Microstructure of shear bands in bulk metallic glasses

Microstructure of shear bands in bulk metallic glasses
Bulk metallic glass; shear band; Transmission electron microscopy (TEM); Plastic deformation; In situ straining
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Bulk metallic glass (BMG) is emerging as one of the promising materials. Various researches on BMGs have brought possibilities for wide potential applications as structural and functional materials due to their high strength and good corrosion/magnetic properties. One of the most attractive properties for BMGs is exotic mechanical properties including ultra high elastic limit and high yield strength compared with those of the corresponding crystalline materials. However, a catastrophic fracture without strain hardening occurs against the external loads, limiting the reliability of the material in practical use. Under a tensile loading condition, the metallic glasses fail catastrophically on one dominant shear plane and show little global plasticity. On the other hand, under the compressive and geometrically constrained loading condition, the metallic glasses deform in an elastic-perfect plastic manner. Therefore, multiple shear bands can be observed when the catastrophic failure is delayed by the geometrical constraint, for example, compression, bending, rolling and nano indentation. During plastic deformation of the metallic glasses, the plastic flow is highly localized into shear bands. When the shear bands propagate, resistance of the material to deformation within the shear band is reduced, and further deformation takes place preferentially along the shear bands. This indicates that mass flow inside the shear bands is enhanced and structural changes occur within the shear bands during deformation. Since the deformation energy is concentrated in the shear bands, it can lead to the formation of nanocrystals either by directly imposing large permanent strain for atomic rearrangement or by inducing adiabatic heating enabling the long-range atomic diffusion. However, the microscopic mechanism for the deformation-induced crystallization is not clearly solved yet.
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