Multi-scale modeling of spatial heterogeneity effect on the shear banding behaviors in metallic glasses

Abstract

Stronger than steels but able to be shaped and molded like plastics, the mechanical properties of metallic glasses (MGs) are proven to be scientific interest and potential applications in industry. However, MGs suffer from negligible plasticity prior to catastrophic failure in the form of a single shear band at room temperature, which precludes their immediate application as structural components. The structural and property heterogeneity have been found recently in MG. These nanoscale heterogeneities may influence the initiation and propagation of the shear band, which would thus improve the plasticity. To understand the structural and property heterogeneity effect on the shear band behaviors, this thesis is divided into three sections. First, Activation-relaxation technique (ART) and dynamic atomic force microscopy (DAFM) are employed to prove the existence of the nanoscale heterogeneity. ART discovers that the activation energy possesses a normal distribution, reflecting the non-uniform local structure. This non-uniformity should correspond to the different motifs found in the molecular dynamics simulation. The energy dissipation resulted from the DAFM also exhibits a normal distribution, thus the inelastic spatial heterogeneity is confirmed. Furthermore, the correlation length of the inelasticity is identified based on the 2D scanning figures from DAFM. Second, a mesoscale modeling technique, shear transformation zone dynamics (STZD) will be employed. A series of configurations with the spatial elastic heterogeneity will be built up. We find that the organization of such nanometer-scale shear transformation events into shear-band patterns is dependent on the spatial heterogeneity of the local shear moduli. A critical spatial correlation length of elastic heterogeneity is identified for the simulated MGs to achieve the best tensile ductility, which is associated with a transition of shear-band formation mechanisms, from stress-dictated nucleation and growth to structure-dictated strain percolation, as well as a saturation of elastically soft sites participating in the plastic flow. Third, a state variable, excess free volume, is incorporated into STZD model in order to introduce the strain softening which is typical during MG deformation. The stress ‘overshoot’ and cyclic hardening of MGs have been successfully captured by the model. We found that it is the dynamic competition between free volume creation and annihilation that give rise to the signature stress overshoot in the homogeneous deformation regime at elevated temperatures during tension and cause the removal of large free volume sites in the confined deformation region during nanoindentation.