Contribution of Lattice Distortion to Solid Solution Strengthening in a Series of Refractory High Entropy Alloys
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We present an experimental approach for revealing the impact of lattice distortion on solid solution strengthening in a series of body-centered-cubic (bcc) Al-containing, refractory high entropy alloys (HEAs) from the Nb-Mo-Cr-Ti-Al system. By systematically varying the Nb and Cr content, a wide range of atomic size difference as a common measure for the lattice distortion was obtained. Single-phase, bcc solid solutions were achieved by arc melting and homogenization as well as verified by means of scanning electron microscopy and X-ray diffraction. The atomic radii of the alloying elements for determination of atomic size difference were recalculated on the basis of the mean atomic radii in and the chemical compositions of the solid solutions. Microhardness (μH) at room temperature correlates well with the deduced atomic size difference. Nevertheless, the mechanisms of microscopic slip lead to pronounced temperature dependence of mechanical strength. In order to account for this particular feature, we present a combined approach, using μH, nanoindentation, and compression tests. The athermal proportion to the yield stress of the investigated equimolar alloys is revealed. These parameters support the universality of this aforementioned correlation. Hence, the pertinence of lattice distortion for solid solution strengthening in bcc HEAs is proven.
The financial support by the Deutsche Forschungsgemeinschaft (DFG), Grant No. HE 1872/31-1, is gratefully acknowledged. One of the authors (AK) thanks the Carl Zeiss Foundation for financial support via a Postdoctoral Grant. IC acknowledges the financial support by the Alexander von Humboldt Foundation. The authors acknowledge the chemical analysis by ICP-OES at the Institute for Applied Materials (IAM-AWP), Karlsruhe Institute of Technology (KIT). We also thank D. Schliephake, U. Hauf, P. Eyer, F. Hinrichs, and M. Swetik for experimental support. We thank Professor W.A. Curtin (EPFL, Lausanne, Switzerland) for fruitful discussions and suggestions. This work was partly carried out with the support of the Karlsruhe Nano Micro Facility (KNMF, www.knmf.kit.edu), a Helmholtz Research Infrastructure at Karlsruhe Institute of Technology (KIT, www.kit.edu).
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