AB2 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. 

In essence, metal/intermetallic hydrides of AB, AB2 (Laves phases), AB5, and A2B have not been improved to any remarkable extent since the end of the 1990s. Nevertheless, there have been some efforts directed to either improvement of synthesis or properties of metal/intermetallic hydrides particularly by application of ball milling associated with nanostructuring. Some of these more recent efforts will be briefly discussed in the following sections. 

Shaltiel, D., Jacob, I., Davidov, D. (1977) Hydrogen absorption and desorption properties of AB2 Laves-phase pseudobinary compounds J.Less-common metals 53, 117-131. 

Only rare-earth system (AB5-type) and zirconium-titanium-vanadium system (AB2 Laves phase-type) hydrogen storage alloys have been used as negative electrode materials for the commercial production of Ni-MH batteries [3, 7, 8], However, these materials have a low hydrogen storage capacity resulting in a low electrode energy density. 

Figure 3.24. The AB2 Laves phases (a) the hexagonal structure (the unit cell is shown by the dashed line) and (b) the cubic structure. The A atoms are the dark circles.

Jacob, 1. and Shaltiel, D., Hydrogen sorption properties of some AB2 Laves phase compounds. Journal of the Less-Common Met., 1979, 65 p. 117-128. 

A traditional strength, and still a major application, of INS is the study of hydrogen-in-metal systems. In these systems hydrogen occupies a weU-defined lattice site, but is often free to move between sites. This proton mobility has led to numerous applications in battery technology, particularly for LaNisHx and the AB2 Laves phase compounds. The simplest model is to consider that the jumps are all of the same length (1), that jumps in any direction are equally probable, and that the jump time is negligibly small compared to the residence time (r). With these assumptions Equation [6] becomes ... 

It is in some measure demonstrated that the formation of A-B and B-B contacts provides the energy for the compression of the A atoms and permits AB2 phases with radius ratios so much larger (up to 1 -67) than the ideal (1-225) to adopt the MgCu2 type structure. At radius ratios somewhat lower than the ideal, the B atoms are insufficiently compressed for A-B and A-A contacts to form. This is probably a consequence of there being twice as many B atoms as A atoms, and it results in fewer known Laves phases with radius ratios below the ideal value than above it. 

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

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. 

Shoemaker D.P., Shoemaker C.B. (1979) Concerning atomic sites and capacities for hydrogen absorption in the AB2 ffiauf-laves phases. Les-Common Met. 668, 43-58. 

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. 

In he Laves phases AB2 typified by MgZn2, MgCu2, and MgNi2, with three closely related structures, the coordination of A by 12 B + 4 A is consistent with the view that the structures are determined primarily by size factors. These phases... 

Table 2.2. Hydrogen-induced amorphization of several types of the AB2 type intermetallic compounds with C15 structure (Laves phase RM2)...

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