Local atomic structure of Ni₆₀Pd₂₀P₂₀ and Ni₆₀Pd₂₀P₁₇B₃ bulk metallic glasses and the origin of glass forming ability

Ni₆₀Pd₂₀P₁₇B₃ alloy has high glass forming ability (GFA) and forms bulk metallic glass (BMG) which exhibits good mechanical properties of a high strength and a large plasticity. Furthermore the plastic strain of the Ni₆₀Pd₂₀P₁₇B₃ BMG increases with lowering temperature by 3.4 times from 295 K to 77 K. In order to know the origin of such mechanical properties and reasons of high glass formability of this alloy from an atomic scale point of view, XAFS and X-ray diffraction measurements for Ni₆₀Pd₂₀P₂₀ and Ni₆₀Pd₂₀P₁₇B₃ metallic glasses have been done using the large scale synchrotron radiation. XAFS results of the Ni and Pd K-edge for the Ni₆₀Pd₂₀P₂₀ metallic glass can be well represented by the Pd₄Se-type structure model. A result of interatomic distances agrees with those of the ab initio calculations. Temperature dependence of the Debye-Waller factor reflects strong Ni-P and Pd-P bondings and a large thermal fluctuation of metal-metal bondings. A high GFA of the Ni₆₀Pd₂₀P₁₇B₃ alloy compared with Ni₆₀Pd₂₀P₂₀ is discussed based on the XAFS and total pair distribution results.