Dislocation Motion in Single-Phase High-Entropy Alloys -- Theory and Simulation

• Applicants: Professor Dr.-Ing. Erik Bitzek; Professor Dr. Michael Zaiser
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 289363470

High-entropy alloys (HEA), loosely defined as metallic solid solutions containing more than four elements in near-equimolar composition, represent an exciting new class of alloys. In particular the combination of high strength and good ductility as well as high hardness, wear and corrosion resistance makes HEAs promising candidate materials for high performance structural applications. The excellent failure resistance of HEAs is commonly attributed to some extreme form of solid solution strengthening. However, as in HEAs "every atom is a solute atom", conventional theories of solid solution hardening cannot be directly applied, and the necessary novel theoretical concepts for analyzing and predicting dislocation motion in HEAs have not yet been established. The aim of this project is to obtain a fundamental understanding of how dislocation glide motion, and hence dislocation plasticity in HEAs, is influenced by their unique underlying atomic structure, and to develop a theoretical framework for predicting the stress and temperature dependence of the dislocation velocity and the concomitant plastic deformation behavior. To this end, we propose a multi-scale modeling approach where atomistic simulations are used to characterize the energy landscape in which dislocations move. The results are analyzed by drawing on methods and theoretical concepts developed in statistical physics to identify the relevant features and spatial correlations of the energy landscape. This information serves as input for the mesoscopic simulation of dislocations using the discrete dislocation dynamics (DDD) method, where atomic-scale information on dislocation-lattice interactions is incorporated in the form of a stochastic pinning field. This combination of atomistic and mesoscale simulations methods allows for the study of thermally activated dislocation motion governed by complex, extended energy barriers, which result from the possibility of the dislocations to adjust their shape to the local pinning energy landscape. In addition, Monte Carlo simulations are used to study how short-range diffusion can lead to ageing by changing the local energy landscape around a dislocation at rest. The changes in energy and the associated length and time scales are then used in a mesoscale framework to investigate the dynamic strain aging and PLC-like phenomena, which have been recently observed in HEAs. Ultimately, this study on model systems for single-phase fcc HEAs will serve to develop a methodological framework which enables the computational prediction of the plastic deformation behavior of HEAs based on their atomic structure and composition. Such a framework is crucial for computational alloy design, which is of particular importance for HEA systems, where the large number of compositional degrees of freedom renders conventional alloy optimization by experimental trial-and-error approaches particularly challenging.

• DFG Programme: Research Grants