Fundamental Properties of Intermetallic Compounds
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MRS BULLETIN/AUGUST 1995
kinds of interatomic bonding, ranging from metallic to covalent or ionic bonding. The ordering of atoms and the strong interatomic bonding result in many attractive properties for intermetallic compounds. Some solid-solution alloys exhibit an order-disorder transition at a critical temperature, below which they have an ordered arrangement of atoms. Some of
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Al (at.%)
Figure 1. Composition dependence of room-temperature elastic moduli for Fe-AI alloys.4 The composition range of single-phase Fe3AI is represented by the shaded band.
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the alloys have a disordered state even at lower temperatures when they are rapidly cooled from temperatures above the critical temperature. The ordered forms are intermetallic compounds. Thus, a difference in characteristics between intermetallic compounds and solid-solution alloys is clarified by comparing the properties in the ordered state and the disordered state of a solid-solution alloy. The characteristics of intermetallics result from both their crystal structures and their interatomic bonding. For example, a characteristic crystal structure like A15, CP8 (a cubic structure with eight atoms in each unit cell) results in diverse superconducting intermetallic compounds with high Tc, for example, Nb3Sn. The anisotropy of a crystal structure causes permanent magnetic materials to have high coercive forces, for example, SmCo5. In intermetallic compounds, two or more elements are distributed uniformly. This is desirable for hydrogen-absorbing alloys like FeTi and LaNi5. The long periodicity of a superlattice in intermetallic compounds causes the high plastic strength. A special example of unique plastic behavior is the shapememory effect, in which deformation caused by a martensitic transformation precedes the plastic deformation, an example being NiTi. The ordered arrangement of different elements also influences the mechanical properties of intermetallic compounds. The bonding energy between unlike atoms in intermetallic compounds is larger than the average of those between atoms of the same element. This causes high elastic coefficients, typically higher than those for ordinary structural metals and alloys like steels, Ti alloys, and Al alloys. The large bonding energy also causes high resistance to atomic displacements induced by irradiation. Thus, intermetallic compounds are desirable for nuclear applications. The ordered lattices of intermetallic compounds make diffusion of atoms difficult because many atomic jumps are required for an atom to diffuse to a neighboring site without disturbing the ordered arrangement of atoms. Therefore, intermetallic compounds are resistant to irradiation-induced swelling. The difficulty in diffusion also causes strong creep resistance at high temperatures. The longer periodicity of the structure for intermetallic compounds generates dislocations with a large Burgers vector. Consequently, intermetallic compounds are difficult to deform and have high strength at high temperatures. The dislo-
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