Local Order in Amorphous Fe-alloys
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CC7.7.1
Local Order in Amorphous Fe-alloys
Despina Louca, Kyungsoo Ahn, V. Ponnambalam and S. J. Poon University of Virginia, Dept. of Physics, Charlottesville, VA 22904.
ABSTRACT The pair density function analysis of neutron diffraction data of Fe-based metallic glasses of the Zr and Mo series shows how the local atomic structure changes by chemical substitution. The results provide evidence for short-range chemical reorganization accompanied by a volume contraction that could in turn be associated with stronger glass forming ability. While the existence of chemical short-range topological ordering is enhanced in both systems by alloying with a transition metal such as Mn, locally, the atomic structure changes in a way that corresponds to an increase in bonding interactions. The shortening of bonds is also related to volume contraction that can in turn be associated with a reduction of the ferromagnetic coupling of the Fe sublattice and to a lower Curie transition temperature.
INTRODUCTION In spite of years of research, the number of systems that can form bulk metallic glass is still quite small. This is partly due to the limited understanding of the basic glass forming ability and partly due to the absence of a predictive mechanism for searching new glasses other than what is derived empirically. In the current understanding glass synthesis is somewhat determined by at least three rules such as the negative heat of formation, a large difference in the atomic size of constituent atoms along with a low liquidus eutectic [1]. Systems have actually been discovered that defy these rules as reported in refs. [2, 3, 4]. In the past decade or so, great efforts have been made to synthesize new kinds of amorphous alloys, especially in zirconiumbased and aluminum-based systems [1, 2, 3]. New amorphous alloys exhibiting a wide supercooled liquid region (> 50K) before crystallization were found to form by melt spinning in ZrAl-M (M = Ni or Cu) systems consisting of constituent elements with significantly different atomic sizes. The largest thickness, up to 7 mm, for glass formation has been found in the Zrbased systems [4]. Glass formation is a very complex phenomenon that has defied rigorous scientific understanding. However, it appears that substantial progress can be made now because of recent advances made in the field. These are the introduction of the concept of glass fragility by Angel [5], the measurement of fragility for bulk glasses by Busch et al. [6], and the development of the theory of atomic-size factor in glass transition and glass formability by Egami [7]. It appears that the key to glass formation is to slow down the kinetics of crystallization in the supercooled liquid where two factors can contribute to such a slowing down: the first is low diffusivity in the supercooled liquid, and the second is a small heat of crystallization, or relative stability of liquid against crystallization.
CC7.7.2
Bulk ferrous metallic glasses investigated in this study are intended for targeted mechanical and corrosion p
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