Short Range Chemical Ordering in Bulk Metallic Glasses
- PDF / 41,007 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 100 Downloads / 288 Views
Short Range Chemical Ordering in Bulk Metallic Glasses P. A. Sterne1, P. Asoka-Kumar1, J. H. Hartley1, R. H. Howell1, T.G. Nieh1, K. M. Flores2, D. Suh2, and R. H. Dauskardt2 1 Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94550 2 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305
ABSTRACT We provide direct experimental evidence for a non-random distribution of atomic constituents in Zr-based multi-component bulk metallic glasses using positron annihilation spectroscopy. The Ti content around the open-volume regions is significantly enhanced at the expense of Cu and Ni, indicating that Cu and Ni occupy most of the volume bounded by their neighboring atoms while Ti and Zr are less closely packed and more likely to be associated with open-volume regions. Temperature-dependent measurements indicate the presence of at least two different characteristic sizes for the open volume regions. Measurements on hydrogencharged samples show that the larger open-volume regions can be filled by hydrogen up to a critical density. Beyond this critical density, local atomic-scale open-volume damage is created in the sample to accommodate additional hydrogen. The onset of this local damage in positron annihilation data coincides with the onset of volume expansion in X-ray diffraction data.
INTRODUCTION A number of zirconium-based multi-component alloys form bulk metallic glasses at relatively slow cooling rates [1,2]. These glasses exhibit a number of interesting physical properties, including large elastic strains to failure, unusual diffusion characteristics, and high tensile strengths [3-5]. Little is known about the detailed atomic arrangement of these metallic glasses, and how any such atomic structure may influence their mechanical properties. The fundamental processes responsible for atomic mobility and rearrangement are different from those for crystalline materials, and may be influenced by defect-like open-volume regions [4]. Positron annihilation spectroscopy (PAS) provides a sensitive probe of open-volume regions and defects in materials [6]. Positrons tend to occupy open-volume regions due to their Coulomb repulsion with the atomic nuclei. Consequently, positrons preferentially annihilate with electrons associated with atoms adjacent to these open-volume regions. Positron lifetimes measure the characteristic size of these regions, and previous work indicates that the metallic glasses studied here have single-component lifetime values that lie between those for the constituent bulk metals and metal vacancies [7,8]. Positron Doppler broadening experiments measure the electron-momentum distribution. The high-momentum part of this distribution results from annihilation with electrons that retain their orbital character in the solid, such as core electrons and atomic-like transition-metal delectrons. The resulting momentum distributions are therefore characteristic of the atom type, and so can be used to determine the chemical species associated with the open-volume
Data Loading...
