Shear-Band Formation and Thermal Activation in Metallic Glasses

Metallic glasses (MGs) usually have high strength, high hardness and high elastic strain limit. However, the deformation mode and mechanism in metallic glasses are radically different from those in conventional crystalline materials with a long-range ordered structure. For crystalline materials, the intrinsic relationship between their mechanical properties and crystal structures has been well described by dislocation theory. In contrast, for amorphous materials, theories on the structures and controlling factors of localized shear-band formation are far from being complete.

In this thesis, shear-banding behavior of MGs under nanoindentation was first reviewed. The hardness of MGs was found to be independent on the shape of indenter tip. The hardness drop during each pop-in was a constant for a given indenter tip. A nanoindentation-based method for measuring the shear resistance of MGs was further developed.

The hardness of MGs was largely affected by residual stresses, especially the tensile residual stress. Significant softening could be caused by tensile residual stress and the softening was attributed to the creation of extra free volume. The hardness of MGs was demonstrated to be extremely sensitive to the initial free volume in the material. Spherical indentation was also conducted on stressed MG sample to study the effect of residual stress on the first shear-band formation. It was found the critical shear stress for the shear-band formation was essentially a constant. The constant critical shear stress was correlated with a critical free volume in the material. Spherical indentation was further carried out at elevated temperature but well below glass transition temperature to explore the temperature effect on shear-band nucleation. Localized shear-banding was observed to be the dominant deformation mode at all temperatures. The shear stress at first pop-in or the onset of yielding decreased with temperature, and the activation energy and the size of shear transformation zone (STZ) were measured. Shear-band nucleus was estimated to be 10~20 nm and independent on temperature.

Micro-compression tests were further performed on micro-sized pillar samples at different temperatures. The strength-temperature relationship could be explained by the constant viscosity concept, suggesting shear-banding was a stress-induced glass transformation.