Effect of Alloy Composition on the Structure of Zr Based Metal Alloys

The structures of Ovonic multi-element, multi-phase Zr-based transition metal alloys for hydrogen storage are studied. The alloys are designed to be multi-phase materials. The hexagonal and diamond cubic structures, known as C14 and C15 Laves structures respectively, are the two major hydrogen storage phases in the alloys. In both C14 and C15 structures, Zr and Ti are the elements typically occupying the hydride former (A) sites and V, Cr, Mn, Fe, Co and Ni the catalytic (B) sites. Based on the application of Ovshinsky's design principles for disordered materials, both A and B sites are compositionally disordered by the corresponding elements in cubic (C15) structure. However, the hexagonal (C14) structure has two distinct B sites, B(I) and B(II) with a 1:3 ratio. Preliminary neutron diffraction studies indicate that the B(I) sites are predominantly occupied by Ni and the B(II) sites are randomly mixed with V, Cr, Mn, Fe, and Ni. It is then proposed that the formula of the hexagonal structure should become A{sub 2}B(I){sub 1}B(II){sub 3} in the current multi-element Zr-based alloys. Minor phases close to the structures of Zr{sub 7}Ni{sub 10}, Zr{sub 9}Ni{sub 11}, and ZrO{sub 2} are also present in some alloys. The average electron concentration factormore » (e/a) derived from the alloy composition dictates the alloy structures. The alloys with higher electron concentration factor (>7.1) favor the diamond cubic structure. On the other hand, the hexagonal structure is associated with the alloys with lower electron concentration factor (<6.5). The alloys having electron concentration factors in between are mixtures of the diamond cubic and hexagonal structures.« less