Micromechanical and microstructural investigations of monocrystalline face-centred cubic high- and medium-entropy alloys

• Applicant: Professor Dr.-Ing. Gunther Eggeler, since 1/2017
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 310751327

Recently, a novel class of materials, high- and medium-entropy alloys (HEAs/MEAs) containing multiple principal elements in roughly equal atomic parts, has taken the scientific world by storm. In polycrystalline HEAs/MEAs, strength depends on the type and number of alloying elements, but a rigorous understanding of basic mechanisms is lacking. To deepen our scientific understanding of alloying effects, it is vital to test single crystals so that friction stresses can be evaluated on the activated slip systems as a function of composition and correlated with the underlying dislocation processes. Since growth of bulk single crystals is tedious and not always feasible, we plan to perform micromechanical tests on micropillar and microtensile specimens prepared by focused ion beam (FIB) milling from individual (single crystal) grains of polycrystalline materials. This will allow us to use conventional processing (melting, casting, homogenization, rolling, recrystallization, grain growth) to make any desired alloy. Initially, face-centered cubic HEAs/MEAs based on various combinations of Cr, Mn, Fe, Co and Ni will be investigated. Effects of elemental substitutions (Pd, Cu, Al, Cu+Al) deliberately chosen to uncover specific mechanisms will then be probed as well as deviations from equiatomic compositions. In other words, the number, concentration, and type of alloying elements, will be varied to evaluate the influence of fundamental parameters such as atomic size/mass, elastic modulus, and stacking fault energy, on microstructure and mechanical properties. Some of the micromechanical tests will be performed in situ in a scanning electron microscope to correlate yield point phenomena on the load-displacement curves with physical slip events on the specimen surfaces in real time. Critical resolved shear stress will be determined as a function of orientation and composition, and the validity of Schmids law will be checked. Microtensile tests will be conducted to check for tension-compression asymmetries. Since microstructural analysis is crucial to the interpretation of these results, TEM foils will be extracted and examined before and after interrupted mechanical tests (different amounts of strain) to characterize the single-phase nature of the alloys, deformation microstructures (twins, stacking faults, dislocations, etc.), and how compositional complexity affects microstructure, slip character and plasticity. Slip behavior will be correlated with heat treatment and chemical composition for possible short range ordering effects. To check whether dislocation core configurations are responsible for higher friction stresses, we will investigate dislocation cores as a function of compositional complexity. The knowledge gained from this project will contribute to a better understanding of alloying effects on the mechanical properties of concentrated, massively alloyed, solid solutions whose behavior cannot be explained by textbook theories.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Professor Easo P. George, Ph.D., until 12/2016