EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The so-called refractory high entropy alloys (RHEAs) attract enormous interest from researchers worldwide due to their encouraging mechanical properties at elevated temperatures. However, most of the RHEAs are quite brittle at room temperature. RHEAs mostly have body-centered cubic (bcc) structure with the possible presence of B2, Laves, and other secondary phases. Here, we have systematically explored the mechanical behavior of various equiatomic bcc refractory alloys to get a better understanding of their fundamental deformation behavior. The NbTiZr, HfNbTa, HfTaTiZr, and HfNbTaTiZr equiatomic alloys with single bcc phase microstructure were examined. At room temperature, the yield strength of the alloys increased with the number of components. Variations in strength agreed reasonably with predicted solid solution hardening. The dislocation motion of different alloys was controlled by a single thermally-activated dislocation glide mechanism, i.e. overcoming the Peierls-Nabarro stress barrier. Further, stress equivalence, i.e. identical values of activation volume in different alloys at the same flow stresses was found between alloys with various chemical compositions and a number of components. Tensile testing of the HfNbTaTiZr alloy at cryogenic temperature has revealed a pronounced increase in strength together with the drop in ductility. Low-temperature plasticity was found preferred on {112} planes. The apparent activation volume analysis has suggested kink-nucleation as a dominant deformation mechanism at 77K. Meanwhile, evidence of dynamic strain aging was found during testing of the NbTiZr at 473K. The effect of chemical complexity and temperature on deformation mechanisms in bcc refractory alloys are discussed.

This study was supported by Russian Science Foundation, grant № 19-79-30066.