Ab initio modeling of the energy landscape for screw dislocations in body-centered cubic high-entropy alloys (original) (raw)

In traditional body-centered cubic (bcc) metals, the core properties of screw dislocations play a critical role in plastic deformation at low temperatures. Recently, much attention has been focused on refractory high-entropy alloys (RHEAs), which also possess bcc crystal structures. However, unlike face-centered cubic high-entropy alloys (HEAs), there have been far fewer investigations into bcc HEAs, specifically on the possible effects of chemical short-range order (SRO) in these multiple principal element alloys on dislocation mobility. Here, using density functional theory, we investigate the distribution of dislocation core properties in MoNbTaW RHEAs alloys, and how they are influenced by SRO. The average values of the core energies in the RHEA are found to be larger than those in the corresponding pure constituent bcc metals, and are relatively insensitive to the degree of SRO. However, the presence of SRO is shown to have a large effect on narrowing the distribution of dislocation core energies and decreasing the spatial heterogeneity of dislocation core energies in the RHEA. It is argued that the consequences of the mechanical behavior of HEAs is a change in the energy landscape of the dislocations, which would likely heterogeneously inhibit their motion. npj Computational Materials (2020) 6:110 ; https://doi. INTRODUCTION Previous investigation of the fundamentals of deformation in body-centered cubic (bcc) transition metals have revealed that the core properties of the ½〈111〉 screw dislocations play an essential role in their plasticity 1 , especially at low temperatures where the deformation is thermally activated through the kink-pair nuclea-tion mechanism 2 , and is expected to be strongly temperature dependent. The high lattice friction associated with such screw dislocation motion is a result of nonplanar core structure 1,3 and is related to the height of the Peierls potential 4. Due to the importance for plastic deformation, extensive atomistic simulation studies have been devoted to computing core structures and corresponding mobilities of screw dislocations in bcc transition metals 3,5-8. In these studies, one of the significant challenges has been the variation in properties derived from different models for the interatomic potentials. For example, early studies based on classical potential models often predicted a metastable split core structure 9-11 , which leads to a camel-hump shape in the Peierls potential. Later density functional theory (DFT) calculations produced symmetric and compact dislocation cores in Mo, Ta, and Fe 12-16 ; similar compact cores have been found in other bcc transition metals, such as W, Nb, and V 17,18. In DFT studies of the energy landscape of screw dislocations in bcc transition metals 18-20 , it was found that nondegenerate cores lead to a single humped curve in the Peierls potential, implying that the split core structure might not be metastable. Alloying effects on the Peierls potential of W have also been explored 21. Recently developed machine learning-based potentials 22-24 and new embedded atom method potentials that consider quantum effects on lattice vibrations 25 and extra constraints 26 all lead to predictions of a single humped curve in the Peierls potential. Due to the dependence of the results for screw dislocations in bcc transition metals on the model for interatomic bonding, DFT-based approaches are of interest to provide benchmarks for subsequent modeling at higher scales.