Discrete-Continuum Dislocation Dynamics at Surfaces and Interfaces with Application to Plasticity of Nanolaminated Composites

• Applicant: Professor Dr. Michael Zaiser
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429421091

We develop improved dislocation dynamics simuation approaches to investigate plastic deformation of nanolaminated metal composites. Such composites combine different metals into layered structures with layer thicknessse in the nanometre range. Their plastic deformation properteies are governed by the interactions of crystal lattice dislocations with the bimaterial interfaces of the laminate. On the atomic scale, these interactions are similar to processes occuring at internal interfaces (grain and twin boundaries), as atomic re-arrangements control the nucleation, absorption or transmission of dislocations. However, in laminates consisting of elastically dissimilar materials the elastic boundary conditions at the interfaces lead to additional, long-ranged elastic interactions: Dislocations of an elastically weaker material are repulsed by an interface with a stronger material, whereas dislocations of the stronger material are attracted (Koehler Effect). At the same time, the dislocation stress fields and stress-induced dislocation interactions are modified by the boundaries. In discrete dislocation dynamics simulations of nanolaminate plasticity it is therefore essential to accurately evaluate the influences of the bimaterial interfaces on the internal stress fields which enter the driving forces for dislocation motion. In recent years, the so-called discrete-continuum method (DCM) has led to important progress in dislocation dynamics simulation of elastic surface and interface effects. DCM uses a hybrid method to evaluate dislocation-related internal stress fields by superimposing the singular stress fields of dislocation segments with a slowly varying internal stress field that derives from solution of a coarse grained eigenstrain problem and accounts for boundary conditions at surfaces and interfaces. However, the singular segment stress fields have always been evaluated using bulk expressions which are inaccurate near surfaces/interfaces, leading to a mismatch with the coarse grained solution and to an inaccurate representation of the forces acting on dislocations. We resolve this problem by accounting for analytical corrections to the singular stress fields near surfaces and interfaces as derived in recent years by Prof. Yuan, who participates in the Project as a Mercator Fellow. Our improved DCM approach will provide an accurate description of interface effects and complex deformation boundary conditions (nanoindentation) in plastic deformation of nanolaminates. We will exploit this capability to understand complex observations such as strength inversion in Cu-Au Laminates (the elastically stronger Cu may show more plastic deformation than the elastically weaker Au), and to analyze and optimize the behavior of graded nanolaminates under different loading conditions.

• DFG Programme: Research Grants
• International Connection: China
• Cooperation Partner: Professor Dr. Xu Zhang