Utilisateur:Yongsheng1994/Brouillon1

histoire

2.2 SURVEY OF EARLIER WORKS ON INTERMETALLIC COMPOUNDS In the beginning of the nineteenth century, when the first systematic studies of alloy systems were being made, many scientists noted that the behaviour of certain alloy compositions was strikingly like that of ordinary chemical compounds and began to speculate as to whether compounds might exist between metals. The first observation of an intermetallic was made by Karl Karsten in 1839 (Westbrook 1967), when he noticed a discontinuity in the action of acids on alloys of copper and zinc at the equi- atomic composition and suggested the formation of a compound. The compound claimed by Karsten does really exist and is now popularly called as fi-brass (CuZn). During the second half of the century, similar discontinuities were observed in electrical, mechanical, magnetic as well as chemical properties in other alloy systems when the components had definite proportions and that led to the suggested formulation of more intermetallic compounds. The definition of an intermetallic compound is very elusive, but mostly it includes metal-metal compounds, both ordered and disordered, binary and multi-component. Upon occasion, when it appears useful to do so, even the metal-metal aspect of the definition is relaxed somewhat in the consideration of some metal-metalloid compounds, such as the silicides or phosphides like MoSi2 and InP ( Samsonov 1959). The intermetallies find diverse applications ranging from high strength compounds like NisAl (Liu et al 1985) to GaAs which has a unique capability for solid-state laser action.

2:3 CONSTITUTIONAL STUDIES The German metallurgist Tamman (Westbrook 1967) made enormous contributions in many different fields of physical metallurgy over a period of almost fifty years following the turn of the century. Much of his work was on the study ofthe occurrence and properties ofintermetallic compounds. His works was aimed at generalising the binary constitution and studying the nature of intermetallic phases that could be feasible for a large number of systems rather than making a careful study of a few. Thermal analysis and analysis of residues after chemical attack were the early methods used for identifying a compound and gradually they were supplemented by studies of the composition dependence of physical properties such as density, resistivity, thermal conductivity, hardness and thermoelectric power. Exploitation of this approach of "Physico-chemical analysis" is duly credited to Kurnakov (Westbrook 1967), the great Russian metallurgist. His contribution 19 to the understanding ofintermetallics begins with the phase-diagram determination and property-composition studies. He also discussed the nature and stability of intermetallics on theoretical grounds. In 1900, a published list on intermetallic compounds contained only thirty sewn confirmed systems. But within two decades the number multiplied nearly tenfold. Most of the early works naturally were concerned with binary systems, though the first ternary compounds CdHgNa and Hg2KNa were reported as early as 1906. Scores of ternary compounds, some quartemaries, and even a few quinary compounds have been identified in the subsequent years.

2.5 EARLIER THEORETICAL WORKS ON INTERMETALLICS The British metallurgist Desch (Westbrook 1967) was the first to summarise the results of the theoretical attempts on the understanding of the constitution of intermetallic compounds. Nevitt (Westbrook 1967) attempted to understand the 20 theory of intermetallics from a knowledge of crystal structure, bonding and composition. It was Hume-Rothery (Westbrook 1967) who in this regard made an outstanding contribution to the understanding of a large group of intermetallics. One of the next attempts at systematization of intermetallic compound types was that of Zintl (Westbrook 1967) who proposed a rule for differentiating intermetallics based on the nature of bonding, namely: ionic, covalent, metallic or combinations of the three. Later Laves (1956) studied the different factors influencing the structures of the intermetallic phases based on the following geometrical principles:

1.Space-filling principle 2.5.1 Space-filling principle In the case of metals the atoms tend to have higher coordination numbers (CN’s) when the metal atoms are not distinguished from one another. Ifthe atoms are assumed to be spherical and in contact then the best space filling will be that of the cubic or hexagonal close packed arrangement. When metals crystallise the tendency to have a good space filling is called the "space filling principle"

2. Symmetry principle 2.5.2 Symmetry principle There are certain factors which operate against the space filling principle such as the temperature and binding factors. Other than the 12 fold coordination the next most commonly found coordination is only 8 in the case of metals. Though the other coordination numbers 9,10 and 11 do occur with higher packing density than CN 8 the arrangements with CN 9, 10 or 11 possess lower symmetry than the 8 fold coordination. The tendency to build configurations with higher symmetries is called the ’symmetry principle’.

