Crystal Laves phases

There have been many reviews and discussions of the crystal chemistry of the Laves phases (among the more recent see Berry Raynor, 1953 Laves, 1956 Elliott Rostoker, 1958 Bardos, Gupta Beck, 1961 Dwight, 1961 Nevitt, 1963) but none of these explicitly discuss what we find to be their most remarkable features ... 

Table 3.7. Summary of the crystal structure data of the three most important Laves phases (see 7.4.33). 

Remarks on the alloy crystal chemistry of the 4th group metals. Selected groups of isostructural phases, pertaining to simple common structural types have been collected in Table 5.25. A number of them (for instance CsCl, AuCu types, Laves phases, AuCu3 type) correspond to more or less extended solid solution... 

GdAh crystallizes in the pure C15 structure (cubic Laves phase, MgCu2 type, space group Fd3m) without vacancies on the Gd sites, which lead to a superstructure of Cl 5 in case of GdNi2 (see section 4.2). 

The structure of YCu4.74Pbo.26 is shown as an example in Figure 19. The copper atoms on the 16c site build up a three-dimensional network of corner-sharing tetrahedra. This network leaves voids of coordination number 16 which are filled by the rare earth and Pb/Cu atoms on the four-fold sites. The large coordination number of this void readily explains the higher lead content on this site. The atoms occupying the 16c site (mainly copper in the RCus-xPb series) have coordination number 12 in the form of a distorted icosahedron. For more crystal chemical details on the Laves phases and related compounds we refer to review articles (Simon, 1983 Nesper, 1991 Johnston and Hoffmann, 1992 Nesper and Miller, 1993). 

X-ray powder diffraction revealed that the crystal structure of the sample was characterized as hexagonal C 14-type Laves phase and no second phase was found. The lattice parameters were a=4.92A and c=8.07A. 

There are several dozen metallic AB2 compounds called Laves phases that are superconducting they have either cubic or hexagonal crystal structures. Some have critical temperatures above 10 K and high upper critical magnetic fields Bc2- For example, Zri/2Hfi/2V2 has rc = 10.1K, B 2 = 24 T, and a compound with a different Zr/Hf ratio has similar and Bc2 values with the critical current density Jc 4 X 10 A/cm. These materials also have the advantage of not being as hard and brittle as some other intermetallics and alloys with comparable transition temperatures. 

RNi2 compounds crystallize in the cubic Laves phase structure. Because of the simplicity of structure and the ease of preparation and characterization these materials have been extensively studied. From magnetic studies of this family of compounds, Skrabek and Wallace (55) established that nickel moment is zero in the ordered state and moreover that the moment of the rare earth atom is considerably reduced in comparison to that expected for a free trivalent rare earth ion. Bleaney (86) and Skrabek and Wallace (55) have interpreted this decrease in the saturation moment as arising from partial crystal field quenching of the orbital contribution to the total moment. In this respect the RNi2 compounds behave like the RA12 compounds described in an earlier section above. 

Fig. 10.4. Rod-shaped precipitates in Cr-Nb alloy (courtesy of Sharvan Kumar). The precipitates are Cr2Nb and have the C15 Laves phase crystal structure.

Table 2.1. Structures of Laves-phase Fe2-rare-earth compounds (CIS) exposed to hydrogen gas at a pressure of 5 MPa and various temperatures for 86.4 ks, including the crystallization temperatures Tx of the hydrogen-induced amorphous alloys [2.33]...

LiAl has been shown to possess a crystal structure of the NaTl type. The crystal structure of RaAL shows that it is actually 6a7Ali3, and belongs to the trigonal space group P3ml. The atomic arrangement is very similar to that in the Laves phase prototype, MgNi2-... 

The Laves phases - sometimes designated as Friauf-Laves phases - with an AB2 composition in the binary case form a very large group of intermetallics which crystallize with the hexagonal C14 structure, the cubic Cl 5 structure, or the dihexagonal C36 structure (Laves, 1967 Wernick, 1967 Livingston, 1992). These structures are topologically close-packed (tcp) structures (Wernick, 1967 Schulze etal., 1973 Watson and Bennet, 1984), i.e. the Laves... 

The partial substitution of the transition element M in MjSi by a second transition metal M leads to ternary silicides of approximate composition MM Si corresponding to (M,M )2Si. Such ternaries are primarily the Si-containing E phases and V phases (Jeitschko etal., 1969 Jeitschko, 1970) and the ternary Si-containing Laves phases (Bardos etal., 1961), which were discussed in Sec. 8, as well as many other phases, which all differ by composition and crystal structure (Nowotny, 1972 a). This is exemplified by the Fe-Nb-Si system with the ternary silicides E, V, Xj, Xj, Xj and the Laves phase Nb(Fe,Si)2 with up to 25 at.% Si (Raghavan, 1987), or the Co-Nb-Si system with the ternary silicides E, T, v, Ti, v i, and the ternary Laves phase Nb(Co,Si)2 with Si contents between about 10 and 20 at.% (Argent, 1984). Finally, it is noted that other phases - in particular a phases and A13 Mn-base phases - dissolve large amounts of Si by which these phases are stabilized (Gupta et al., 1960 Bardos et al., 1966). 

The present monograph was first written as a chapter for Volume 8 of the series Materials Sdence and Technology A Comprehensive Treatment , edited by Robert W. Cahn, Peter Haasen, and Edward J. Kramer (Volume Editor Dr. Karl Heinz Matucha). Its aim is to give an overview of intermetallics, which is both detailed and comprehensive and which includes the fundamentals as well as applications. The result is an extended, critical review of the whole field of intermetallics with an emphasis on those intermetallic phases which have already been applied as functional or structural materials or which are currently the subject of materials developments. A historical introduction and a discussion of the relationship between atomic bonding, crystal structure, phase stability and properties is followed by a discussion of the major classes of intermetallics. The titanium aluminides, nickel aluminides, iron aluminides, copper phases, A15 phases. Laves phases, beryllides, rare earth phases, and siliddes are reviewed. In particular, the crystal structures, phase diagrams, and physical properties as well as the mechanical and corrosion behavior are treated. The state of developments as well as prospects and problems are discussed in view of present and future applications. The publisher has decided to publish the review as a separate monograph in order to make it accessible to a wider audience. 

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