Atomic structure of amorphousAl100−2xCo

Abstract

The local atomic structure of Co, Fe, and Ce in aluminum-rich amorphous alloys of Al&00 2"Co"Ce" (x=8, 9, and 10) and Ce in A180FeloCelo has been investigated by x-ray-absorption fine structure (XAFS) spectroscopy. The materials, 1and 3-p,m-thick films, were prepared by vapor quenching using the dc magnetron sputtering method. %'e show that the composition of the glass-forming region of these magnetron-sputtered alloys strictly follows the theoretical limit calculated on the basis of the atomic size criterion. From analyses of the XAFS data at the E edges of Co and Fe and the L3 edge of Ce, the following conclusions with regard to local structure are made. The first coordination sphere of Co in Alloo 2"Co"Ce"(x=8, 9, and 10) consists of 5.8 to 6.4 Al atoms at a distance of 2.44 A. The local coor-0 dination sphere for Fe in A180FeloCelo consists of 6.4 Al atoms at a distance of 2.47 A. Ce in both sys-0 tems appears to be coordinated with roughly 5 and 9 Al atoms at distances of 2.95 and 3.15 A, respectively. These results are discussed in light of the dense random packing (DRP) of hard spheres model. Both Co-Al and Fe-Al bond lengths are anomalously short (9 and 8%%uo contraction, respectively) with also anomalously low coordination numbers (45% reduction) from values based on the DRP model using the metallic state radii. These anomalous changes indicate a strong interaction between Co or Fe atoms and the Al atoms which perhaps may be a result of a covalently bonded environment. The Ce-Al dis-0 tance in both the Al-Co-Ce and Al-Fe-Ce systems, on the other hand is smaller by only 0.17 A (a contraction of only 5%) and the coordination number is reduced by only 13% from expected values based on the DRP model. The contraction in the distance and reduction in the coordination number for Ce are much smaller than those values of the Co or Fe and, hence, the local bonding for Ce is likely to be metallic in character. Ihf+RODUtmrONAluminum-based metallic glasses with remarkably high Al content are relatively new materials that were discovered independently by He, Poon, and Shiflet, ' and Tsai, Inoue, and Masumoto. Typical composition is Al-TM-R where TM is a late transition element such as iron, cobalt, or nickel and 8 is yttrium or a rare-earth element such as gadolinium or cerium. These amorphous alloys can be prepared with extremely high aluminum content either by vapor quenching using the dc magnetronsputtering method3 or rapid solidification from the liquid phase using the melt-spinning method. ' The amorphous phase is formed with an Al content as high as 84 at. %%u o forsample sprepare db y th emagnetron-sputtering process and as high as 90 at. %%u o forsample sprepare dby the melt-spinning process. These materials are truly noncrystalline alloys which combine the properties of a metal with the short-range order of a glass. They are very homogeneous and lack the defects such as grain boundaries and dislocations typical of a crystalline material. The homogeneity and the lack of grain boundaries have led to a number o...

Amorphous and nanostructured Al85Ni5Y6Co2Fe2 powder prepared by nitrogen gas-atomization

“…Al-TM-RE amorphous alloys [19][20][21][22]. A shortening in the Al-TM (TM = transition metal) bond length was observed, a characteristic of covalent bonding, whereas the Al-RE bond length is just slightly contracted compared with the metallic state radii, likely to be metallic in nature.…”

Hierarchical densification and negative thermal expansion in Ce-based metallic glass under high pressure

“…1c). Al-Co pair-centred extended cluster is also possible due to their strong interaction with covalent bondinglike characteristics 41 . Even the solute-solute connection (Al-Al or Co-Co or Al-Co) could become network-like due to a high concentration (35%) of Al and Co.…”

Amorphization and crystallization in Al–Ni–La during mechanical milling

“…This deviation from experimental results (i.e., the minimum concentration for Fe plus Ce is ∼10%) would be even higher if we consider the fact that the strains required to destabilize the Al solid solution caused by Ce and Fe have opposite signs in this model. On the basis of neutron scattering [28], X-ray scattering [29] and EXAFS [30] data, Egami [26] then suggested that if a short-range order (Al 6 Fe, where Fe atoms are coordinated by Al octohedra) is imposed, the minimum concentration of Fe for glass formation is estimated to be ∼11% using Eq. (8).…”