Laves cubic

In addition to this cubic Laves phase, a variant with magnesium atoms arranged as in hexagonal diamond exists in the MgZn2 type, and further polytypes are known. 

When the atomic size ratio is near 1.2 some dense (i.e., close-packed) structures become possible in which tetrahedral sub-groups of one kind of atom share their vertices, sides or faces to from a network. This network contains holes into which the other kind of atoms are put. These are known as Laves phases. They have three kinds of symmetry cubic (related to diamond), hexagonal (related to wurtzite), and orthorhombic (a mixture of the other two). The prototype compounds are MgCu2, MgZn2, and MgNi2, respectively. Only the simplest cubic one will be discussed further here. See Laves (1956) or Raynor (1949) for more details. 

Figure 8.2 Structure (C15) of cubic Laves phases. MgCu2 is the prototype. Top—Mg sub-structure with the pattern of the diamond structure. Bottom—Cu sub-structure with four tetrahedral clusters in the tetroid holes of the diamond structure. The stars indicate the centers of the patterns.

An especially hard and stable Laves-type compound is cubic HfW2. Its melting point is 2650 °C, and its hardness at room temperature is 1900kg/mm2 (Stone, 1977). However, it has a high mass density, so its usefulness is limited. 

The importance of the geometrical factor in determining the stability of these phases has been pointed out (Pearson 1972). In a simplified description, Laves phases AM2 of the MgCu2 type may be presented as cubic face-centred packing of large spheres A which form tetrahedral holes that are occupied by tetrahedra of smaller spheres M. The ideal value of the radius ratio rA/rM is 1.225. The values experimentally observed for the various Laves types range from 1.05 to 1.7. 

If in a binary system, at a specific composition, both a cubic and a hexagonal Laves phase are formed, and then generally it is the Cu2Mg type which is stable at low temperature and the MgZn2 (or Ni2Mg) type at higher temperature. 

Alloys with the 7th to 9th group elements. Several Laves phases are formed either as the hexagonal MgZn2 type (with Re, Fe, Ru, Os) or as the cubic Cu2Mg type (with Fe, Co, Rh, Ir). CsCl-type compounds are given mainly by Yb. 

Figure 7.56. Characteristic sections of the cubic unit cell of MgAl204 (spinel). Compare the metal positions with those depicted in Fig. 7.30 for the atoms in the Cu2Mg-type Laves phases.

Zr-based AB2 Laves phase alloys consist of cast polycrystalline and nanocrystalline structures. The nanocrystalline microstructure could be obtained for quenching the melt-spun alloys after annealing151. Figure 1 shows SEM Micrographs of the cast Polycrystalline ABi Alloys. Microstructures of AB2-1 alloys consist of cubic C15 Laves phase, hexagonal C14 Laves phase and of AB2-4 is only Cl5. The white one of non Laves in AB2-I and AB2-4 is the phase Zr9Nil 1 and Zr7NilO respectively. 

AB2 ZrV2, ZrMn2, TiMn2 ZrV2H5.5 Laves phase, hexagonal or cubic... 

Table 10. Magnetic properties of cubic AnX laves phases intermetallics with non-magnetic elements... 

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). 

Eu—Ir system.—Bozorth et al. [238] have prepared Eulr2 by sintering the components between 800 and 1050° G. Eulr2 possesses cubic laves phase C15 and does not show [239] any EPR signal. It is believed that the europium in Eulrz is in the trivalent state. Gdlra was found to be isomor-phous with Eulr2. 

Irodova A.V. (1980) Directional order (k=0) in hydrogen solid solution based on Cubic (C15) Laves phases. (Preprint IAE-3308/9), Moscow, 16. 

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