Structural Relaxation in Fe-B Glasses
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STRUCTURAL RELAXATION IN Fe-B GLASSES
Z. Altounian , J.O. Strom-Olsen and N. Olivier *Department of Physics, McGill University, 3600 University Street, Montreal, Quebec, Canada. H3A 2T8. **Division of Chemistry, National Research Council, Montreal Road, Ottawa, Ontario, Canada. KIA 0R6.
ABSTRACT We present results on structural relaxation in Fe-B glasses. Relaxation effects were studied through measurements of the Curie temperature (Tc) of the glasses annealed at different temperatures (Ta). We observe a monotonic increase in Tc with Ta in all alloys due to irreversible structural relaxation. This increase continues till the start of surface/ bulk crystallization. The observed decrease in Tc upon further annealing is attributed to the strain introduced in the partially glassy system. We see no evidence for reversible structural relaxation in Fe-B glasses.
INTRODUCTION It is well known that metallic glasses on being annealed relax to a structure closer to a local minimum of free energy. Structural relaxation causes changes to many physical properties which are themselves used to monitor the relaxation process. Relaxation has been classified as of two types. The first type gives irreversible, often monotonic, changes to a property and is generally attributed to a decrease in free volume by the disappearance of voids, resulting in an increase in topological short range order (TSRO). The second type is less widely seen and is characterized by reversible changes of property -
i.e.
the system can be cycled between
different glassy structures by annealing at different temperatures. There have been suggestions [1] that reversible relaxation is a result of changes in chemical or compositional short range order (CSRO). Both irreversible and reversible structural relaxations have been demonstrated in a number of magnetic metallic glasses, mainly through measurements of the Curie temperature, Tc, which is a simple and sensitive measure of the structural state. The alloys investigated were usually of the metal-metalloid type, containing more than two metallic elements (e.g. Fe4 0 Ni 4 0 B2 0 ). Quite recently structural relaxation results on binary Fe-B glasses were presented by So et al. [2] and interpreted as a reversible structural relaxation, the driving force being the deviation of the CSRO in the glass from that in crystalline Fe 3 B. The extensive work of Nold et al. [3] on the structure of amorphous Fe8oB2O by X-ray and neutron diffraction provided for the first time reliable experimental partial structure factors. The results showed that in Fe-B glasses, the B atoms are at the centre of a coordination sphere containing nine Fe atoms with a very well defined Fe-B distance and with no B-B atoms in contact. The B coordination is independent of concentration [4] and implies that the B atoms force a special arrangement of the Fe atoms by chemical interaction rather than just fill up the Bernal holes in the dense random packing of the Fe atoms, and indeed Nold et al. [3] find that the normalized CSRO parameter is 1.0 c
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