Composition, Laves phases

The term Laves phases is used for certain alloys with the composition MM3, the M atoms being bigger than the M atoms. The classical representative is MgCu2 its structure is shown in Fig. 15.4. It can be regarded as a superstructure of the CsCl type as in Fig. 15.3, with the following occupation of the positions a, b, c, and d ... 

Examples of true ternary phases may be observed in systems such as Nb-Ni-Al, Ta-Ni-Al, where no Laves phases exist in any of the boundary binary systems and a MgZn2-type phase Ta(Ni,Al)2 (or Nb(Ni,Al)2) is formed within the composition rangeTa(Nij xA1 c)2 (with0.15 [Pg.180]

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 10th group elements. Typical compositions and structure types of the binary phases formed with Ni, Pd, Pt are 1 1 (CsCl type or FeB or CrB), 1 2 (Laves phases), 2 7 (Gd2Co7 or Ce2Ni7 type), 1 5 (CaCu5), etc. 

Considering then the phase composition as a significant parameter, we obtain the histogram shown in Fig. 7.1(a) for the distribution of the intermetallic phases according to the stoichiometry of binary prototypes. For instance, the binary Laves phases, the A1B2, Caln2, etc., type phases are all included in the number reported for the 66-67.99 stoichiometry range, even if the real stoichiometry of the specific phase is different, see Fig. 7.1(b). We may note the overall prevalence of phases and, to a certain extent, of structural types, which may be related to simple (1 2, 1 1, 1 3, 2 3, etc.) stoichiometric ratios. 

X-ray analyses have been shown that the sample NbVCo is single phase and has C14 structural type with cell parameters (table 1) corresponding references [7]. Specific nature of IMC NbVCo is that X-ray make weak difference between V and Co atoms with the close atomic numbers and for neutron V atoms are invisible. Using both methods allowed to determine (table 2,3 Fig. 1,2) that in NbVCo Nb atoms located in 4f site (site of A atoms in AB2) and V and Co atoms distributed in accordance with composition in 2a, 6h - B site in the hexagonal structure of Laves phase. 

The magnetic moments of rare earth elements are caused by the unpaired electrons in their 4f shells. These shells" are shielded by the outer shells so that chemical bonding has relatively little effect on the magnetic moments of these elements. The rare earths form a series of Laves phases of composition MB2 (M = rare earth, B = precious metal) which share the characteristic of ferromagnetic coupling at low temperatures (7). Figure 13 shows the Curie temperatures of a series of com-... 

A large number of alloys (nearly 100 are known) at a composition MN2 display one or other of three closely related structures, often termed Laves phases. The simplest and most common of these, of which MgCu2 is an example, is illustrated in fig. 13.04. If we consider first the arrangement of the smaller copper atoms we see that they are disposed... 

Besnus et al. (1982) have performed magnetization, diffuse neutron scattering, 57Fe Mossbauer and NMR studies on these C15 Laves phase compounds over a wide range of composition. The Curie temperatures and average moments are reduced with increasing Mn content and it was concluded from the neutron and NMR data... 

The transition from antiferromagnetism to ferromagnetism at stoichiometric composition is ascribed to Fe atoms entering Ti sites in the C14 (Laves phase) structure... 

Up to now the fracture toughness at room temperature is 2-4 MNm (see data in Sauthoff, 1990 a, b) for these alloys, and the brittle-to-ductile transition temperature is between about 500 °C and 700 °C depending on composition and preparation method. Analysis of the microstructures of the deformed alloys has shown that in the brittle-to-ductile transition range cracks in the Laves phase are stopped at the Laves phase-NiAl interface by producing a plastic zone in the NiAl phase ahead of the crack (Machon, 1992 Wunderlich etal., 1992). 

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