There is a growing interest in High Entropy Alloys (HEAs) due to their outstanding mechanical properties. Most simulation studies have focused on face-centered cubic (fcc) HEAs; however, bcc HEAs can offer a larger elastic modulus and plastic yielding, thus, becoming possible candidates for the next generation of refractory materials. In this work, we focus on molecular dynamics (MD) simulations of bcc HfNbTaZr nanocrystalline samples, with a grain size (d) between 5 and 17 nm, deformed under tension at 300 K. The elastic modulus increases with the grain size and reaches a plateau near 10 nm. We find the typical inverse Hall–Petch (HP) behavior with yield strength, ultimate tensile stress (UTS), and flow stress increasing with d. Up to 12 nm, there are contributions from dislocations and twins; however, grain boundary (GB) activity dominates deformation. For the 5 nm grains, the GB disorder extends and leads to extensive amorphization and grain size reduction. For $d>10$ nm, there is a HP-type behavior with dislocations and twinning controlling deformation. For this regime, there is hardening at large strains. Compared to bcc single metal samples, the HP maximum of this HEA appears at a lower grain size, and this could be related to the chemical complexity facilitating dislocation nucleation. We use machine learning to help understand deformation regimes. We also compare our results to a single crystal (SC) HfNbTaZr HEA deformed along [001] and find that the single crystal is weaker than the nanocrystalline samples. The single crystal deforms initially by twinning and then rapidly by dislocation multiplication, leading to strong hardening. It has been proposed that edge dislocations play a major role in bcc HEA plasticity, and we also analyze the relative contributions of edge versus screw dislocations during deformation for both single crystal and nanocrystalline samples.

Noble-metal-based catalysts are the most used nanomaterials to carry out electrochemical reactions, which are commonly applied in fuel cells. This kind of catalyst is expensive and it is worth mentioning that noble metals are scarce. So, nanocatalysts based on cheaper metals, such as Ag and Cu, are highly desired. Here, we report the natural arrangement of different Ag_xCu_y nanostructures through oleylamine reduction. Firstly, an experimental study was carried out in order to study the crystallographic structure, size, and shape of each synthesized nanostructure. The samples were fully characterized via powder X-ray diffraction, while scanning-transmission electron microscopy with a high-angle annular dark-field (HAADF) was applied to investigate the morphological features. Interestingly, the HAADF images of the AgCu NPs mostly show a Janus-type configuration, instead of a core–shell architecture, which is the most stable atomic arrangement. Given this, we subsequently performed classical molecular dynamics simulations under the NVT canonical ensemble to further deepen our study. The theoretical results pointed out that the nanostructure with a core–shell morphology is the one with the lowest energy. However, it also indicates an energy decrease in the Janus configuration, as long as the NP size increases. Therefore, for nanostructures with a large number of atoms, this could lead to a strong competition between Janus and core–shell arrangement. Finally, considering the AgCu NP size, it is worth noting that the theoretical data supports the experimental results, making these systems interesting not only because of their properties but also due to the relatively easy synthesis procedure.