The Evolution of Short-Range Order in Ti-Hf-Ni Alloys with Hydrogenation
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The Evolution of Short-Range Order in Ti-Hf-Ni Alloys with Hydrogenation Anne Sadoc1,2, Van T. Huett3, K. F. Kelton3 1 LPMS, Université de Cergy-Pontoise, Neuville sur Oise, 95031 Cergy-Pontoise Cedex, France 2 LURE, Centre Universitaire Paris-Sud, BP 34, 91898 Orsay Cedex, France 3 Department of Physics, Washington University, St. Louis, MO 63130, USA.
ABSTRACT The evolution of the short-range order in a Ti-Hf-Ni 3/2 approximant phase, has been studied as a function of hydrogenation by means of extended X-ray absorption fine structure (EXAFS). The local structure of the nonhydrogenated alloy has been derived from that of 1/1 Ti-Zr-Ni. After hydrogenation, drastic changes are observed in the local structure of Ti-Hf-Ni. Preliminary results of the EXAFS analysis are also presented for an amorphous TiHf-Ni alloy. INTRODUCTION A way to reduce emissions from internal combustion engines is to use cars that run on an electric motor. A promising technology is the fuel cell. A fuel cell generates energy by oxidizing molecular hydrogen, and its only byproduct is water. It is therefore of great importance to improve our understanding of the mechanism of hydrogen absorption in metal-hydrogen systems. In particular, Ti- based quasicrystals and related crystals might be excellent materials for hydrogen storage because of their high number of tetrahedrally coordinated sites suitable for interstitial hydrogen and their favourable hydrogen chemistry. Ti-Zr-Ni and Ti-Hf-Ni quasicrystals can absorb up to two hydrogen atoms for each metallic atom, making them candidates for novel hydrogen storage materials [1, 2]. The extended X-ray absorption fine structure (EXAFS) experiment is a convenient method for probing the structure of matter at the atomic and nanometer scales. Although, like X-ray diffraction, EXAFS is not directly sensitive to the presence of hydrogen, it has allowed the study of the average change in local order induced by hydrogen in Ti-Zr-Ni alloys [3]. However, the formation of a crystal hydride, which is detrimental to hydrogen cycling, occurs in the Ti-Zr-Ni alloys. Since the growth of this irreversible hydride phase is less in the Ti-Hf-Ni alloys, we have undertaken an EXAFS study of the evolution of the short range ordering as a function of the hydrogen content in both crystal and amorphous Ti-Hf-Ni alloys. This will provide a complement to the information on the averaged, long-ranged ordering available from diffraction measurements, thereby advancing the understanding of the hydrogenation. EXPERIMENTAL DETAILS Samples of Ti40Hf40Ni20, a 3/2 cubic rational approximant phase, and of Ti25Hf50Ni25 , an amorphous alloy, were prepared and characterized at Washington University [4]. The samples were loaded at 250oC with hydrogen from the gas phase to H/M = 0.8, 1.2, 1.4 and 1.7 for the approximant phase and 1.2 and 1.4 for the amorphous phase. In the following discussion, the non-hydrogenated 3/2 and amorphous samples will be designated as ‘3/2’ and ‘a’ respectively, while the hydrogenated ones will be called ‘
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