Laves phases binary

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. 

Pseudo-binary Laves phases. The relatively easy formation of solid solutions of binary Laves phases has resulted in an intensive investigation of various pseudo-binary series, especially those for U-3d compounds. They provide us with the possibility to trace the formation or the destruction of moments which accompanies the variations of the electronic structure in a continuous way. The pseudo-binary series can be divided into four groups which will be discussed separately below. 

The formed hydrides in these binary Laves phases are too stable for easy hydrogen desorption, which is a prerequisite for hydrogen storage (Ivey and Northwood, 1986 b). The hydride stability can be reduced and adjusted to practical hydrogen storage conditions by deviations from stoichiometry, substitution of the B element primarily by Fe, Co, Ni, Cu, Mn, or Cr, or substitution of the A element primarily by Ti, or any combination of these alloying possibilities. 

Structure data are summarized in Table 2. No ternary phase has been reported in the Cr-Fe-Zr system. However binary Laves phases show extended solubihty ranges along the ZrFc2-ZrCr2 quasibinary section. The variation of the lattice parameters with composition along the section are reported in Fig. 1 according to [1970Kan]. 

The cubic Laves phase compounds RFc2 were first studied by Wertheim and Wernick (1962) who found a low temperature hyperfine field of roughly 230 kOe at the iron nucleus, essentially independent of the rare earth element. in the compound. Although this value is considerably smaller than in 3d metal alloys ( =320kOe), the isomer shifts are similar in the two cases, indicating that the electronic configuration of the iron is the same. The conclusion is that conduction electron polarization is responsible for the decreased hyperfine field in the rare earth compounds. Examples of NGR work on the pseudo-binary Laves phases with iron include R(FeCo)2 [Guimaraes and Bunbury (1973)] and Dy(Fe jNii i)2 [Burzo et al. (1975)]. These results are summarized in table 18.13. 

According to Rajasekharan and Girgis on a A t>, A ws1/3 map, considerable resolution is obtained among the binary systems in which different structure types occur. The points corresponding to the systems in which the Laves phases (or the phases of types such as Cr3Si, TiAl3, etc.) occur show linear relationships on the... 

Typically, binary Laves compounds AM2 are formed in several systems of A metals such as alkaline earths, rare earths, actinides, Ti, Zr, Hf, etc., with M = Al, Mg, VIII group metals, etc. Laves phases are formed also in several ternary systems either as solid solution fields extending from one binary phase (or possibly connecting the binary phases of two boundary systems) or as true ternary phases, that is forming homogeneity fields not connected with the boundary systems. 

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]

A systematic analysis of structure and stability of binary and ternary Laves phases, or Friauf-Laves phases, has been performed by Stein et al. (2004, 2005). By reviewing a large number of experimentally determined phase diagrams with Laves phases, a number of general conclusions have been obtained. These may be summarized in the following points ... 

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. 

Laves phases occurring in ternary systems, the binary subsystems of which contain no Laves phases, generally pertain to the hexagonal MgZn2-type structure. 

Stein et al. (2005) observed also that, in binary and ternary systems, the homogeneity regions of different Laves phases are generally separated by two-phase fields which are very narrow and of difficult determination. 

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. 

Notice, moreover, that for a family of binary and complex phases such as the Laves phases (Cu2Mg, MgZn2, Ni2Mg types) an overall number of about 1400 has been estimated. The restriction of the phase concentration to a limited number of stoichiometric ratios is also valid (and, perhaps, more evident) for the ternary phases. We may notice in Fig. 7.2, adapted from a paper by Rodgers and Villars (1993), that seven stoichiometric ratios (1 1 1, 2 1 1, 3 1 1, 4 1 1, 2 2 1, 3 2 1, 4 2 1) are the most prevalent. According to Rodgers and Villars they represent over 80% of all known ternary compounds. 

Among the actinide compounds the interest is concentrating on binary compounds of simple structure (e.g. 1 1 compounds with elements of the groups V and VI of the periodic table) for which the theoretical treatment is rather advanced, and on intermetal-lic (e.g. Laves-) phases. 

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... 

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