Coupling of irreversible plastic rearrangements and heterogeneity of the local structure during deformation of metallic glasses

• Applicants: Professor Dr. Fathollah Varnik; Professor Dr.-Ing. Gerhard Wilde
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2017 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 325408982

One of the major difficulties in the applications of metallic glasses as structural materials lies in their propensity to localized plastic deformation and the resulting catastrophic failure under load. Yet, although of both, scientific as well as application related interest, the early stages of strain localization are not well understood. The present project aims, therefore, at a study of the initial stages of shear localization in metallic glasses. An important issue here is the role played by small scale structural heterogeneity, inherent to amorphous solids, and fluctuations of the local stress for the occurrence of an elementary shear transformation event. These structure-stress effects are also of great importance for the onset of the transition from a homogeneous plastic deformation to shear bands in metallic glasses. The following questions are of key interest here. (i) How do the local structure and the locally acting stress influence irreversible particle rearrangements and (ii) under which conditions may they lead to the formation of system-spanning shear bands? In order to address these issues adequately and in a comprehensive way, the present project brings together expertise in experimental investigations of metallic glasses and computer simulations of model glassy systems. Molecular dynamics simulations, on the one hand, will provide detailed information about the collective particle rearrangement on small scales, thereby helping to identify elementary plastic events, the so-called shear transformation zones. Moreover, the stress tensor will be determined on the local scale, relevant for elementary shear events. Using the thus obtained simulated data, correlations of stress and plastic strain will be evaluated both in space and time. Similarly, an analysis of the mutual connection between plastic strain and the local structure will be performed. Experiments, on the other hand, will reveal static and dynamic properties of metallic glasses under deformation on bridging scales, from atomistic ones, on which advanced TEM methods such as electron correlation microscopy will address the structural heterogeneities, to meso- and macroscopic scales, providing sample-averaged information on the enhancement of atomic transport related to earlier stages of deformation localization, prior to shear banding. The above mentioned experiments and computer simulations will shed light from different angles onto the role of the local structure and stress fluctuations for the onset of shear banding. The overarching goal of the present collaborative work lies in assessing the results from the complementing experimental and theoretical work packages to obtain a physically consistent picture of the transition between elasticity and plasticity in metallic glasses.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Sergiy Divinski