Origin of the plasticity in bulk amorphous alloys
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Eric Fleury Division of Advanced Metals, Korea Institute of Science and Technology, Seoul 130-136, Korea
Byeong-Joo Lee Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea
Masato Wakeda Department of Mechanical Engineering, Osaka University, Osaka 565-0871, Japan
Yoji Shibutani Department of Mechanical Engineering, Osaka University, Osaka 565-0871, Japan; and Center of Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan (Received 2 February 2007; Accepted 27 June 2007)
Unlike the dislocation-based plasticity in crystalline metals, which can be readily explained by their crystal structure and the presence of defects, the nature of the plasticity in amorphous alloys is not completely understood. Experiments have shown that the plasticity in amorphous alloys is strongly dependent on their atomic packing density. This study, based on the combination of experimental and computational techniques, examines the origin of the plasticity in amorphous alloys considering characteristics of the inherent atomic-scale structure as defined by short-range ordered (SRO) clusters. The role of various SRO atomic clusters in creating free volume during shear deformation is discussed. We report that the plasticity exhibited by amorphous alloys is very sensitive to the characteristics of the atomic packing state, which can be described by various SRO atomic structures and quantified by the effective activation energy for crystallization.
I. INTRODUCTION
Amorphous alloys are characterized by a random distribution of atoms with short-range atomic ordering,1–4 and their properties, the plasticity in particular, are strongly dependent on this atomic-scale structure. However, understanding of the connection between the global plasticity of amorphous alloys and their quantifiable structural characteristics is generally lacking. Many earlier studies implicitly suggested that the plasticity of amorphous alloys is intimately related to the free-volume fraction, i.e., the packing density.5–9 The global plastic deformation achieved by the formation of the shear band is accompanied by an increase in free volume.6 Cold rolling of amorphous alloys promotes the creation of the excess free volume, providing a substantial increase in the plasticity.7–9 In contrast, annealing
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0382 J. Mater. Res., Vol. 22, No. 11, Nov 2007
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below the glass-transition temperature causes the reduction in free volume, resulting in embrittlement.10–13 With increasing size, amorphous alloys become brittle due to small free volume associated with the slower cooling rate.14 A common feature of these reports viewed from a microstructural perspective is that the plasticity of the amorphous alloy is strongly influenced by the amount of free volume and hence the mobility of atoms. Deng et al.15,16 pioneered d
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