Ternary Laves phases

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 ... 

Stein et al. (2005) observed on the basis of their systematic analysis that the site occupancies in ternary Laves phases cannot be predicted simply from the atomic... 

N. Hanada, S. Orimo, H. Fujii, Hydriding properties of ordered-/disordered-Mg-based ternary Laves phase structures, J. Alloys Compd. 356-357 (2003) 429-432. 

Since the transition metal elements in these ternary Laves phases can substitute for each other freely, and since the distribution of Laves phase can be controlled by thermomechanical treatments there are possibilities for optimizing such NiAl alloys with Laves phases with respect to creep resistance and the brittle-to-ductile transition temperature (BDTT) by controlling the composition and phase distribution, and this is being studied presently... 

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 ordered ternary Laves phases are discussed with the 6725 Sc2C03Si structure. The disordered Laves phases can be found with the 67(50) Sc(Ni 5Al 5)2 compound. A tendency toward a partial ordering of the non-rare-earth elements is found to be related to the T/M ratio. See for example Oesterreicher (1973a). 

Sc2RuA13 ( Type Mg2Cu3Si ternary Laves phase) ... 

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]

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. 

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. 

Another example is given in Fig. 27, which illustrates the strengthening of NiAI by ternary Cl4 Laves phases. The data points for TaNiAl-NiAl seem to follow a rule of mixtures, but the data for NbNiAl-NiAl clearly indicate a significant deviation from a lineal superposition for the creep resistances. Furthermore, the extrap-... 

Two solid solutions, a (bcc (A2) and y (fee (Al), and four intermediate binary phases a, p (FeyMog), (Laves phase, Fe2Mo) and the high temperature R phase, all with signifieant solubility of the third element, exist in this system. One ternary phase, Ti, was found. Miseibility gaps are found in the a phase in both the Cr-Fe and Cr-Mo systems. The crystallographic information available is shown in Table 2. 

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