Structural prediction and surface-induced short-range order in HfNbTaZr high-entropy alloy nanoparticles

High entropy alloy (HEA) nanoparticles (NPs), finite-size nanostructures that combine multiple elements coexistent with a nanoscale surface, have emerged as a new material due to their non-trivial properties. While there exist several studies addressing the behavior of body-centered cubic (BCC) HEAs, there is still missing information devoted to predicting the structural and morphological configurations that HEA NPs can adopt. In this work, using molecular dynamic simulations and Monte Carlo algorithms, HfNbTaZr HEA NPs of 2 and 3 nm were studied. For all simulated scenarios, HEA NPs showed a significant precipitation of elements while maintaining their BCC crystalline structure. The presence of the surface drives the diffusion of some elements, leading to the formation of a core-shell structure with an HfZr rich shell and a near equiatomic HEA core. A topological change in the typical short-range order of HfNbTaZr is induced by the surface, since the formation of HfZr bands is replaced by the formation of an HfZr rich surface. It was observed that BCC HEA NPs have an irregular shear stress, a consequence of the chemical complexity, which at the same time induces surface reconstruction favoring the formation of highly faceted nanoparticles. The {111} planes were identified as the most probable surface facets on the NP, mostly due to the enthalpy of mixing and cohesive energy. Finally, a phase separation of the Hf and Zr surfaces is observed, which is also in accordance with the positive enthalpy of mixing that favors the repulsion between both elements.