Computational analysis of shear band initiation and propagation in Zr-Cu based bulk metallic glass

Amorphous materials such as bulk metallic glass (BMG) lack an orientational long-range order unlike their crystalline counterparts. The disordered structure results in unusual deformation and structural characteristics of BMGs, such as their superior strength and hardness combined with excellent corrosion and wear resistance. The scientific interest to BMGs stems from their unique inelastic deformation demonstrated where the typical carriers of plasticity in crystalline metallic materials, namely, dislocations, are absent. Deformation in BMGs is observed to occur via highly localised heterogeneous shear bands. Initiation and propagation of these shear bands control the degree of brittleness or ductility observed in BMGs at different length scales. In this thesis, a mechanism for shear band initiation and propagation in BMGs is employed, which is elaborated via two techniques. First, a strain gradient theory was implemented (weakly nonlocal model) and secondly, a nonlocal plasticity approach employing a Vermeer–Brinkgrave model (strongly nonlocal model) was employed. Experimental studies in wedge indentation (localised and non-homogeneous deformation) and micro pillar compression (homogeneous deformation) were used to calibrate and validate the models. For this study, a commercial FE code with an implicit time-integration scheme was used with the extensive utilisation of user defined subroutines. Numerical studies with the weakly nonlocal models indicate that strain gradients from the internal material constituent caused an early onset of yield. This mechanism dominates under uniaxial homogeneous deformation conditions such as micropillar compression. The effect is less significant in wedge indentation due to stronger gradient effects caused by the tip geometry. Thus, based on this study it can be concluded that even in the absence of geometric inhomogeneity (due to component geometry or loading scenario) strain gradients influence initiation and propagation of shear bands. So, the weakly nonlocal models are suitable to assess evolution of shear bands for homogeneous and nonlocalised deformation conditions. II Results obtained with the strongly nonlocal model demonstrated that, for localised deformation studies local and nonlocal effects caused the onset of yield to be occurred at the similar time, but still elucidate earlier initiation of yield than the conventional yield criterion. However, for homogeneous deformation, it predicts a much earlier onset of yield with activation of only nonlocal effects, than the conventional yield criterion. From our studies we conclude that for indentation studies, shear -band initiation is independent of local and nonlocal effects but the technique is still efficient to elucidate the earlier onset of yield than the conventional approach. The technique is suitable for capturing the immediate yielding due to nonlocal effects for homogenous deformation conditions as well. The onset of yield occurred sooner when strong nonlocal model was implemented for both localised and homogeneous deformation studies, whereas for the weakly nonlocal models the effect was not significant for localised deformations. Hence, the strongly nonlocal model is deemed appropriate to predict the onset and propagation of shear bands on homogeneous and non-homogeneous deformations more efficiently than the weakly nonlocal model. However, this technique is found to be computationally expensive.