Effect of a minor element with a large mixing enthalpy difference on the plasticity of amorphous alloys
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Hwi-Jun Kim Division for Bulk Amorphous and Nano Materials, Korea Institute of Industrial Technology, Incheon 406-130, Korea
Jae-Chul Leea) Department of Materials Science and Engineering, Korea University, Seoul 136-701, Korea (Received 31 January 2007; accepted 8 May 2007)
In this study, we investigated the role of a minor alloying element in improving the plasticity of amorphous alloys. The plasticity of the amorphous alloys, Cu60−x Zr30Ti10Bex , was drastically improved with increasing amount of Be and reached a maximum of 23% at Cu53Zr30Ti10Be7. It was observed that an atomistic-scale phase separation existed within the alloy, which resulted from the large difference in mixing enthalpy between the binary pairs (Be–Cu, Be–Zr). This atomistic-scale phase separation resulted in an open structure in which atomic rearrangements in the form of the creation of free volume and crystallization were facilitated during deformation. Here we discuss the origin of the enhanced plasticity by clarifying the effect of an additional element, whose mixing enthalpies with the major elements are significantly different, on the structural change of the amorphous alloy.
I. INTRODUCTION
The plastic deformation of amorphous alloys is obtained mostly from the generation of shear bands, and the degree of plasticity depends on the total number of shear bands that are generated. It is well known that structural inhomogeneities such as excess free volume,1,2 nanocrystals,3–5 and phase-separated particles6,7 are preferential sites for the generation of shear bands. Therefore, to improve the plasticity of an amorphous alloy, the generation of structural inhomogeneities during deformation should be facilitated. The rate of formation of structural inhomogeneities is closely related to the degree of atomic mobility. In other words, excess free volume and nanocrystals can be generated with ease in alloys when the atoms move easily and are readily rearranged. From this perspective, it is necessary to facilitate the atomic movement within an amorphous alloy, to promote its plasticity. Because the atomic mobility mainly depends on the packing state, the structure should have a low packing density to obtain a high atomic mobility. In general, the nonuniform mixing of the constituent elements lowers the packing density of an amorphous alloy. Therea)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0327 2558 J. Mater. Res., Vol. 22, No. 9, Sep 2007 http://journals.cambridge.org Downloaded: 14 Mar 2015
fore, inhomogeneous mixing of the elements can enhance the plasticity of amorphous alloys due to the high atomic mobility associated with the resultant low packing density. Recently, several attempts have been made to enhance the plasticity of amorphous alloys. One of the methods attempted is the addition of a minor element having a positive enthalpy of mixing with the major elements,8–10 leading to the development of inhomogeneous mixing between the constituent elements. However, this technique has a li
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