Phase Transitions in Al 87 Ni 7 Nd 6
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Phase Transitions in Al87Ni7Nd6 Despina Louca1, K. Ahn1, A. K. Soper2, S. J. Poon1 and G. J. Shiflet3 1 University of Virginia, Dept. of Physics, Charlottesville, VA 22904, USA. 2 ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK. 3 University of Virginia, Dept. of Materials Science and Engineering, Charlottesville, VA 22904, USA.
ABSTRACT The local atomic structure of Al87Ni7Nd6 amorphous metallic glass was determined upon heating by neutron diffraction and the pair density function (PDF) analysis. Two isotopes of Ni with very different scattering intensities were used (58Ni and 60Ni) to separate the local environment of the transition metal. A distinct pre-peak observed in reciprocal space arises because of chemical clustering of Ni atoms. With increasing temperature the coherence length of this peak increases indicating an enhancement in atomic clustering. In addition, precipitation of Al metal is first observed at 200 oC with heating. Binary and ternary Al phases form as the temperature increases further to 500 oC although the majority crystalline phase is Al. The local atomic topology at the intermediate temperatures can be represented with a model that is a combination of the atomic structure at room temperature plus Al.
INTRODUCTION A group of Al-TM-RE (TM = transition metal, RE = rare earth) metallic glasses with remarkably high Al content (up to 90 at. %) was synthesized independently by He, Poon and Shiflet, and Tsai, Inoue and Masumoto [1, 2]. Previously studied compositions for the Al-TMRE glass, where TM is a late transition metal such as iron, cobalt or nickel and RE is a rare-earth metal such as yttrium, gadolinium or cerium, include Al87Ni5Y8, Al90Fe5Ce5 and Al87Fe8.7Gd4.3 [1, 3]. The amorphous phase is formed with an Al content as high as 84 at. % for samples prepared by the magnetron sputtering process and as high as 90 at. % for samples prepared by the melt spinning process. These materials are truly noncrystalline alloys and combine the properties of a metal with the short-range order effects of a glass. They are very homogeneous and lack defects such as grain boundaries and dislocations typical of crystalline materials. The homogeneity and lack of grain boundaries have led to a number of remarkable mechanical and magnetic properties [4]. Earlier neutron and x-ray structural studies of melt spun Al87FexCe10-x alloys revealed a strong interaction in the vicinity of the first coordination sphere of Fe [5, 6]. The Fe-Al bond lengths were found to be anomalously short with a suggested 8 % contraction and a low coordination number, a 45 % reduction from values based on dense-random-packing (DRP) of a hard spheres model. However, the Ce-Al bonds show relatively smaller changes with a 5 % reduction in the bond length and a 13 % reduction in the coordination number from the expected values
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based on DRP models. These anomalous changes for the Fe-Al bonds in the amorphous phase indicate a strong interaction between the Fe and Al atoms that may be in re
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