Chang Myeon Lee

Structural disordering process of an amorphous alloy driven by the elastostatic compression at room temperature

This study demonstrates that permanent deformation results when an amorphous alloy is subjected to a stress below yield at room temperature. The resultant deformation was observed to be homogeneous and is thought to occur as a result of the structural disordering, in which densely packed short-range ordered clusters break down to form new, loosely packed ones. It was found that this disordering process is irreversible even if the applied stress is removed, resulting in the creation of excess free volume. These findings were analyzed using the molecular dynamics simulations in the light of atomic-scale structural changes.

Paradoxical phenomena between the homogeneous and inhomogeneous deformations of metallic glasses

Experiments in binary alloys demonstrate that metallic glasses exhibiting more plastic strain during homogeneous deformation tend to show lower global plasticity during inhomogeneous deformation. Testing of Cu–Zr binary alloys supports the hypothesis that the formation energy of a shear transformation zone, as extracted from the experimental data, is related to the homogeneous flow rate. We also report the microstructural aspects that control the global plasticity of metallic glasses in the light of structural disordering, softening, and shear localization.

Medium-range orders in amorphous alloys and their role on the plasticity: A molecular dynamics viewpoint study

The local structural states of amorphous alloys have been depicted previously via short-range orders (SROs). However, the concept of SROs alone is inadequate and sometimes insufficient to explain the structure-property relation of the amorphous alloys. In this study, we propose new types of medium-range building structures that affect the mechanical properties, plasticity in particular. Using a combination of molecular dynamics simulations and the Voronoi tessellation method, we demonstrate a three-dimensional configuration of icosahedral medium-range orders (I-MROs) and elucidate how these icosahedral orders evolve by the application of shear deformation. It was observed that the structural stability of the icosahedral orders relies largely on how they are linked via percolation and this linking is explained in detail. (Received June 9, 2009)

Understanding the Plasticity of Amorphous Alloys Via the Interpretation of Structural Evolution Inside a Shear Band

)Abstract The effect of the initial packing structure on the plasticity of amorphous alloys was investigated bytracing the structural evolution of the amorphous solid inside a shear band. According to the moleculardynamics simulations, the structural evolution of the amorphous solids inside the shear band was more abruptin the alloy with a higher initial packing density. Such a difference in the structural evolution within the shearband observed from the amorphous alloys with different initial packing density is believed to cause differentdegrees of shear localization, providing an answer to the fundamental question of why amorphous alloys showdifferent plasticity. We clarify the structural origin of the plasticity of bulk amorphous alloys by exploring themicrostructural aspects in view of the structural disordering, disorder-induced softening, and shear localizationusing molecular dynamics simulations based on the recently developed MEAM (modified embedded atommethod) potential.Key wordsamorphous alloys, free volume, structural disorder, molecular dynamics.