/processing methods are presented, including ingot and powder-metallurgy approaches and use of hydrogen as a temporary alloying element. Composite concepts as a method to enhance not only “forgiveness” but also elevated-temperature behavior are discussed. Consideration of present and projected applications of both monolithic and composite material are then presented. It is concluded that while the titanium aluminides in monolithic form can be used now in nondemanding applications, much further research and development is required before this material class can be used in critical applications, especially in composite concepts.
F. H. Froes, C. Suryanarayana

Chapter 9. Iron Aluminides

Iron aluminides have been of interest since the 1930s when the excellent corrosion resistance of compositions with more than about 18 at% Al was first noted (DeVan 1989; Ziegler 1932). These alloys offer relatively low material cost, conservation of strategic elements, and a lower density than stainless steels. Their tensile strength also compares favorably with many ferritic and austenitic steels. These property advantages have led to the consideration of iron-aluminum alloys for many applications including those listed in Table 9–1 (McKamey et al. 1991; Nachman and Buehler 1956; Sikka et al. 1993). However, limited ductility at ambient temperatures and a sharp drop in strength above 600° C have been major deterrents to their acceptance for many structural applications. More recent studies have demonstrated that improved engineering ductility (to 10–15% in Fe3Al) can be achieved in wrought Fe3Al-based iron aluminide alloys through control of composition and microstructure (Bordeau 1987; Culbertson and Kortovich 1986; Sikka 1991b; Sikka et al. 1993). Accompanying this improvement has been an increased understanding of the causes for ambient temperature embrittlement in this system (Liu et al. 1989, 1990; McKamey and Liu 1992). Because of these advances, iron-aluminide alloys (especially those of up to 50 at% Al) are again being considered for structural uses, especially for applications where their excellent corrosion resistance can be exploited.
C. G. McKamey

Chapter 10. Advanced Intermetallics

The need for materials for very high-temperature applications with a simultaneous desire for low density, a balance of mechanical properties (e.g., creep resistance and low-temperature damage tolerance), and environmental resistance has directed research beyond the Fe-, Ni-, and Ti-aluminides. Thus, for example, there is considerable ongoing research in the area of suicides with interest residing in obtaining not only a fundamental understanding of the structure and deformation behavior but also in learning how to produce, fabricate, machine, join, and test components of such materials. For this reason, there is ample justification in dedicating a separate chapter in this book to the suicides and therefore, this area is not covered in this chapter. The scope of this chapter includes the trialuminides (Al3 X) with the DO22 and L12 structures, some aluminides that are frequently not included under other headings (e.g., CoAl, Nb3Al, and RuAl), beryllides, and chromides based on the AB2 stoichiometry (Laves Phase).
K. S. Kumar

Chapter 11. Silicides

The formation of silicides is reviewed with the focus on the disilicides, 5–3 silicides and monosilicides, as the three principal useful groups. For the refractory metal-based disilicides, the relationship between the crystal structures C11b, C40, C49, and C54 is examined in terms of stacking sequences and contrasted in relation to the structures of aluminides. The role of interstitial elements and the in situ composite approaches are emphasized for the complex 5–3 silicides and the monosilicides as being the most adjacent phases to refractory metal-solid solutions. For most silicides with a noncubic crystal structure, the effect of associated anisotropy of the coefficient of thermal expansion (CTE) on the mechanical integrity through processing and application of the material is brought forth as a potentially critical issue. Molybdenum disilicide has received much attention for use as a high-temperature structural material. Recent efforts to produce MoSi2-base composites are reviewed.
Michael J. Maloney, Dilip Shah

Environmental Effects


Chapter 12. Environmental Embrittlement

Recent studies have shown that low ductility and brittle fracture in intermetallics are caused not only by intrinsic factors (such as lack of sufficient deformation modes, poor cleavage strength, etc.) but also by extrinsic factors. Environmental degradation, an extrinsic factor, is found to be a major cause of brittle fracture in many ordered intermetallics, particularly those with high crystal symmetries (i.e., cubic Ll2, and B2 and hexagonal DO19).
N. S. Stoloff

Chapter 13. Aqueous Corrosion of Intermetallic Alloys

New engineering alloys based on intermetallic compounds continue to undergo extensive development primarily for high-temperature applications. Accordingly, their strengths and oxidation/sulfidation resistances at elevated temperatures are of paramount importance. However, these alloys will not be continuously exposed to elevated-temperature conditions, and if any detrimental aqueous corrosion occurs near room temperature, their overall performances could be compromised. Consequently, the aqueous-corrosion properties are also of major concern. Moreover, the aqueous-corrosion properties of certain intermetallics in certain aqueous solutions at and near room temperature are sufficiently promising that principal applications may be warranted in these areas.
R. A. Buchanan, J. G. Kim, R. E. Ricker, L. A. Heldt



Chapter 14. Processing of Aluminides

Intermetallic compounds of nickel, iron, and titanium with aluminum have been studied extensively. Research on these materials has been published in the proceedings of several symposia and in journal articles. Detailed articles describing the development of nickel aluminides (Liu 1994; Noebe, Bowman, and Nathal 1994), iron aluminides (McKamey 1994), and titanium aluminides (Froes and Suryanarayana 1994) are the subject of other chapters in this book. In order to take advantage of their unique properties in various applications, methods for their processing need to be identified. In addition, the selected processing methods need to be implemented in the real-world manufacturing sector for the economical production of these materials. The processing of these materials is possible by both casting and powder metallurgy methods. The powder metallurgy processing is described in detail in a separate chapter (German and Iacocca 1994) of this book. This chapter deals with the processing techniques involving melting and casting into net or near-net shapes or ingots for secondary breakdown.
V. K. Sikka

Chapter 15. Powder Metallurgy Processing

The processing of intermetallic compounds has similarities to ceramic processing, as both ceramics and intermetallics are stoichiometric, with limited compositional ranges and brittle behavior. Generally, the limited ductility forces a reliance on powder techniques for shaping and consolidation. The high melting points of many intermetallics make them attractive for high-temperature service, but this same attribute contributes to difficulty in fabricating and consolidating intermetallic powders.
R. M. German, R. G. Iacocca

Chapter 16. Joining

In recent years, ordered intermetallic alloys have attracted considerable interest because of the unique properties that make them attractive candidates for high-temperature structural applications (Stoloff and Davies 1966; Kear et al. 1970; Westbrook 1959, 1967; Lipsitt 1985; Liu 1984; Stoloff 1989). Some of the alloy systems that are currently under development or in use include those based on the following compounds: Ni3Al, Fe3Al, Ti3Al, Co3V, Ni3V, TiAl, NiAl, and FeAl. Many of these compounds have low density and high modulus coupled with excellent high-temperature strength, which derives from the unique dislocation dynamics in ordered lattices and slow atomic mobility. Often they have unique oxidation and corrosion properties (Liu and Stiegler 1984). Intermetallic compounds, however, tend to have limited ductility, and this characteristic has deterred their widespread use for structural applications. Recent work has shown that the ductility and fabricability of several ordered intermetallic compounds can be substantially improved using physical metallurgy principles (Liu 1984; Stoloff 1989; Rhodes, Hamilton, and Paton 1987; Liu, White, and Horton 1985). Because of the unique properties of these intermetallic alloys, a wide range of applications is anticipated, ranging from aerospace to land-based transportation systems, energy systems, chemical systems, heating elements, and so on.
S. A. David, M. L. Santella


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