Microstructures and elementary deformation mechanisms of single-phase fcc and bcc high-entropy alloys

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The proposed research aims at contributing to a better understanding of a new class of metallic materials, the so-called high-entropy alloys (HEAs). These are compositionally complex alloys consisting of four, five or more metallic components in near-equiatomic concentrations. For specific element combinations, it has been shown that HEAs can be produced that consist of a single-phase solid solution with a cubic crystal structure, e.g., face-centered cubic or body-centered cubic, and with a seemingly high degree of thermodynamic stability. First assessments of the mechanical properties of single-phase HEAs revealed interesting mechanical properties, such as high mechanical strength and ductility. Moreover, detailed studies on an equimolar, fcc-structured HEAs consisting of Co, Cr, Fe, Mn and Ni showed that the characteristic mechanical properties exhibit strong temperature dependencies. In particular the strong yield stress increase with decreasing temperature is unusual for alloys with this crystal structure. It is yet unclear whether the observed peculiarities reflect intrinsic properties of HEAs or whether they are merely related to this specific alloying combination.As a consequence, the present project focuses solely on alloys that exhibit single-phase microstructures. Besides the investigation of HEA compositions which were demonstrated to yield single-phase microstructures, new bcc- and fcc-structured HEAs will be produced whose compositions will be altered with the goal of directly controlling intrinsic properties, such as the stacking fault energy. Focus will be placed on the identification of suitable thermomechanical processing routes that allow for establishing chemically and microstructurally homogeneous materials, an aspect which has often been neglected in previous studies. Homogeneous microstructures are, however, a prerequisite for a meaningful characterization of the mechanical properties. The latter is another major goal of the proposed work. On the one hand, open questions which were raised in previous experimental studies dealing with mechanical properties of HEAs will be addressed by well-designed experiments. An example of such an open question is the nature of the strongly temperature dependent yield stress in fcc-structured HEAs which will be investigated by a first accurate assessment of the activation parameters. On the other hand, a first thorough characterization is planned for other single-phase HEAs whose mechanical behavior has not yet been characterized in a systematic fashion, i.e., most bcc-structured alloys. In combination, the obtained results will help greatly in determining whether HEAs possess unique intrinsic properties as is often hypothesized. In addition, the proposed experiments will significantly extend the mechanical property data base for HEAs, something that is necessary to identify potential candidate materials for further research efforts or even practical applications.