High-Frequency Dynamics in Metallic Glasses

Abstract

Using Inelastic X-ray Scattering we studied the collective dynamics of the glassy alloy Ni33Zr67 in the first pseudo Brillouin zone, an energy-momentum region still unexplored in metallic glasses. We determine key properties such as the momentum transfer dependence of the sound velocity and of the acoustic damping, discussing the results in the general context of recently proposed pictures for acoustic dynamics in glasses. Specifically, we demonstrate the existence in this strong glass of well defined (in the Ioffe Regel sense) acoustic-like excitations well above the Boson Peak energy. The physics of disordered materials has attracted considerable interest during recent years [1]. This is mainly due to new theoretical concepts that can handle systems lacking long-range order, to large scale computer simulations based on realistic interactions and to the simultaneous development of new experimental techniques. Disordered materials show several kinds of universal behaviors that differ from those of the ordered state. In particular, great interest has been devoted to their high frequency (THz) dynamics in respect to the corresponding crystalline counterpart. In this context, Inelastic Neutron Scattering (INS) investigations of the collective dynamics in simple metallic glasses, which may be regarded as model systems, have played a leading role [2,3,4,5]. Because of their comparably simple atomic structure and their nearly isotropic interaction, metallic glasses (and glasses with Lennard-Jones (L-J) interaction), were the first tackled in extended computer simulations, and oneto-one comparisons with experiments were often undertaken. Simple systems like Mg 70 Zn 30 belong however to the minority of metallic glasses and interest has therefore moved soon to more abundant and characteristic representatives like the Zr-based transition metal-transition metal (TM-TM) or the TM-metalloid glasses, which are characterized by a strong topological and chemical short range order [6].An important role in this context plays the TM-TM glass NiZr, which can be produced by melt spinning in a large range of compositions. Of these, the Zr-rich concentrations, for which also crystalline NiZr 2 exists, have played a prominent role in the investigation of the dynamics of metallic glasses [6,7]. Concerning collective dynamics, in particular, the dispersion of modes around Q p , (the Q position of the main peak of the static structure factor), i.e. in the region of maximal diffuse umklapp scattering [8], was measured first for glassy Ni 24 Zr 76 [9] and then in Ni 67 Zr 33 [10]. In these pioneering experiments it was possible to prove the existence of Pseudo Brillouin Zones with clear Zone Boundaries near the maxima of the static structure factor, these latter acting like "smeared out lattice points" [3]. However, important questions concerning the properties of the collective excitations remained unanswered on the experimental side: the existence of a low Q linear dispersion, the wave-vector and energy transfer dependence of the velocit...

Inelastic neutron scattering applied to the investigation of collective excitations in topologically disordered matter

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An atomic mechanism for the boson peak in metallic glasses

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