3. Connection principle 2.5.3 Connection principle The shortest link between the atoms in a solid is said to form a connection. If it connects between structurally equivalent atoms it is called homogeneous and that between inequivalent atoms is called heterogeneous. The connections can be one, two or three dimensional; accordingly they are named as islands, chains and nets or lattices. *1116 tendency to form multi-dimensional connections is said to constitute the connection principle.

ref:http://shodhganga.inflibnet.ac.in/bitstream/10603/76947/10/10_chapter%202.pdf

fabrication

Intermetallic compounds are produced by direct reaction of theircomponents upon heating or by double decomposition reactions. The formation of intermetallic compounds is observedduring the separation of an excess of a component from metallic solid solutions or as a result of positional ordering of theatoms of the components in solid solutions.

ref:http://encyclopedia2.thefreedictionary.com/Intermetallic+Compounds

type

The fact that intermetallic compounds are rarely found in hep, ccp or bcc structures only leads to the conclusion that there are additional interactions present. One of them is to study the nature of bonding between different kinds of atoms. Accordingly the intermetallic compounds are classified as follows.

1. Valence compounds. 2.5.4 Valence compounds The main factor influencing valence compounds is sharing or transfer of electrons between atoms and thus forming the ionic or covalent bonding. The general trends observed among the valence compounds are discussed in detail by Girgis (1983). Most of the valence compounds are iono-covalent compounds with a bonding intermediate between ionic and covalent.

2. Electron compounds and Interstitial compounds. 2.5.5 Electron compounds and interstitial compounds Electron compounds with particular valence electron concentrations (VEC) are found to crystallise in one group and they are referred to as Hume- Rothery phases. Table 2.1 shows some representatives of Hume- Rothery phases. Interstitial compounds are defined as compounds of the transition metals (T) with relatively large atomic radii with non-metals (X) of small radii (H,B,C,N,0). The X atoms occupy the interstices of the T atom host structure.

3. Laves phase (size factor) compounds. 2.5.6 Laves phase (size factor) compounds Laves phase compounds denote a large group of intermetallic compounds crystallising in any one of the three closely related structures MgCu2 (C15), MgZn2 (C14) and MgNi2 (C36). It is believed that one of the main factors contributing to the existence of Laves phase compounds is of the geometrical origin (size factor) i.e. filling the space in convenient way. If for maximum filling of space, the A atoms are made to contact each other and B atoms are also made to contact each other, the ratio of the atomic radii iJrB that permits this is 1.225. In practice, for the known Laves phases, this ratio varies from 1.05 to 1.68. On the otherhand, there are AB2 combinations whose radius ratio lie within the typical range which do not form Laves phases.

ref:http://shodhganga.inflibnet.ac.in/bitstream/10603/76947/10/10_chapter%202.pdf

1.2.1 Mechanical Properties high-temperature applications: Ni3Al, NiAl, Ti3Al, TiAl, Fe3Al and FeAl 1.2.2 Thermoelectric Properties high thermoelectric efficiency: PbTe and other compounds like hexaborides LaB6; Bi2Te3 1.2.3 Electronic Properties Superconductivity: Nb3Sn; Nb3Os or Mo3Ge 1.2.4 Thermal Properties High thermal conductivity 1.2.5 Magnetic Properties MnBi with extremely high coercivities, and SmCo5 (Brooks et al 1991), were found to be useful as permanent magnets.

ref:http://shodhganga.inflibnet.ac.in/bitstream/10603/39427/6/06_chapter1.pdf

Ces composés présentent des propriétés intéressantes pour de nombreuses applications. Les composés intermétalliques sous forme de particules dispersées dans une matrice métallique (phénomène de précipitation) modifient de façon notable les propriétés de cette matrice. De nombreux alliages sont durcis par précipitation : précipitation des carbures (aciers spéciaux), des composés intermétalliques (alliages légers, aciers à hautes performances). Les eutectiques contrôlés possèdent de remarquables propriétés mécaniques : ce sont des composés intermétalliques sous forme de longues baguettes ou de grandes lamelles, régulièrement disposées parallèlement à une même direction, dans une matrice plus ductile (principe du renforcement par fibres). Les composés à grande maille ont souvent une très grande dureté, accompagnée d'une fragilité très marquée ; ils sont souvent utilisés sous forme de pièces frittées (carbures pour outil de coupe). Des composés covalents comme InSb et GaAs sont utilisés comme semiconducteurs, d'autres (de type A3B) ont des températures critiques de supraconduction élevées : 18,05 K pour Nb3Sn et 17,9 K pour V3Si.

ref:https://www.universalis.fr/encyclopedie/composes-intermetalliques/