Exafs and Exelfs Study of the Structure of Pd-Ni-P Bulk Metallic Glasses
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EXAFS and EXELFS Study of the Structure of Pd-Ni-P Bulk Metallic Glasses Faisal M. Alamgir, Himanshu Jain, David B. Williams, Gilles Hug*, Ricardo B. Schwarz**, Ou Jin** Dept of Material Science & Engr., Lehigh University, Bethlehem, PA 18015, USA * LEM, ONERA-CNRS, B.P. 72, F-923722, Chãttilon, France ** Los Alamos National Lab, Los Alamos, NM 87545, USA ABSTRACT We have explored the short-range order around all three constituent atoms in (Pd-Ni)80P 20 bulk metallic glasses (BMGs), a system that is a prototype for a whole class of BMG formers containing 80% transition metal and 20% metalloid. We have examined the changes in the nearest neighbor environments around the transition metals in (Pd-Ni)80P 20 glasses using extended X-ray absorption fine structure (EXAFS) in comparison to their binary counterparts. We have done similar studies around the coordination of P using extended energy loss fine structure (EXELFS). The environment around the all the atoms in Pd 60Ni20P 20 and Pd30Ni50P 20 are very similar to those of the binary phosphides at the ends of the composition range. However, the (Pd-Ni)80P 20 glasses are not simply solid solutions of the phases. The nearest neighbor distances of the metals are reduced in the ternary alloys with respect to those of the binary phosphides. The best glass former in this series, Pd40Ni40P20 is nearly isostructural to Pd30Ni50P 20 but shows shorter distances at the 2nd and 3rd coordination shells, which we believe is due to more efficient packing in this glass. The metal environments in Pd40Ni40P20, on the other hand, are better described by a weighted average of those of Pd 30Ni50P 20 and Pd 60Ni20P 20.
INTRODUCTION There has been a rapid expansion in the field of metallic glasses ever since the discovery of alloy compositions and processing techniques which allow bulk glass formation [1,2]. By definition, bulk metallic glasses (BMGs) are characterized by a critical cooling rate < 10 3 K/s or a minimum dimension > 1mm [3]. Binary alloys of pure transition metals (TMs), or transition metals-metalloids (TM-M), can undergo a liquid-glass transition using high enough cooling rates, but they do not form BMGs. The (Pd-Ni)80P 20 system, on the other hand, has a critical cooling rate lower than 1 K/s and is one of the simplest prototype of a TM-M BMG. With three constituent elements a study of the structure of these glasses remains tractable in comparison to other BMGs that often contain five or more elements. This system offers a wide range of bulk glass formability, with Pd40Ni 40P 20 as the most stable glass composition [4]. A basic question that remains to be answered about TM-M BMGs is this: what determines their easy glass forming ability? This has been explained, to some extent, for other BMG systems by different criteria that are based on the suppression of the nucleation of crystals [5], the suppression of the kinetics via the “confusion principle” [6], and atomic size effects of L2.4.1
the constituent elements [7]. What can the atomic and electronic structure of a BMG sys
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